Download Regulation of macronutrient transport

Review
Blackwell
Oxford,
New
NPH
1469-8137
0028-646X
?October
0
Tansley
???
Tansley
?? Phytologist
review
Review
UK
2008
Publishing Ltd
Tansley review
g
y
P
w
iN
p
lcm
a
).Jn
8
0
rs(2
to
u
eA
h
T
©
Tansley review
Regulation of macronutrient transport
Author for correspondence:
Anna Amtmann
Tel: +44 141 3305393
Fax: +44 141 3304447
Email: [email protected]
Anna Amtmann and Michael R. Blatt
Plant Sciences Group, Faculty of Biomedical and Life Science, University of Glasgow, Glasgow G128QQ,
UK
Received: 9 July 2008
Accepted: 10 September 2008
Contents
Summary
35
III.
Molecular mechanisms of regulation
40
I.
Introduction
36
IV.
Conclusions and outlook
46
II.
Stimuli and signals
37
References
47
Summary
New Phytologist (2009) 181: 35–52
doi: 10.1111/j.1469-8137.2008.02666.x
Key words: environmental stress, ion
transport, nutrient signalling, plant nutrition,
regulation.
© The Authors (2008).
Journal compilation © New Phytologist (2008)
In addition to light, water and CO2, plants require a number of mineral nutrients, in
particular the macronutrients nitrogen, sulphur, phosphorus, magnesium, calcium
and potassium. After uptake from the soil by the root system they are either immediately assimilated into organic compounds or distributed within the plant for usage
in different tissues. A good understanding of how the transport of macronutrients
into and between plant cells is adjusted to different environmental conditions is
essential to achieve an increase of nutrient usage efficiency and nutritional value in
crops. Here, we review the current state of knowledge regarding the regulation of
macronutrient transport, taking both a physiological and a mechanistic approach.
We first describe how nutrient transport is linked to environmental and internal cues
such as nutrient, carbon and water availability via hormonal, metabolic and physical
signals. We then present information on the molecular mechanisms for regulation of
transport proteins, including voltage gating, auto-inhibition, interaction with other
proteins, oligomerization and trafficking. Combining of evidence for different nutrients,
signals and regulatory levels creates an opportunity for making new connections
within a large body of data, and thus contributes to an integrative understanding of
nutrient transport.
New Phytologist (2009) 181: 35–52 35
www.newphytologist.org 35
36 Review
Tansley review
I. Introduction
1. Nutrient transport
Plants are modest organisms, their basic requirements for
life being largely satisfied by light, CO2 and water, with the
addition of a supply of mineral nutrients in the soil.
Macronutrients, which are required in comparatively large
quantities, include the elements nitrogen (N), potassium (K),
sulphur (S), phosphorus (P), magnesium (Mg) and calcium
(Ca). Our understanding of plant nutrient requirements
has led to enormous progress in agricultural food production,
and most farmers in Western Europe and the USA routinely
apply N, P and K as fertilizers. Nevertheless, nutrient
deficiencies regularly occur even in fertilized fields as
chemical and physical properties of the soil can lead to
reduced mobility and absorbance or leaching of individual
nutrients. Financial constraints, especially in developing
countries, often necessitate that farmers prioritize some
nutrients over others (e.g. N over K). As a result of such
imbalanced fertilizer input, crops do not reach their full
potential and soils are depleted of specific nutrients. With
fertilizer prices increasing and agriculture (as well as biofuel
production) moving into marginal soils, a good understanding
of nutrient uptake and usage by the plant becomes ever more
important. In particular, molecular information concerning
the regulation of nutrient transport will be an essential
prerequisite for biotechnological efforts to increase nutrient
usage efficiency.
For the uptake of macronutrients and their allocation in
different cellular compartments and tissues, plants employ
a number of transport proteins (‘transporters’), which differ
from each other not only in their tissue and membrane
location but also in their mode of energization, substrate
affinity and specificity (Blatt, 2004). The enormous variety
of features displayed by transport proteins provides an
invaluable pool for plants from which to select those transporters that are best suited to fulfil their nutritional demands
in particular conditions. Approximately 1000 genes (5% of
the entire genome) of Arabidopsis thaliana have known or
putative functions in membrane transport (Maathuis et al.,
2003). Clearly they are not all active everywhere and at all
times. Rather their expression and activity are regulated in
response to a number of external and internal stimuli. The
degree to which plants make use of the specific features of
individual transporters has been revealed in a recent study.
For example, the spatial arrangement of four different
members of the ammonium transporter (AMT) family
within the A. thaliana root, in combination with their distinct
substrate affinities, provides a highly sophisticated system
for directed ammonium transport within the root tissue
(Yuan et al., 2007). Differential regulation of the individual
transporters by external factors further increases the complexity
of such systems.
New Phytologist (2009) 181: 35–52
www.newphytologist.org
2. Function and regulation
Regulation of nutrient transporters is closely related to the
functions of the transported macronutrients, which can
be roughly divided into metabolic, osmotic and energetic
functions (Marschner, 1995; Epstein & Bloom, 2005). Apart
from K and Ca, all macronutrients are integrated into important
organic compounds such as amino acids and proteins (N
and S), nucleic acids (N and P), phospholipids (P) and
chlorophyll (Mg). P and Mg have additional functions in
energy conservation and conversion (Mg-ATP). In accordance
with these functions, regulation of N, P, S and Mg transporters
has the overall goal to supply the plant with essential
molecules for the build-up of dry matter and energy during
growth and development. K and Ca differ in several aspects
from other macronutrients. They are not metabolized but
remain in their ionic elementary form throughout plant
growth and development. In their capacity as osmolytes they
drive growth by securing a steady increase in the plant water
content, and regulation of their transport is therefore again
under the diktat of growth and related stimuli. However,
transport of K+ and Ca2+ is not only a means for uptake
and allocation of these nutrients but serves other essential
functions. Notably, K+ flux counterbalances other ion fluxes,
for example, fluxes of protons, thereby enabling the activity of
ATP synthase, H+-ATPase and H+ co-transport systems. As
a result, regulation of these systems is often linked to the
regulation of K+ transport. Furthermore, the osmotic function
of K+ is not limited to growth but extends to the reversible
movement of plant organs, most importantly the opening and
closing of stomata (Willmer & Fricker, 1996; Blatt, 2000).
Consequently, K+ channels in guard cells are regulated by
environmental factors such as light, humidity and CO2. Ca2+
transport is closely related to the role of cytoplasmic Ca2+ as a
second messenger. Influx of Ca2+ into the cytoplasm through
ion channels and its subsequent removal by Ca2+-pumps
create oscillations of cytoplasmic Ca2+ that constitute a signal
that is in many cases essential for linking environmental stimuli
to downstream events inside the cell (Sanders et al., 1999,
2002). Consequently, regulation of Ca2+ transporters is at the
centre of many cellular responses to the environment.
3. Physiology and mechanistics
Regulation of nutrient transport can be discussed from a
physiological and a mechanistic point of view. In this review
we do both. Section II reviews current knowledge regarding
external and internal stimuli that regulate nutrient transporters
and the signalling pathways involved. While it is impossible to
include all existing evidence here, we present a number of
examples that we consider representative for the different
situations that require regulation of nutrient transport. Section
III describes molecular mechanisms underlying regulation
of nutrient transporters. Investigation of these mechanisms is
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
greatly aided by techniques that directly measure nutrient
movement through the respective transport proteins and
high-resolution structural models on which to base structure–
function analysis. The former are available for ion channels
(e.g. patch clamp analysis), and the latter exist for some ion
channels and pumps. Much of the information collated in
Section III therefore relates to the transport of K, Ca or
protons. To enhance the potential usefulness of this section in
terms of our understanding of the regulation of other nutrient
transporters, we present only those mechanisms that operate
in more than one type of transporter or that functionally link
different types of transporters. The overall aim of this review
is to provide an incentive for knowledge transfer between
different lines of research, for example concerning different
stimuli (nutrients, metabolites and water status), nutrients
(N, S, P, K and Ca), types of transporters (channels, pumps
and co-transporters), and levels of regulation (transcript,
protein and submolecular). The future challenge will be to
experimentally establish for each stimulus–response pair a
functional continuum between receptor and targeted transporter,
and to understand how simultaneously occurring stimulatory
inputs are integrated into distinct mechanistic outputs.
II. Stimuli and signals
1. Nutrient availability
Transcript abundances of ion transporters often vary with the
concentration of their substrate in the growth medium. While
some transporters are induced by a decrease in substrate
concentration from high to low supply, others are induced by
an increase in substrate concentration from nil to low supply.
For example, abundances of transcripts encoding high-affinity
sulphate (e.g. A. thaliana AtSULTR1;2 and AtSULTR1;2;
Buchner et al., 2004) and phosphate (e.g. AtPT1 and AtPT2;
Al-Ghazi et al., 2003) transporters rise upon removal of S or
P from the growth medium. By contrast, up-regulation of
members of the high-affinity nitrate transporter (NRT2) family
is observed after adding small amounts of nitrate (10–50 μM)
to an N-depleted medium (Krapp et al., 1998; Filleur & DanielVedele, 1999). Responsiveness to external supply can even differ
between homologous genes in different species. For example,
ammonium transporters of the AMT family in A. thaliana
(e.g. AtAMT1;1 and AtAMT1;3) show increased expression
during N deficiency, whereas some AMT homologues in tomato
(Lycopersicon esculentum) (LeAMT1;2) and rice (Oryza sativa)
(OsAMT1;1 and OsAMT1;2) are induced by N supply (Loque
& von Wiren, 2004). AMT transporters also differ in their
temporal response to N deficiency, suggesting that they respond
to plant nutrient status rather then external concentrations.
Thus, transfer of plants to N-free medium induces the expression
of AtAMT1;1 and AtAMT1;3 within 3 d, whereas induction of
AtAMT1;2 and AtAMT2;1 requires more extended periods of
N deficiency (Gazzarrini et al., 1999; Sohlenkamp et al., 2000).
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
The observation that some transporters are induced by a
change from high to low nutrient supply and others by a change
from nil to low supply indicates a fundamental difference
in the underlying signalling pathways that allows plants to
respond to different environmental situations, that is, progressive depletion of the nutrient in the soil or resupply after
a period of scarcity. In both cases the regulatory events result in
high expression levels of specific transporters under conditions
that require their function as high-affinity systems in nutrient
uptake. Whether differences observed at the mRNA level are
still apparent at the protein level is often questioned. A recent
study in which myc-tagged SULTR1;1 and SULTR1;2
proteins were expressed under the control of the endogenous
promoters showed that the response to S starvation is not only
apparent but stronger and faster at the protein level (Yoshimoto
et al., 2007).
Astonishingly few transporters involved in K+ transport
respond to varying K supply at the level of transcripts
(Maathuis et al., 2003). Out of some 50 genes expected to
have K+ transport capacity (Mäser et al., 2001) only HAK5, a
putative high-affinity K+ uptake system, has consistently been
reported as being induced by K starvation (Armengaud
et al., 2004; Shin & Schachtman, 2004; Gierth et al., 2005).
Transcript abundances of K+ channels, although responding
to several environmental and hormonal stimuli (Pilot et al.,
2003), are not affected by external K supply. It appears that
adaptation of K+ channels to K availability and plant K status
occurs primarily at the protein level (see Section III).
The molecular elements targeted by nutritional stimuli
have been identified in some cases. A GATA transcription factor
was found to be inducible by nitrate (Bi et al., 2005), and a
150-bp region in the promoter of AtNRT2.1 contains a GATA
motif (Girin et al., 2007). The 150-bp cis-acting element is
required for up-regulation of NRT2.1 by nitrate and its
repression by N metabolites. The GATA motif, while necessary
for regulation of NRT2.1, is not sufficient, and it has been
suggested that a DOF element in the promoter regions plays
an additional role in the regulation of NRT2.1. Interestingly,
activity of the 150-bp region is further modulated by sucrose, and
DOF elements, as well as the GATA transcription factors,
play a role in regulating carbon (C) metabolism (Bi et al., 2005;
Girin et al., 2007). These regulatory units are therefore possible
points of integration between C and N metabolism. Analysis of
mutants with deletions in the upstream region of AtSULTR1;1
identified a 16-bp sulphur-responsive element (SURE)
between −2777 and −2762 that is sufficient and necessary for
enhanced expression of SULTR1;1 in response to S starvation
(Maruyama-Nakashita et al., 2005). Regulation of phosphate
transporters and other P-responsive genes is under the control
of transcription factors of the MYB-CC family such as PHR1
and PHR2, which act as positive regulators (Rubio et al.,
2001; Todd et al., 2004). AtPHR1 recognizes a GnATATnC
motif, the P1BS element. However, although the P1BS motif
is present in the promoters of many P-regulated genes it is not
New Phytologist (2009) 181: 35–52
www.newphytologist.org
37
38 Review
Tansley review
Fig. 1 Transcriptional regulation of nutrient transport. Depicted are
stimuli and pathways (shown as arrows) regulating genes encoding
phosphate transporters (PHT, green), sulphate transporters (SULTR,
yellow), ammonium transporters (AMT, light blue), nitrate
transporters (NRT, dark blue) and potassium transporters (HAK, red;
note that the depicted transporters represent several different
proteins and that the exact transport mode is not shown).
Transporters are generally up-regulated by low concentrations of the
respective nutrient (phosphorus (P), sulphur (S), nitrogen (N) and
potassium (K)) in the soil, but may be inhibited by deficiency in other
nutrients (e.g. NRT by K). Feedback control is exerted through
primary assimilates such as glutathione (GSH), O-acetyl-serine (OAS)
cysteine (Cys), glutamine (Gln), asparagine (Asn) and arginine (Arg).
Nutrient uptake is linked to the photosynthetic rate (light) through
sugar signals (Suc, sucrose; Fru, fructose; Glu, Glucose; G6P, glucose6-phosphate). Formation of reactive oxygen species (ROS) is
necessary for the K-deficiency response of HAK5. In some cases there
is evidence for the involvement of specific hormones (Ckn,
cytokinin), components of ubiquitination complexes (PHO, SIZ),
micro RNAs (miR), kinases (CRE1, Sac3) and transcription factors
(PHR) in the signalling pathway. P1BS and SURE are P- and
S-responsive promoter cis elements respectively. For further details
and references see text.
overrepresented in P-regulated genes (Hammond et al., 2003).
This could indicate that other promoter elements are required
for P-specific responses or that P regulates PHR genes at the
post-transcriptional level (Amtmann et al., 2006). For example,
a recent study provided evidence for post-transcriptional
regulation of PHR1 by sumoylation. SIZ1, a small ubiquitinlike modifier (SUMO) E3 ligase, transiently activates PHR1
during P starvation (Miura et al., 2005). Finally, protein
degradation by ubiquitination has emerged as an important
regulatory mechanism for plant adaptation to N- and P-limiting
conditions (Fujii et al., 2005; Peng et al., 2007). Particularly
interesting is the case of PHO2, an E2 ubiquitin conjugating
enzyme, which underlies the locus of a P-hyperaccumulating
New Phytologist (2009) 181: 35–52
www.newphytologist.org
A. thaliana mutant. Upon P starvation, PHO2 mRNA is
degraded by a complementary micro RNA (miR399), resulting
in de-repression of downstream targets including phosphate
transporters PHT1.8 and 1.9 (Fujii et al., 2005; Aung et al.,
2006; Bari et al., 2006; Chiou et al., 2006).
Identification of upstream events linking changes in nutrient
availability to gene expression of the respective transporters
has been an area of intensive research, the results of which
are summarized in Fig. 1. Transcriptional regulation of sulphate
transporters involves phosphorylation (e.g. by the Snf1-like
Ser/Thr kinase Sac3; (Davies et al., 1999) and dephosphorylation events (Maruyama-Nakashita et al., 2004a). Production
of reactive oxygen species (ROS) occurs in roots in response
to K, N, P and S deprivation (albeit differing in origin and
location; Shin et al., 2005; Schachtman & Shin, 2007), and
is necessary for the induction of some downstream responses
including de-repression of HAK5 (Shin & Schachtman, 2004).
Several studies indicate a role for plant hormones in mediating
between nutritional stimuli and nutrient transporters. Thus,
K starvation enhances the expression of enzymes involved in
the biosynthesis of ethylene (Shin & Schachtman, 2004) and
jasmonic acid (Armengaud et al., 2004), and concentrations
of the two hormones increase in roots and shoots of K-starved
plants, respectively (Shin & Schachtman, 2004; Cao et al., 2006).
However, the exact position of ethylene and jasmonate signals
within the K starvation response remains to be elucidated.
Expression of sulphate and phosphate transporters is repressed
by cytokinin and application of cytokinin suppresses their
induction by S or P starvation (Martin et al., 2000; MaruyamaNakashita et al., 2004c; Hou et al., 2005). Signal transduction
of cytokinin-dependent responses involves the histidine kinase
CRE1 (Inoue et al., 2001), as cre1 mutants no longer show a
response of sulphate transporters to cytokinin (MaruyamaNakashita et al., 2004b). The observation that both cytokinin
and CRE1 transcript abundances decrease during P starvation
(Franco-Zorrilla et al., 2005) further supports the notion of
a CRE1/cytokinin signalling pathway in nutrient responses.
2. Primary assimilates and other nutrients
Primary assimilates often exert feedback control on nutrient
uptake (Fig. 1). For example, expression of the nitrate
transporter NRT2.1 is repressed by high concentrations of
the amino acids arginine, asparagine and glutamine (Krapp
et al., 1998; Zhuo et al., 1999; Vidmar et al., 2000). In
A. thaliana, induction of the ammonium transporter
AMT1;1 in response to resupply of ammonium only occurs
if assimilation of ammonium into glutamine is inhibited by
methionine sulphoximine (Rawat et al., 1999). By contrast,
in rice, glutamine induces the expression of OsAMT1;1
(Sonoda et al., 2003), indicating that the same amino acid can
act as a metabolic trigger for both down- and up-regulation of
AMT genes, depending on isoform and plant species (Loque
& von Wiren, 2004). Sulphate transporters in barley (Hordeum
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
vulgare) (HvST1) and A. thaliana (SULTR1;1 and 1;2) are
repressed by glutathione (GSH) and cysteine (Smith et al.,
1997; Maruyama-Nakashita et al., 2004c). O-acetyl-L-serine
(OAS, a precursor of cysteine synthesis) overrides the negative
feedback regulation of HvST1 by GSH (Smith et al., 1997).
Both A. thaliana genes are also up-regulated by OAS, albeit with
different sensitivities (Maruyama-Nakashita et al., 2004c).
The extent to which a plant can utilize a particular nutrient
depends on the availability of all other nutrients. It can therefore
be expected that nutrient transporters will also be regulated by
nonsubstrate nutrients. Very few studies have directly addressed
this question, but the effect is clearly visible in microarray
experiments where removal of one nutrient almost always
results in transcript changes of genes mediating the transport
of other nutrients (Wang et al., 2002; Hammond et al., 2003;
Maathuis et al., 2003; Nikiforova et al., 2003). One example
that has been investigated in some detail concerns the response
of sulphate transporters to N supply. It was found that low N
attenuates the induction of SULTR1;1 and SULTR1;2 by S
starvation, and suggested that the signal resides in the OAS
pool (Maruyama-Nakashita et al., 2004b). OAS production
involves an amino transfer reaction and therefore depends on
the supply of N. Another example of cross-regulation is the
effect of P supply on the expression of AMTs and NRTs (Wang
et al., 2002; Wu et al., 2003), which may involve a systemic
sucrose signal. NRT2.1 was also found to be down-regulated
during K starvation (Armengaud et al., 2004), which is
surprising in the light of increased sugar concentrations in Kstarved roots (Amtmann et al., 2008). In this case, regulation
is likely to occur through changes in organic and amino acid
concentrations, which in turn may be the result of allosteric
regulation of several glycolytic enzymes by K+ and pH
(Amtmann et al., 2006).
3. Carbon status
Nutrient uptake is tightly linked to the C status of the plant, and
indirectly controlled by environmental factors that determine
the photosynthetic rate, such as light (Fig. 1). Dependence
on C metabolism was established for sulphate transporters of
the SULTR family. Addition of glucose and sucrose enhanced
the transcriptional response of SULTR1;1 and SULTR1;2 to
S starvation (Maruyama-Nakashita et al., 2004c). Conversely,
depletion of C sources from the growth media attenuated the
induction of these two genes in S-free medium. Lejay and
co-workers (Lejay et al., 2003) tested a number of root ion
transporters for regulation by photosynthesis. The A. thaliana
genes AMT1.1, AMT1.2 and AMT1.3, NRT1.1 and NRT2.1,
HST1, AtPT2 and AtKUP2, encoding ammonium, nitrate,
sulphate, phosphate and potassium transporters, respectively,
were all repressed in the dark. This repression was prevented
by adding sucrose at the beginning of the dark period,
indicating a link to the photosynthetic rate rather than the
circadian clock. The authors found a strong correlation between
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
the stimulating effects of light and sucrose, and measured an
increase in the concentration of soluble sugars in the root
tissue during the light period. Lejay and co-workers went
on to investigate a possible role of known sugar signalling
pathways in the regulation of these transporters. Hexokinase
(HXK) has been postulated to be a sugar sensor and a regulatory
element for crosstalk between C and N metabolism (Jang
et al., 1997; Moore et al., 2003). Previous experiments with
sugar analogues that are phosphorylated by HXK but poorly
metabolized by glycolysis (i.e. 2-deoxyglucose (2-DOG) and
mannose) had suggested that HXK was necessary and
sufficient for the creation of sugar signals, independent of its
function in sugar metabolism (Jang & Sheen, 1994). However,
this was not the case for light regulation of nutrient transporters
(Lejay et al., 2003). Expression of NRTs, AMTs and HST1
was repressed rather than stimulated by 2-DOG or mannose,
and glucosamine, an inhibitor of HXK, decreased mRNA levels
of NRT2.1 even when sucrose was applied. Further evidence
against a role of HXK signalling in light regulation of transporters
came from experiments with sugar sensing mutants (rsr1, sun6,
gin1-1 and hxk), none of which showed an altered transcriptional
response of NRT2.1 to sucrose and light. Induction of NRT2.1
by sucrose and glucose was, however, abolished in HXK
antisense plants, suggesting that catalytic activity of HXK is
required for sugar regulation of this transporter. Recently, the
same group identified glucose-6-phosphate (G6P) as the sugar
signal (Lejay et al., 2008). G6P regulation operates downstream
of the metabolic function of HXK and requires in most but
not all cases an active oxidative pentose phosphate pathway
(Lejay et al., 2008). It should be noted that the sensitivity of
NRT2.1 to nitrogen, light and sugars at the level of the
transcript is reflected in high-affinity nitrate uptake but not in
protein abundance at the target membrane (Wirth et al., 2007),
which suggests that additional post-translational regulatory
mechanisms are involved in adjusting nitrate uptake to the
nutritional and metabolic requirements of the plant.
4. Water status
Over the past three decades the stomatal guard cell has risen
to the status of the premier model for study of membrane
transport and its regulation in plants, especially in relation to
those characteristics associated with ion channels. Stomata are
pores that provide the major route for gaseous exchange across
the impermeable cuticle of leaves and stems (Hetherington
& Woodward, 2003). They open and close in response to
exogenous and endogenous signals – the most important of
these being light, CO2 and the hydration status of the plant –
and thereby control the exchange of gases, most importantly
water vapour and CO2, between the interior of the leaf and
the atmosphere.
Guard cells control stomatal aperture through changes in
turgor pressure, and hence in cell volume, which are driven by
uptake and loss of the osmotically active solute (mainly K+
New Phytologist (2009) 181: 35–52
www.newphytologist.org
39
40 Review
Tansley review
and Cl–, and possibly also nitrate; Guo et al., 2003). These
events are critically dependent on signalling to coordinate
membrane transporters with the primary environmental
stimuli and with each other (Blatt, 2000; Schroeder et al.,
2001; Hetherington & Woodward, 2003; Willmer & Fricker,
1996; Sokolovski & Blatt, 2007). In the latter context, the
voltage sensitivities of the main ion channels contribute significantly to their regulation (see Section III), but coordinated
regulation of the Cl– and K+ channel currents also occurs
through voltage-independent signalling pathways, in particular
those that lead to a rise in cytosolic free Ca2+ concentration
([Ca2+]i). The coupling of changes in [Ca2+]i to abcisic acid
(ABA), a ubiquitous water-stress hormone, is well established
(Davies & Jones, 1991; McAinsh et al., 1997; Blatt, 2000;
Webb et al., 2001). However, the mechanisms leading to a rise
in [Ca2+]i and its downstream targets continue to yield new
insights as well as throw up new problems. It is clear now that
ABA influences [Ca2+]i through at least two complementary
processes. At the plasma membrane, ABA affects the voltage
threshold for activation of Ca2+ channels mediating Ca2+
entry (Hamilton et al., 2000) and thereby modulates [Ca2+]i
oscillations (Allen et al., 2000, 2001), possibly through a
NADPH oxidase-dependent process (Kohler et al., 2003;
Kwak et al., 2003). Within the guard cell, ABA promotes
Ca2+-induced Ca2+ release from internal stores via at least one
well-defined mechanism in which nitric oxide stimulates
cyclic GMP- and cyclic ADP-ribose-activated Ca2+ channels
within endomembranes (Garcia-Mata et al., 2003; Neill
et al., 2003; Sokolovski et al., 2005). Other internal, Ca2+associated pathways include inositol-1,4,5-trisphosphate
release and [Ca2+]i elevation through the actions of phospholipase C (Blatt et al., 1990; Gilroy et al., 1990; Hunt et al.,
2003; Tang et al., 2007), as well as the actions of inositolhexakiphosphate, sphingosine and other membrane lipid metabolites (Lee et al., 1996; Ng et al., 2001; Coursol et al., 2003;
Lemtiri-Chlieh et al., 2003). Much less is known about the
interactions and origins of the rise in cytosolic pH in response
to ABA, although this signal is known to be an important
factor in ion channel control both at the plasma membrane
and at the tonoplast (Blatt, 1992; Blatt & Armstrong, 1993;
Miedema & Assmann, 1996; Grabov & Blatt, 1997).
While the Ca2+ signal predominates in regulating Cl– channels
and inward-rectifying K+ channels, it is curiously absent in the
control of outward-rectifying K+ channels in the guard cell,
for example GORK in A. thaliana and its counterpart in Vicia
(Hosy et al., 2003; Dreyer et al., 2004). Instead, current
through these channels is strongly increased by increasing
cytosolic pH (pHi; Grabov & Blatt, 1997) consistent with the rise
in pHi evoked by ABA (Irving et al., 1992; Blatt & Armstrong,
1993). Unlike the situation for Ca2+, virtually nothing is
known of the mechanism behind this rise in pHi nor of its site
of action, although its kinetics are sufficiently slow (Blatt &
Armstrong, 1993) to be accommodated by cation exchange and
charge balancing events during solute efflux from the vacuole
New Phytologist (2009) 181: 35–52
www.newphytologist.org
Fig. 2 Signalling pathways coordinating membrane transport, water
status and abscisic acid (ABA). ABA triggers a network of interacting
second messengers that converge on two major ionic intermediates,
the cytosolic free concentrations of Ca2+ and H+ (pH). All of the
intermediates identified here increase in the presence of ABA to affect
Ca2+ channels (blue), inward- and outward-rectifying K+ channels
(red), anion/Cl– channels (yellow) and H+ pumps. (For simplicity,
transporters have been grouped by predominant permeant ion,
although there are obvious overlaps such as the slow vacuolar (SV)
channel of the tonoplast, which is both Ca2+ and K+ permeable.) The
interconnections between many of the upstream elements affecting
[Ca2+]i have been defined, including inositol phosphates (InsPn) and
their metabolism, sphigosine-1-phosphate (S1P), reactive oxygen
species (ROS) and nitric oxide (NO), as well as cyclic ADP-ribose
(cADPR). The rise in cytosolic pH is a major factor in control of K+ and
anion/Cl– channels at the plasma membrane as well as the SV
channel at the tonoplast.
(MacRobbie, 2000). Figure 2 summarizes the signalling
pathways mediating between plant water status and transport
activity in stomatal guard guard cells.
III. Molecular mechanisms for regulation
1. Voltage gating
Membrane voltage can affect ion permeation through transport
proteins in two ways. First, as an electromotive force on ion
(charge) movement across the membrane, the membrane
voltage acts as an electrical analogue of concentration in
driving ion flux through ion channels, carriers and pumps.
Secondly, membrane voltage serves as a regulatory factor in
controlling ion flux. Many ion channels open and close – or
gate – in response to membrane voltage, thereby controlling
the flux of ions on timescales of milliseconds. In general, gating
is associated with conformational changes of the channel
protein coupled to a ‘voltage sensor’ that effectively control the
opening of the pore (Hille, 2001). Best characterized are the Kv
(Shaker) K+ channels, which incorporate a voltage sensor
domain comprising a charged transmembrane helix which
moves in response to changes in the membrane voltage and is
thought thereby to pull directly on the closure mechanism of
the channel pore (Sigworth, 2003; Dreyer et al., 2004; Tombola
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
et al., 2005). Several members of this channel family have
important functions in plant K nutrition as they mediate uptake
of K+ from the soil (AKT1; Hirsch et al., 1998) or its longdistance transport in the xylem (SKOR; Gaymard et al., 1998)
and phloem (AKT2/3; Marten et al., 1999). While gating of
AKT2/3 is only weakly voltage-dependent, the open probability
of AKT exhibits a steep increase at negative membrane
potentials. Such ‘inward rectification’ gives the channel ‘valve’
type characteristics favouring uptake of K+ when an inward
driving force exists while minimizing its loss when the driving
force is directed outwards (Maathuis & Sanders, 1995).
Regulation of channels often targets the voltage sensitivity
of the gating. Most importantly, for many plant K+ channels, the
voltage dependence of gating is modulated by the availability
of K+ outside (Blatt, 1991). For example, in stomatal guard
cells, outward rectifying channels open only when the driving
force for net K+ flux is directed outwards. This regulation
ensures that K+ efflux – which is needed to drive stomatal
closure and control gas exchange – can occur even when
extracellular K+ varies over concentrations from 10 nM to
100 mM (Blatt & Gradmann, 1997). The ability to respond
to extracellular K+ is integral to the K+ channel protein itself
and therefore represents one of the very few examples in
which we know of the mechanism for ‘nutrient sensing’. From
a molecular standpoint, gating by K+ implies that cation binding
with the channel protein stabilizes a closed conformation of
the channel pore. Recent studies identified a site deep within
the S6 transmembrane helix of the channel protein, adjacent
to the so-called pore helix that is essential for this K+ sensitivity
(Fig. 3; Johansson et al., 2006). The finding is significant,
because analogous interactions between the pore helix and the
S6 helix are known to affect gating of mammalian K+ channels
Review
(Alagem et al., 2003; Seebohm et al., 2003, 2006), but in
animals these interactions are favoured by cation occupation
of the pore. One recent study (Li et al., 2008) builds on the
findings of Johansson and co-workers, demonstrating that,
with a few additional mutations, the voltage dependence for
SKOR gating shifts sufficiently to yield inward rectification
behaviour. It is still not clear, however, whether the channel in
this form still retains a coupling to external K+ concentration.
So, the results of Li and co-workers leave open the most
important question about the interplay in the mechanics of
gating by K+ and voltage that accounts for the response of the
channel to the K+ environment.
External pH is another important modulator of channel
activity. K inward currents in the plasma membrane of guard
and root cells are activated by acidification of the external
medium (Blatt, 1992; Ilan et al., 1996; Amtmann et al., 1999).
Activation is achieved by a combined effect of external pH on
channel gating and transport rate. It was further demonstrated
in barley roots that the pK of channel activation is close to the
apoplastic pH of barley roots, which in turn is determined by
the activity of the plasma membrane proton pump (Amtmann
et al., 1999). Hence, regulation of K+ channels by external pH
directly links K+ uptake to the activity of the primary H+ATPase. Analysis of mutations in the potato (Solanum tuberosum)
K+-channel KST1 identified a histidine residue in the outer
pore region of the channel as crucial for the effect of pH on
channel gating (Hoth et al., 1997).
2. Auto-inhibition
Auto-inhibitory domains play an important role in the
post-translational regulation of many transporters, including
Fig. 3 Residues comprising the S6 gating domain of the SKOR K+ channel. SKOR senses the K+ concentration outside by coupling K+-dependent
movement of the pore helix to the channel gate at the base of the S6 helix (at bottom in (a)) via the S6 gating domain (after Johansson et al.,
2006, reproduced with permission). Shown are cross-sectional (a) and ‘bird’s eye’ (b) views of segments from one of the four subunits forming
the channel pore. In (a) the S6 helix (the ribbon on the right) that lines the pore and the pore helix (the shorter ribbon on the left) that links to
the GYGD selectivity filter (in green; the permeation pathway is just to the left as shown) illustrate the juxtaposition of residues M286 at the
base of the pore helix and the S6 gating domain comprising residues D312, M313, I314 and G316. The outer surface of the membrane is at
the top of the image, and the lower surface near the bottom. Packing of the same residues is seen in (b) as viewed from above in a slice through
the centre of the membrane. The protein segment of SKOR shown corresponds to residues from D270 to G330 and was mapped by comparative
modelling to the crystal structure of the corresponding sequence of the KvAP K+ channel in the open conformation using SWISS-MODEL [http:/
/www.expasy.org/] with amino acid side chains in space filling format to highlight their proximity and position.
© The Authors (2008).
Journal compilation © New Phytologist (2008)
New Phytologist (2009) 181: 35–52
www.newphytologist.org
41
42 Review
Tansley review
Fig. 4 Post-translational regulation of transporters. Regulatory
domains for auto-inhibition, protein–protein interaction and ligand
binding are shown for plasma membrane H+-ATPases (AHA2 and
PMA2), plasma membrane and endomembrane Ca2+-ATPases
(ACA2 and ACA8), cyclic nucleotide gated channels (CNGC1 and
CNGC2) and the vacuolar Na+/H+ antiporter NHX1. Transporter
topology is shown as suggested by Baekgaard et al. (2005; H+ and
Ca2+ pumps), Véry & Sentenac (2002; CNGC) and Yamaguchi et al.
(2003; NHX1). Transmembrane spanning domains as well as
extracellular (out), vacuolar (vac), endoplasmic reticulum-luminal
(ER) and cytoplasmic (cyt) loops of the proteins are shown as grey
lines. Regulatory domains are represented by different symbols as
shown in the box (AID, auto-inhibitory domain; 14-3-3BD, 14-3-3
protein-binding domain; FC, fusicoccin; CaM, calmodulin; CaMBD,
calmodulin-binding domain; CNBD, cyclic nucleotide-binding
domain). Individual residues outside the AID involved in AID
interaction are shown as small black boxes. For details and
references see text.
Ca pumps and antiporters (for ammonium transporters, see
Section III.4). Auto-inhibitory domains in Ca pumps are closely
related to those in proton pumps, and we will therefore
include a description of the latter here. Auto-inhibitory
domains of P-type H+ and Ca2+ ATPases reside in the C- and
N-termini, respectively (Geisler et al., 2000a; Morsomme &
Boutry, 2000; Baekgaard et al., 2005), and interact both
intramolecularly with other parts of the pump and extramolecularly with activating proteins including 14-3-3 proteins,
protein kinases and calmodulin (Fig. 4 and Section III.3).
The observations that cleavage of a C-terminal fragment
by trypsin treatment led to H+-ATPase activation, and that
removal of the C-terminus was necessary to achieve functional
complementation of a yeast strain lacking endogenous proton
pump activity provided the first evidence for auto-inhibition of
plant P-type H+-ATPases (Palmgren et al., 1991; Palmgren &
Christensen, 1993; DeWitt & Sussman, 1995; Baunsgaard
et al., 1996). A number of point mutations that release
New Phytologist (2009) 181: 35–52
www.newphytologist.org
auto-inhibition and improve the coupling ratio between proton
pumping and ATP hydrolysis were subsequently identified
in the tobacco (Nicotiana plumbaginifolia) H+-ATPase PMA2
(Morsomme et al., 1996, 1998). The majority of these mutations
are concentrated in two regions located close to one another
within the first half of the C-terminal domain. Similarly,
systematic alanine scanning of the C-terminus of the plasma
membrane proton pump AHA2 from A. thaliana revealed two
regions (RI and RII) as being important for auto-inhibition
(Axelsen et al., 1999).
An auto-inhibitory R-domain is also present in plant Ca2+
pumps of the IIB type (Harper et al., 1998; Chung et al., 2000;
Curran et al., 2000; Geisler et al., 2000b; Luoni et al., 2004;
Baekgaard et al., 2006). Here it forms part of the N-terminus,
thus differing from animal counterparts (Fig. 2). However,
relocation of the C-terminal R-domain in the animal PMCA4b
to the N-terminus had only minor effects on auto-inhibition
(Adamo & Grimaldi, 1998), indicating that the position of
this domain in the C- or N-terminus is not important for
its auto-inhibitory function. In addition to the R-domains,
several amino acid residues outside the auto-inhibitory domains
are involved in auto-inhibition of H+ and Ca2+ pumps
(Curran et al., 2000; Morsomme et al., 1996, 1998), possibly
by providing intramolecular recognition sites of the autoinhibitory domain.
Auto-inhibitory domains also feature in co-transporters. For
example, CAX1, a vacuolar Ca2+/H+ antiporter of A. thaliana,
contains an N-terminal regulatory region of 36 amino acid
residues that interacts with a neighbouring domain, thereby
inhibiting Ca2+ transport (Pittman & Hirschi, 2001). Expression
of N-terminally truncated versions of CAX1 as well as versions
with several point mutations within the 36-amino acid region
suppresses Ca2+ sensitivity in yeast, but expression of the
full-length cDNA does not (Pittman et al., 2002). The
function of such regulation in planta has now been shown for
the first time. Mei and co-workers (Mei et al., 2007) selected
a number of N-terminal variants of CAX1 based on their
inhibitory effect in yeast, and subsequently analysed the effects
of these variants – expressed under the control of the 35-S
promoter – on ion contents in tobacco. All transgenic lines
showed high levels of CAX1 transcript but they clearly differed
with respect to the concentrations of several mineral nutrients,
and this difference was in accordance with the inhibitory
impact of the mutations previously determined in yeast.
3. Protein–protein interaction
14-3-3 proteins 14-3-3 proteins regulate the activities of a
wide range of targets via direct protein–protein interaction,
which depends on the phosphorylation status of the targets
(Roberts, 2003). The interaction involves binding of short
amino acid motifs, containing phospho-serine or phosphothreonine, of the target protein and a conserved amphipathic
region in each monomer of a dimeric 14-3-3 protein (Roberts,
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
2003). 14-3-3 proteins are highly conserved among kingdoms,
and are involved in many cellular processes (Aitken, 1996). In
plants, 14-3-3 proteins interact with metabolic enzymes (Huber
et al., 2002; Comparot et al., 2003), transcription factors
(Schultz et al., 1998), protein kinases (Camoni et al., 1998)
and ion transporters (de Boer, 2002).
Regulation of plant P-type H+-ATPases by 14-3-3 proteins
was discovered through the effects of fusicoccin (FC), a fungal
toxin that activates the proton pump, thereby provoking
membrane hyperpolarization and acidification of the external
medium ( Johansson et al., 1993; Lanfermeijer & Prins, 1994;
De Boer, 1997). The mystery of how FC regulates plant proton
pumps was solved when a 30-kDa protein doublet present in
FC receptor preparations was cloned and identified as member of
the 14-3-3 protein family (Korthout & de Boer, 1994; Oecking
et al., 1994). It was subsequently found that FC stabilizes the
binding of 14-3-3 proteins to a unique region in the C-terminal
end of the H+-ATPase (Jahn et al., 1997; Oecking et al., 1997;
Fullone et al., 1998; Fig. 4). The 14-3-3 binding domain partly
overlaps with the RII auto-inhibitory domain and includes two
phosphorylation sites (Jelich-Ottmann et al., 2001; Fuglsang
et al., 2003). Phosphorylation of the penultimate threonine
residue of the C-terminus is required to stabilize 14-3-3-binding
(Fuglsang et al., 1999) but the kinase that phosphorylates
the threonine residue remans to be identified.
Effects of 14-3-3 proteins on ion channel currents have
been observed in a number of plant systems including tomato,
barley and A. thaliana (Saalbach et al., 1997; van den Booij
et al., 1999; van den Wijngaard et al., 2005; Latz et al., 2007).
Recent studies have provided clues to the physiological
relevance of this type of ion channel regulation. For example,
patch clamp experiments in barley embryos uncovered a
functional link between the role of ABA in seed dormancy
and 14-3-3 regulation of K uptake channels in the emerging
radicle (van den Wijngaard et al., 2005). The current model
is that ABA causes channel dephosphorylation (by activating
protein serine/threonine phosphatases ABI1 and 2; Armstrong
et al., 1995), which leads to dissociation of a 14-3-3 protein
and inhibition of a K+ inward current that is required for
radicle emergence (van den Wijngaard et al., 2005). The likely
molecular target of this regulatory ensemble is the K+ channel
AKT1, which contains C-terminal 14-3-3 binding motifs
both in A. thaliana and in barley (A. H. De Boer, pers. comm.).
Ion channels in the tonoplast are also subject to regulation by
14-3-3 proteins. Latz and co-workers (Latz et al., 2007) measured
opposite effects of 14-3-3 on K+ currents mediated by TPK1
(Gobert et al., 2007) and TPC1 (Peiter et al., 2005). These
channels differ not only in voltage dependence but also in ion
selectivity, and therefore differential regulation by 14-3-3 could
provide a means to alter the overall ion selectivity of the tonoplast.
The future challenge is to identify the signalling pathways that link
this regulation to environmental or developmental stimuli.
14-3-3 proteins might also modulate nitrate transport.
NRT2.1 from tobacco and NRT2.4 from A. thaliana contain
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
perfect 14-3-3 binding motifs, albeit in different positions
(Miller et al., 2007). 14-3-3 regulation of nitrate transporters
is an intriguing prospect as several key enzymes of N assimilation (e.g. nitrate reductase and glutamine synthetase) are
regulated by 14-3-3 (Moorhead et al., 1996; Finnemann &
Schjoerring, 2000).
Calmodulin Calmodulins (CaMs) are small Ca2+-binding
proteins that can translate intracellular Ca2+ signals into a
variety of cellular responses. In accordance with this function,
CaMs are involved in plant responses to a large number of
environmental stimuli (Snedden & Fromm, 2001). Targets of
this large gene family (approx. 28 members in A. thaliana)
include Ca2+-ATPases, cyclic nucleotide gated channels (CNGCs)
and the vacuolar Na+/H+ antiporter NHX1 (Fig. 4).
The relative kinetics of the influx of Ca2+ and its removal
by Ca2+ pumps shape the Ca2+ signal, thereby endowing it
with some specificity (Allen et al., 2001; Harper, 2001). The
interaction among Ca2+, CaM and Ca2+-ATPases plays an
important role in this process because it creates a negative
feedback loop that instigates removal of Ca2+ from the cytoplasm
as soon as cytoplasmic Ca2+ concentrations start to rise (Fig. 5).
Upon binding of Ca2+, CaM interacts with the N-terminus of
IIB-type Ca2+ pumps and leads to increased activity. There is
no consensus CaM binding site but they are usually 15–30
amino acids long and form an alpha helix containing two
Fig. 5 Overview of the mechanisms and pathways involved in
cyoplasmic Ca2+ signals and the regulation of K+ channels.
Membranes (PM, plasma membrane; TP, tonoplast; EM,
endomembrane, e.g. tonoplast or endoplasmic reticulum) are shown
as grey lines. K+ and Ca2+ channels are shown as cylinders, H+ and
Ca2+ pumps as circles and the Ca2+/H+ antiporter as an oval. Solid
arrows show ion movement (red for K+, blue for Ca2+, and green
for H+), and dotted arrows indicate regulation. Regulatory
proteins include calcineurin-like Ca2+-binding proteins (CBLs),
CBL-interacting protein kinases (CIPKs and SOS2) and calmodulin
(CaM). Ψ is the membrane potential.
New Phytologist (2009) 181: 35–52
www.newphytologist.org
43
44 Review
Tansley review
bulky hydrophobic residues that function as anchors for CaM
(Crivici & Ikura, 1995; Yap et al., 2003). N-terminal CaM
binding domains (CaMBD) have been identified in the
cauliflower (Brassica oleracea) Ca2+-ATPase BCA1 (Malmstrom
et al., 1997) and in the Arabidopsis Ca2+ pumps ACA8 and
ACA9 (Bonza et al., 2000; Schiott et al., 2004). Interference of
CaM binding with auto-inhibition involves six residues in
the CaMBD including hydrophobic anchor residues, which
suggests that these residues have a dual function in CaM
recognition and in auto-inhibition (Baekgaard et al., 2006).
Kinases are again important modulators of this regulation
but in contrast to their stabilizing effect on the C-terminal
14-3-3 protein complex in proton pumps they seem to inhibit
CaM action on Ca2+ pumps. For example, it was shown for the
endomembrane Ca2+-ATPase ACA2 that phosphorylation
of a serine site near the CaMBD by a Ca2+-dependent kinase
inhibits CaM stimulation and basal activity (Hwang et al.,
2000). The physiological meaning of this apparently inverse
effect of intracellular Ca2+ on ACA2 via CaM and Cadependent protein kinase (CDPK) remains to be elucidated.
Experiments with the vacuolar H+/Na+ antiporter AtNHX1
have raised the surprising possibility that CaM also acts in the
vacuole (Yamaguchi et al., 2003, 2005). Yeast two-hybrid
assays showed that a CaM-like protein AtCaM15 interacts in
a Ca2+- and pH-dependent manner with the C-terminus of
AtNHX1. Progressive deletions of the C-terminus mapped
the binding site to a region that has indeed the potential to
form a positively charged amphiphilic helix. Previously the
group had shown that the C-terminus of AtNHX is located
in the vacuolar lumen (Yamaguchi et al., 2003), and that its
deletion increases the Na+/K+ selectivity of the antiport. In
accordance with these findings, CaM binding to NHX1
decreases the maximal transport rate for Na+ but not for K+.
Hence, regulation of NHX1 by vacuolar CaM might provide
a molecular switch between Na+ and K+ transport.
CaM-binding sites are also present in the C-termini of
CNGCs. CNGCs display a wide range of substrate specificity
(ranging from K selectivity to no selectivity among cations)
and are involved in a number of physiological processes (Talke
et al., 2003; Demidchik & Maathuis, 2007) but their specific
functions remain obscure. From a mechanistic viewpoint it is
interesting that in plant CNGCs the CaM-binding domain
overlaps with the cyclic nucleotide-binding domain (Arazi
et al., 2000; Fig. 4), and there is some evidence that binding
of CaM at the C-terminus interferes with ligand binding and
activation (Hua et al., 2003).
Protein kinases The importance of [Ca2+]i for plant responses
to environmental stimuli is well established but how [Ca2+]i
signals are translated to alterations in ion transport activities
has long been uncertain, although several strings of evidence
suggested that phosphorylation events were involved (Luan
et al., 1993; Thiel & Blatt, 1994; Grabov et al., 1997). The
molecular components of one pathway linking [Ca2+]i with
New Phytologist (2009) 181: 35–52
www.newphytologist.org
transport activity were discovered in salt-oversensitive (sos)
mutants of A. thaliana (Wu et al., 1996; Liu & Zhu, 1998;
Halfter et al., 2000; Shi et al., 2000; Qiu et al., 2004). In the
SOS pathway, a salt-induced rise in [Ca2+]i is translated
into enhanced activity of a Na+/H+ antiporter (SOS1) via a
calcineurin-B-like (CBL) protein (SOS3, aka CBL4) and a
CBL-interacting protein kinase (SOS2, aka CIPK24). Over
recent years it has become clear that CBLs and CIPKs are
ubiquitous functional modules that are involved in many
signalling pathways, including those regulating nutrient
transport. Thus a forward screen for growth of A. thaliana
plants on low K+ concentrations identified a regulon consisting
of CBL1 (or CBL9), CIPK23 and AKT1 (Xu et al., 2006).
Knockout mutants for CIPK23 (lks1) and double knockout
mutants for the CBLs (cbl1cbl9) mimicked the akt1 phenotype
(hypersensitivity to low K+ in the presence of ammonium;
Hirsch et al., 1998) and displayed reduced K+ uptake compared
with wild-type plants. Furthermore, co-expression of CBL1
and CIPK23 with AKT1 in Xenopus oocytes resulted in
measurable K+ inward currents, while AKT1 alone did not
produce currents in this expression system (Li et al., 2006; Xu
et al., 2006). Based on these findings, the response of plants
to low K comprises the following steps (Fig. 5). A decrease in
the external K+ concentration hyperpolarizes the membrane
potential and leads to Ca2+ influx through hyperpolarizationactivated Ca2+ channels (Allen et al., 2001). Upon a rise of
[Ca2+]i, CBL1 or CBL9 binds to CIPK23 and recruits it to
the plasma membrane (in analogy to the SOS pathway;
Quintero et al., 2002; Quan et al., 2007). Here the kinase
domain of CIPK23 interacts with the C-terminal ankyrin
domain of AKT1, thereby phosphorylating and activating the
channel (Li et al., 2006; Xu et al., 2006). Dephosphorylation
with subsequent deactivation is achieved by the phosphatase
AIP1 (Lee et al., 2007). Two-hybrid analysis showed that
several other members of the relevant gene families interact
with each other, thereby creating a large network of CBL/
CIPK/AKT1 complexes (Lee et al., 2007). However, which of
the potential protein combinations are co-expressed and active
in a particular cell type or condition in planta is unknown. For
example, SOS2 specifically interacts with CBL4 (SOS3) in
roots and CBL10 (SCABP8) in shoots (Quan et al., 2007). In
addition to CIPK23, another CIPK homologue, CIPK9, has
been found to play a role in plant adaptation to K+ deficiency
(Pandey et al., 2007). cipk9 knockout plants show reduced
growth on low K but unlike lks1 they do not differ from wildtype plants with respect to K+ content and K+ influx. One
possibility to explain this phenotype is that CIPK9 interacts
with vacuolar K+ transporters facilitating K+ release from the
vacuole (e.g. TPK1; Gobert et al., 2007), thereby assisting
cellular K+ homeostasis under K+ deficiency (Amtmann &
Armengaud, 2007). Targets of CBL/CIPK regulation also include
the vacuolar transporters NHX1 and CAX1, both of which
interact with SOS2 (Cheng et al., 2004; Qiu et al., 2004).
NHX1 is a Na+/H+ antiporter, which under certain conditions
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
also transports K+ (see previous section on Calmodulin),
while CAX1 is a Ca2+/H+ antiporter. Both are likely to play a
role in cellular and whole-plant homeostasis of K+ and Ca2+.
4. Oligomerization
Membrane transporters are often assembled from several
subunits. These can either be identical (homo-oligomers)
or different (hetero-oligomers). Both homo- and heterooligomerization can provide a means for regulation of ion
transport. Trans-activation within a homomeric complex of
three AMT1;1 subunits has recently been discovered through
functional analysis of structural mutants based on the crystal
structure of a homologous ammonium transporter from
Archaeoglobus fulgidus (Loque et al., 2007). Each of the AMT
subunits provides a functional pore for ammonium. The study
identified specific sites within the soluble C-terminus and the
pore that are required for allosteric regulation of the homomer,
and provided evidence that post-translational modification
(e.g. phosphorylation) of the C-terminus of a single monomer
leads to a conformational change resulting in cooperative
closure of all three pores in the complex. The authors suggest
that this mechanism allows rapid inactivation of the multipore
complex to protect against over-accumulation of potentially
toxic ammonium at high external ammonium concentrations
or during depolarization.
While trans-activation within homomers allows for rapid
responses of a transport pathway, differential regulation of
individual subunits within a heteromer has the potential to
introduce a high degree of plasticity into ion transport within
a single cell. In fact, regulation of AMT transporters may use
both mechanisms, as several isoforms of the AMT1 family can
interact with each other (Ludewig et al., 2003; Loque &
von Wiren, 2004; Neuhauser et al., 2007). The best-studied
example of heteromeric protein assembly concerns K+ channels
of the Shaker family. Functional channels consist of four αsubunits, which in the simplest case form homo-tetramers.
Biochemical experiments and yeast two-hybrid studies have
revealed that interaction between the individual subunits involves
three C-terminal domains. The KHA domain at the extreme
C-terminus cross-interacts with a region just downstream of the
hydrophobic core, and the putative cyclic nucleotide-binding
domain interacts with itself (Daram et al., 1997).
The possibility of functional heteromerization between different α-subunits was uncovered by co-expression of different
plant Shaker channel mRNAs in Xenopus oocytes. Dreyer and
co-workers (Dreyer et al., 1997) showed that the currents
produced by co-expression could not be explained by simply
adding homomeric channel currents. Analysis of expression
patterns in plants shows that many tissues express at least two
types of Shaker channels previously shown to interact in
heterologous systems (Dreyer et al., 1997; Baizabal-Aguirre
et al., 1999; Pilot et al., 2001, 2003; Zimmermann et al.,
2001), thus providing the opportunity for the formation of
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
hybrid channels in planta. For example, KAT1 and KAT2 are
both expressed in guard cells and interact in heterologous
expression systems (Pilot et al., 2001). Formation of KAT1/2
heteromers provides a good explanation for the apparently
contradictory observations that over-expression of mutant
KAT1 channels affects stomatal function (Kwak et al., 2001)
while KAT1 knockout does not (Szyroki et al., 2001). However,
in this as in most other cases, in vivo co-localization of the
subunits to the same membrane is still elusive.
The question remains whether heteromerization provides a
physiological means for regulation. An interesting case is
AtKC1, which does not form functional homo-tetrameric
channels in heterologous expression systems, but appears to
act as a modulator of AKT1 currents (Reintanz et al., 2002;
Duby et al., 2008). Both genes are expressed in root hairs but
knockout of AKT1 alone is sufficient to completely abolish
K+ inward current, thus confirming AtKC1 as a ‘silent channel’.
Disruption of the AtKC1 gene does, however, strongly suppress the inward current and alters its Ca2+ and pH sensitivity
(Reintanz et al., 2002). These findings, together with recent
evidence that AKT1 and AtKC1 preferentially associate in vivo
(Duby et al., 2008), indicate that AKT1/AtKC1 heteromers
underlie physiological K+ uptake by root hairs.
5. Trafficking
Finally, transmembrane ion and solute transport is subject
to regulation via membrane traffic, if only through its impact
on the population of transport proteins available at the
membrane surface. Thus, exocytosis and endocytosis of ion
and solute transporters serve to control transport capacity,
albeit not necessarily the intrinsic kinetic characteristics for
transport across the membrane. Among mammalian cells,
the best-characterized model is that of Na+-coupled glucose
transport via the GLUT4 transporter which cycles between
the apical membrane and a pool of cytosolic vesicles in
intestinal epithelial cells (Simpson et al., 2001; Ishiki &
Klip, 2005). Insulin stimulates the exocytosis of GLUT4 and,
as a consequence, stimulates the rate of glucose uptake. Fusion
of GLUT4 vesicles depends on a number of membrane
trafficking proteins, including so-called SNARE complexes
of mammalian SNAP-23, Syntaxin 4 and VAMP2 proteins
(Volchuk et al., 1996; Chamberlain & Gould, 2002; Williams
& Pessin, 2008). In turn, GLUT4 transporters are recovered
from the apical plasma membrane by endocytosis and sequestered
in specialized GLUT4 vesicles before recycling.
There is no doubt that membrane targeting of nutrient
transporters is important for nutrition. For example, a mutation
in an SEC12-type phosphate traffic facilitator (PHF1) impairs P
transport, resulting in the constitutive expression of many P
starvation-induced genes (Gonzalez et al., 2005). Nitrate uptake
through AtNRT2.1 depends on interaction of AtNRT2.1
with the smaller AtNAR2 protein, which carries an N-terminal
‘secretory pathway signal’ (Orsel et al., 2006). The observation
New Phytologist (2009) 181: 35–52
www.newphytologist.org
45
46 Review
Tansley review
that in nar2 knockout mutants green fluorescent protein
(GFP)-tagged AtNRT2.1 protein is not correctly targeted to
the plasma membrane suggests a role of NAR2 in trafficking
(Orsel et al., 2007). The interesting question is whether
reversible recruitment of transporters to the target membrane
can be a means for adjusting nutrient uptake to nutrient availability and demand.
In A. thaliana, a number of integral membrane proteins
have now been found to traffic to, and be recovered from, the
plasma membrane (e.g. KOR1 (Robert et al., 2005), PIN1
(Geldner et al., 2001), BRI1 (Geldner et al., 2007) and FLS2
(Robatzek et al., 2006)). Of particular interest in this context
is the boron transporter BOR1, which is essential for xylem
loading and boron translocation to the shoot under nutrient
limitation. Boron resupply leads to BOR1 endocytosis and
degradation in the vacuole (Takano et al., 2005). Each of
these examples entails traffic characterized to varying degrees
by changes in the constitutive turnover of the integral membrane protein. Unlike GLUT4 traffic, however, on endocytosis
these plant proteins enter a one-way path that leads to their
sequestration in the vacuole and degradation.
Traffic of the Kv-like K+ channel KAT1 presents a different
picture. Turnover of KAT1 at the plasma membrane of intact
epidermal and guard cells is tightly controlled through a
mechanism evoked by ABA and leads to recycling in true
exchange with an endomembrane pool distinct from known
degradation pathways to the vacuole (Sutter et al., 2006). The
close parallel with GLUT4 traffic and its role in transmembrane
solute transport is self-evident. Furthermore, studies using
dominant-negative (so-called Sp2) fragments of SYP121, a
plasma membrane SNARE previously shown to have a role in
guard cell ion channel control (Leyman et al., 1999), have
indicated that export of KAT1 to the plasma membrane is
dependent on SYP121 function. Sutter and co-workers (Sutter
et al., 2007) found that co-expression of the SYP121 Sp2
fragment selectively suppressed KAT1 delivery to the plasma
membrane and altered its distribution and anchoring within
microdomains in the plasma membrane.
Recent work from the same laboratory has yielded direct
evidence for SYP121 as a key structural element determining
the gating of another K+ channel (Honsbein et al., 2007).
Significantly, the interaction was found to be essential for K+
channel gating and K+ uptake in the A. thaliana root. Thus,
the SNARE appears to be essential for channel-mediated K+
nutrition, a function wholly distinct from any role in membrane traffic. It will be of interest now to determine whether
similar SNARE interactions contribute directly to other ion
transport, signalling and homeostatic functions in plant
mineral nutrition.
IV. Conclusions and outlook
Transport proteins mediate ion fluxes across biological membranes that underlie nutrient acquisition, cell volume changes
New Phytologist (2009) 181: 35–52
www.newphytologist.org
and transpiration. Regulation of ion transporters is essential to
adjust these parameters to plant development and environmental
challenges. The combination of electrophysiological, biochemical, molecular and genetic approaches has created a wealth
of information on responses of macronutrient transporters to
environmental stimuli, the signalling pathways leading to
these responses and the molecular mechanisms involved.
Many studies have provided evidence for the involvement of
signalling molecules such as Ca2+, cyclic nucleotides, ROS and
nitric oxide, hormones such as ABA, ethylene and cytokinin,
and regulatory proteins such as kinases and phosphatases,
calmodulins and 14-3-3 proteins in these processes. Nevertheless
there are still substantial gaps in our understanding of the
molecular mechanisms that link signalling pathways to
transporter abundance and activity. The data collated in this
review seem to suggest that the transport of nitrate, phosphate
and sulphate is primarily regulated at the transcript level,
whereas transport of K+ and Ca2+ is predominantly regulated
at the protein level. Because anionic nutrients are taken up
by proton co-transporters while cationic nutrients in most
situations enter the cells passively through ion channels, this
could indicate different strategies for regulating active and
passive transport. Indeed, responsiveness at the transcript level
to external nutrient supply was observed for genes encoding
active transporters for K+ and Ca2+ (e.g. HAK5 and CAX;
Maathuis et al., 2003; Armengaud et al., 2004). However, it
is also possible that the mechanisms of regulation observed for
different nutrient transporters merely reflect preferences for
certain experimental approaches within research communities
(e.g. mineral nutrition vs guard cell biology). Several recent
publications indicate that post-translational mechanisms are
also important for regulation of N, P and S transporters, and
this line of research is now gaining momentum (Liu & Tsay,
2003; Orsel et al., 2006; Loque et al., 2007). Surprisingly,
there is hardly any information on Mg transport and its
regulation (Schock et al., 2000). Another area that still awaits
progress is the identification of the receptor elements that
perceive nutritional stimuli, and in many cases the exact nature
of the primary stimulus itself remains to be characterized.
Active research in the area of transport regulation will continue
to fill these gaps, supported by the increasing willingness of
scientists to investigate interdependences between different
nutrients, signalling pathways and physiological processes. The
success of this research relies on continuous improvements in
experimental techniques. In particular, interaction screens and
phospho-proteomics with membrane proteins (Nuhse et al.,
2007; Lalonde et al., 2008), fluorescent indictors for a number
of key ions, metabolites and signalling molecules (Looger
et al., 2005; Belousov et al., 2006; Gu et al., 2006) and tools
for studying recombinant proteins and their interactions in
planta (Walter et al., 2004; Capanoni et al., 2007) are likely to
uncover much-sought information on in vivo cellular and
molecular events involved in the regulation of nutrient
transporters under various environmental and nutritional
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
conditions. Identification of the regulatory elements for
nutrient uptake will not only increase our understanding of
how plants adapt to conditions of nutrient shortage but also
provide potential targets for future bioengineering efforts
aimed at improving crop performance on marginal soils.
References
Adamo HP, Grimaldi ME. 1998. Functional consequences of relocating
the C-terminal calmodulin-binding autoinhibitory domains of the plasma
membrane Ca2+ pump near the N-terminus. Biochemical Journal
331: 763–766.
Aitken A. 1996. 14-3-3 and its possible role in co-ordinating multiple
signalling pathways. Trends in Cell Biology 6: 341–347.
Alagem N, Yesylevskyy S, Reuveny E. 2003. The pore helix is involved in
stabilizing the open state of inwardly rectifying K+ channels. Biophysical
Journal 85: 300–312.
Al-Ghazi Y, Muller B, Pinloche S, Tranbarger TJ, Nacry P, Rossignol M,
Tardieu F, Doumas P. 2003. Temporal responses of Arabidopsis root
architecture to phosphate starvation: evidence for the involvement of auxin
signalling. Plant, Cell & Environment 26: 1053–1066.
Allen GJ, Chu SP, Harrington CL, Schumacher K, Hoffmann T,
Tang YY, Grill E, Schroeder JI. 2001. A defined range of guard cell
calcium oscillation parameters encodes stomatal movements. Nature
411: 1053–1057.
Allen GJ, Chu SP, Schumacher K, Shimazaki CT, Vafeados D, Kemper A,
Hawke SD, Tallman G, Tsien RY, Harper JF et al. 2000. Alteration
of stimulus-specific guard cell calcium oscillations and stomatal closing
in Arabidopsis det3 mutant. Science 289: 2338–2342.
Amtmann A, Armengaud P. 2007. The role of calcium sensor-interacting
protein kinases in plant adaptation to potassium-deficiency: new answers
to old questions. Cell Research 17: 483 –485.
Amtmann A, Hammond JP, Armengaud P, White PJ. 2006. Nutrient
sensing and signalling in plants: potassium and phosphorus. In: Callow JA,
ed. Advances in botanical research incorporating advances in plant pathology.
San Diego, CA, USA: Academic Press, 209–257.
Amtmann A, Jelitto TC, Sanders D. 1999. K+-selective inward-rectifying
channels and apoplastic pH in barley roots. Plant Physiology
120: 331–338.
Amtmann A, Troufflard S, Armengaud P. 2008. The effect of potassium
nutrition on pest and disease resistance in plants. Physiologia Plantarum
133: 682–691.
Arazi T, Kaplan B, Fromm H. 2000. A high-affinity calmodulin-binding
site in a tobacco plasma-membrane channel protein coincides with
a characteristic element of cyclic nucleotide-binding domains. Plant
Molecular Biology 42: 591–601.
Armengaud P, Breitling R, Amtmann A. 2004. The potassium-dependent
transcriptome of Arabidopsis reveals a prominent role of jasmonic acid
in nutrient signaling. Plant Physiology 136: 2556 –2576.
Armstrong F, Leung J, Grabov A, Brearley J, Giraudat J, Blatt MR. 1995.
Sensitivity to abscisic acid of guard-cell K+ channels is suppressed by
abi1-1, a mutant Arabidopsis gene encoding a putative protein
phosphatase. Proceedings of the National Academy of Sciences, USA
92: 9520–9524.
Aung K, Lin SI, Wu CC, Huang YT, Su CL, Chiou TJ. 2006.
pho2, a phosphate overaccumulator, is caused by a nonsense
mutation in a MicroRNA399 target gene. Plant Physiology
141: 1000–1011.
Axelsen KB, Venema K, Jahn T, Baunsgaard L, Palmgren MG.
1999. Molecular dissection of the C-terminal regulatory domain
of the plant plasma membrane H+-ATPase AHA2: mapping of residues
that when altered give rise to an activated enzyme. Biochemistry
38: 7227–7234.
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
Baekgaard L, Fuglsang AT, Palmgren MG. 2005. Regulation of plant
plasma membrane H+- and Ca2+-ATPases by terminal domains. Journal
of Bioenergetics and Biomembranes 37: 369–374.
Baekgaard L, Luoni L, De Michelis MI, Palmgren MG. 2006. The plant
plasma membrane Ca2+ pump ACA8 contains overlapping as well as
physically separated autoinhibitory and calmodulin-binding domains.
Journal of Biological Chemistry 281: 1058–1065.
Baizabal-Aguirre VM, Clemens S, Uozumi N, Schroeder JI. 1999.
Suppression of inward-rectifying K+ channels KAT1 and AKT2 by
dominant negative point mutations in the KAT1 alpha-subunit.
Journal of Membrane Biology 167: 119–125.
Bari R, Datt Pant B, Stitt M, Scheible WR. 2006. PHO2, microRNA399,
and PHR1 define a phosphate-signaling pathway in plants. Plant
Physiology 141: 988 –999.
Baunsgaard L, Venema K, Axelsen KB, Villalba JM, Welling A,
Wollenweber B, Palmgren MG. 1996. Modified plant plasma membrane
H+-ATPase with improved transport coupling efficiency identified
by mutant selection in yeast. Plant Journal 10: 451– 458.
Belousov VV, Fradkov AF, Lukyanov KA, Staroverov DB, Shakhbazov KS,
Terskikh AV, Lukyanov S. 2006. Genetically encoded fluorescent
indicator for intracellular hydrogen peroxide. Nature Methods 3: 281–286.
Bi YM, Zhang Y, Signorelli T, Zhao R, Zhu T, Rothstein S. 2005. Genetic
analysis of Arabidopsis GATA transcription factor gene family reveals a
nitrate-inducible member important for chlorophyll synthesis and glucose
sensitivity. Plant Journal 44: 680 –692.
Blatt MR. 1991. Ion channel gating in plants – physiological implications
and integration for stomatal function. Journal of Membrane Biology
124: 95 –112.
Blatt MR. 1992. K+ channels of stomatal guard-cells – characteristics
of the inward rectifier and its control by pH. Journal of General Physiology
99: 615–644.
Blatt MR. 2000. Cellular signaling and volume control in stomatal
movements in plants. Annual Review of Cell and Developmental Biology
16: 221–241.
Blatt MR. 2004. Membrane transport in plants. Oxford, UK: Blackwell
Publishing.
Blatt MR, Armstrong F. 1993. K+ channels of stomatal guard-cells –
abscisic-acid-evoked control of the outward rectifier mediated by
cytoplasmic pH. Planta 191: 330– 341.
Blatt MR, Gradmann D. 1997. K+-sensitive gating of the K+ outward
rectifier in Vicia guard cells. Journal of Membrane Biology
158: 241–256.
Blatt MR, Thiel G, Trentham DR. 1990. Reversible inactivation of K+
channels of Vicia stomatal guard-cells following the photolysis of caged
inositol 1,4,5-trisphosphate. Nature 346: 766–769.
Bonza MC, Morandini P, Luoni L, Geisler M, Palmgren MG, De Michelis
MI. 2000. At-ACA8 encodes a plasma membrane-localized calciumATPase of Arabidopsis with a calmodulin-binding domain at the N
terminus. Plant Physiology 123: 1495–1506.
Booij PP, Roberts MR, Vogelzang SA, Kraayenhof R, De Boer AH.
1999. 14-3-3 proteins double the number of outward-rectifying K+
channels available for activation in tomato cells. Plant Journal
20: 673– 683.
Buchner P, Takahashi H, Hawkesford MJ. 2004. Plant sulphate
transporters: co-ordination of uptake, intracellular and long-distance
transport. Journal of Experimental Botany 55: 1765–1773.
Camoni L, Harper JF, Palmgren MG. 1998. 14-3-3 proteins activate a plant
calcium-dependent protein kinase (CDPK). FEBS Letters 430: 381–384.
Cao S, Su L, Fang Y. 2006. Evidence for involvement of jasmonic acid in the
induction of leaf senescence by potassium deficiency in Arabidopsis.
Canadian Journal of Botany 84: 328–333.
Capanoni P, Sutter JU, Davis CS, Littlejohn GR, Blatt MR. 2007.
A generalized method for transfecting root epidermis uncovers endosomal
dynamics in Arabidopsis root hairs (vol 51, pg 322, 2007). Plant Journal
51: 738–738.
New Phytologist (2009) 181: 35–52
www.newphytologist.org
47
48 Review
Tansley review
Chamberlain LH, Gould GW. 2002. The vesicle- and target-SNARE
proteins that mediate Glut4 vesicle fusion are localized in detergentinsoluble lipid rafts present on distinct intracellular membranes. Journal of
Biological Chemistry 277: 49750–49754.
Cheng NH, Pittman JK, Zhu JK, Hirschi KD. 2004. The protein kinase
SOS2 activates the Arabidopsis H+/Ca2+ antiporter CAX1 to integrate
calcium transport and salt tolerance. Journal of Biological Chemistry
279: 2922–2926.
Chiou TJ, Aung K, Lin SI, Wu CC, Chiang SF, Su CL. 2006. Regulation
of phosphate homeostasis by microRNA in Arabidopsis. Plant Cell
18: 412–421.
Chung WS, Lee SH, Kim JC, Heo WD, Kim MC, Park CY, Park HC,
Lim CO, Kim WB, Harper JF et al. 2000. Identification of a calmodulinregulated soybean Ca2+-ATPase (SCA1) that is located in the plasma
membrane. Plant Cell 12: 1393 –1407.
Comparot S, Lingiah G, Martin T. 2003. Function and specificity of
14-3-3 proteins in the regulation of carbohydrate and nitrogen
metabolism. Journal of Experimental Botany 54: 595–604.
Coursol S, Fan LM, Le Stunff H, Spiegel S, Gilroy S, Assmann SM. 2003.
Sphingolipid signalling in Arabidopsis guard cells involves heterotrimeric
G proteins. Nature 423: 651– 654.
Crivici A, Ikura M. 1995. Molecular and structural basis of target
recognition by calmodulin. Annual Review of Biophysics and Biomolecular
Structure 24: 85 –116.
Curran AC, Hwang I, Corbin J, Martinez S, Rayle D, Sze H, Harper JF.
2000. Autoinhibition of a calmodulin-dependent calcium pump involves
a structure in the stalk that connects the transmembrane domain to
the ATPase catalytic domain. Journal of Biological Chemistry
275: 30301–30308.
Daram P, Urbach S, Gaymard F, Sentenac H, Cherel I. 1997.
Tetramerization of the AKT1 plant potassium channel involves its
C-terminal cytoplasmic domain. EMBO Journal 16: 3455–3463.
Davies JP, Yildiz FH, Grossman AR. 1999. Sac3, an Snf1-like serine
threonine kinase that positively and negatively regulates the responses
of chlamydomonas to sulfur limitation. Plant Cell 11: 1179–1190.
Davies WJ, Jones HG. 1991. Abscisic acid: physiology and biochemistry.
Oxford, UK: Bios Scientific.
De Boer AH. 2002. Plant 14-3-3 proteins assist ion channels and pumps.
Biochemical Society Transactions 30: 416– 421.
De Boer B. 1997. Fusicoccin – a key to multiple 14-3-3 locks? Trends in Plant
Science 2: 60 –66.
Demidchik V, Maathuis FJM. 2007. Physiological roles of nonselective
cation channels in plants: from salt stress to signalling and development.
New Phytologist 175: 387–404.
DeWitt ND, Sussman MR. 1995. Immunocytological localization of an
epitope-tagged plasma membrane proton pump (H+-ATPase) in phloem
companion cells. Plant Cell 7: 2053–2067.
Dreyer I, Antunes S, Hoshi T, MullerRober B, Palme K, Pongs O, Reintanz
B, Hedrich R. 1997. Plant K+ channel alpha-subunits assemble
indiscriminately. Biophysical Journal 72: 2143 –2150.
Dreyer I, MullerRober B, Kohler B. 2004. Voltage-gated ion channels.
In: Blatt MR, ed. Membrane transport in plants. Oxford, UK: Blackwell
Publishing, 150–192.
Duby G, Hosy E, Fizames C, Alcon C, Costa A, Sentenac H, Thibaud JB.
2008. AtKC1, a conditionally targeted Shaker-type subunit, regulates the
activity of plant K+ channels. Plant Journal 53: 115–123.
Epstein I, Bloom EJ. 2005. Mineral nutrition of plants: principles and
perspectives, 2nd edn. Sunderland, MA, USA: Sinauer Associates.
Filleur S, Daniel-Vedele F. 1999. Expression analysis of a high-affinity nitrate
transporter isolated from Arabidopsis thaliana by differential display.
Planta 207: 461– 469.
Finnemann J, Schjoerring JK. 2000. Post-translational regulation
of cytosolic glutamine synthetase by reversible phosphorylation and
14-3-3 protein interaction. Plant Journal 24: 171–181.
Franco-Zorrilla JM, Martin AC, Leyva A, Par-Ares JP. 2005. Interaction
New Phytologist (2009) 181: 35–52
www.newphytologist.org
between phosphate-starvation, sugar, and cytokinin signaling in
Arabidopsis and the roles of cytokinin receptors CRE1/AHK4 and AHK3.
Plant Physiology 138: 847–857.
Fuglsang AT, Borch J, Bych K, Jahn TP, Roepstorff P, Palmgren MG.
2003. The binding site for regulatory 14-3-3 protein in plant plasma
membrane H+-ATPase: involvement of a region promoting
phosphorylation-independent interaction in addition to the
phosphorylation-dependent C-terminal end. Journal of Biological
Chemistry 278: 42266– 42272.
Fuglsang AT, Visconti S, Drumm K, Jahn T, Stensballe A, Mattei B, Jensen
ON, Aducci P, Palmgren MG. 1999. Binding of 14-3-3 protein to the
plasma membrane H(+)-ATPase AHA2 involves the three C-terminal
residues Tyr(946)-Thr-Val and requires phosphorylation of Thr(947).
Journal of Biological Chemistry 274: 36774– 36780.
Fujii H, Chiou TJ, Lin SI, Aung K, Zhu JK. 2005. A miRNA involved
in phosphate-starvation response in Arabidopsis. Current Biology
15: 2038–2043.
Fullone MR, Visconti S, Marra M, Fogliano V, Aducci P. 1998. Fusicoccin
effect on the in vitro interaction between plant 14-3-3 proteins and plasma
membrane H+-ATPase. Journal of Biological Chemistry 273: 7698 –7702.
Garcia-Mata C, Gay R, Sokolovski S, Hills A, Lamattina L, Blatt MR.
2003. Nitric oxide regulates K+ and Cl− channels in guard cells through
a subset of abscisic acid-evoked signaling pathways. Proceedings of the
National Academy of Sciences, USA 100: 11116–11121.
Gaymard F, Pilot G, Lacombe B, Bouchez D, Bruneau D, Boucherez J,
Michaux-Ferriere N, Thibaud JB, Sentenac H. 1998. Identification and
disruption of a plant shaker-like outward channel involved in K+ release
into the xylem sap. Cell 94: 647– 655.
Gazzarrini S, Lejay T, Gojon A, Ninnemann O, Frommer WB, von Wiren
N. 1999. Three functional transporters for constitutive, diurnally
regulated, and starvation-induced uptake of ammonium into arabidopsis
roots. Plant Cell 11: 937– 947.
Geisler M, Axelsen KB, Harper JF, Palmgren MG. 2000a. Molecular
aspects of higher plant P-type Ca2+-ATPases. Biochimica et Biophysica
Acta–Biomembranes 1465: 52–78.
Geisler M, Frangne N, Gomes E, Martinoia E, Palmgren MG. 2000b. The
ACA4 gene of Arabidopsis encodes a vacuolar membrane calcium pump
that improves salt tolerance in yeast. Plant Physiology 124: 1814 –1827.
Geldner N, Friml J, Stierhof YD, Jurgens G, Palme K. 2001. Auxin
transport inhibitors block PIN1 cycling and vesicle traficking. Nature
413: 425 –428.
Geldner N, Hyman DL, Wang XL, Schumacher K, Chory J. 2007.
Endosomal signaling of plant steroid receptor kinase BRI1. Genes &
Development 21: 1598–1602.
Gierth M, Maser P, Schroeder JI. 2005. The potassium transporter
AtHAK5 functions in K+ deprivation-induced high-affinity K+ uptake
and AKT1 K+ channel contribution to K+ uptake kinetics in Arabidopsis
roots. Plant Physiology 137: 1105–1114.
Gilroy S, Read ND, Trewavas AJ. 1990. Elevation of cytoplasmic calcium
by caged calcium or caged inositol trisphosphate initiates stomatal closure.
Nature 346: 769 –771.
Girin T, Lejay L, Wirth J, Widiez T, Palenchar PM, Nazoa P, Touraine B,
Gojon A, Lepetit M. 2007. Identification of a 150 bp cis-acting element
of the AtNRT2.1 promoter involved in the regulation of gene expression
by the N and C status of the plant. Plant, Cell & Environment
30: 1366–1380.
Gobert A, Isayenkov S, Voelker C, Czempinski K, Maathuis FJM. 2007.
The two-pore channel TPK1 gene encodes the vacuolar K+ conductance
and plays a role in K+ homeostasis. Proceedings of the National Academy
of Sciences, USA 104: 10726–10731.
Gonzalez E, Solano R, Rubio V, Leyva A, Paz-Ares J. 2005. PHOSPHATE
TRANSPORTER TRAFFIC FACILITATOR1 is a plant-specific SEC12related protein that enables the endoplasmic reticulum exit of a highaffinity phosphate transporter in Arabidopsis. Plant Cell 17: 3500 –3512.
Grabov A, Blatt MR. 1997. Parallel control of the inward-rectifier K+
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
channel by cytosolic free Ca2+ and pH in Vicia guard cells. Planta
201: 84 –95.
Grabov A, Leung J, Giraudat J, Blatt MR. 1997. Alteration of anion channel
kinetics in wild-type and abi1-1 transgenic Nicotiana benthamiana guard
cells by abscisic acid. Plant Journal 12: 203–213.
Gu H, Lalonde S, Okumoto S, Looger LL, Scharff-Poulsen AM, Grossman
AR, Kossmann J, Jakobsen I, Frommer WB. 2006. A novel analytical
method for in vivo phosphate tracking. FEBS Letters 580: 5885–5893.
Guo FO, Young J, Crawford NM. 2003. The nitrate transporter AtNRT1.1
(CHL1) functions in stomatal opening and contributes to drought
susceptibility in arabidopsis. Plant Cell 15: 107–117.
Halfter U, Ishitani M, Zhu JK. 2000. The Arabidopsis SOS2 protein kinase
physically interacts with and is activated by the calcium-binding protein
SOS3. Proceedings of the National Academy of Sciences, USA
97: 3735–3740.
Hamilton DWA, Hills A, Kohler B, Blatt MR. 2000. Ca2+ channels
at the plasma membrane of stomatal guard cells are activated by
hyperpolarization and abscisic acid. Proceedings of the National Academy
of Sciences, USA 97: 4967– 4972.
Hammond JP, Bennett MJ, Bowen HC, Broadley MR, Eastwood DC,
May ST, Rahn C, Swarup R, Woolaway KE, White PJ. 2003. Changes
in gene expression in Arabidopsis shoots during phosphate starvation and
the potential for developing smart plants. Plant Physiology 132: 578–596.
Harper JF. 2001. Dissecting calcium oscillators in plant cells. Trends in Plant
Science 6: 395 –397.
Harper JF, Hong BM, Hwang ID, Guo HQ, Stoddard R, Huang JF,
Palmgren MG, Sze H. 1998. A novel calmodulin-regulated Ca2+-ATPase
(ACA2) from Arabidopsis with an N-terminal autoinhibitory domain.
Journal of Biological Chemistry 273: 1099–1106.
Hetherington AM, Woodward FI. 2003. The role of stomata in sensing
and driving environmental change. Nature 424: 901– 908.
Hille B. 2001. Ion channel of excitable membranes. 3rd edn. Sunderland, MA,
USA: Sinauer Associates.
Hirsch RE, Lewis BD, Spalding EP, Sussman MR. 1998. A role for the
AKT1 potassium channel in plant nutrition. Science 280: 918–921.
Honsbein A, Campanoni P, Sokolovski S, Pratelli R, Paneque-Corralles M,
Johansson I, Blatt MR. 2007. A plasma membrane SNARE protein
is essential for potassium transport in Arabidopsis root epidermis and
growth. International Plant Membrane Biology Workshop 14: 173.
Hosy E, Vavasseur A, Mouline K, Dreyer I, Gaymard F, Poree F, Boucherez
J, Lebaudy A, Bouchez D, Very AA et al. 2003. The Arabidopsis outward
K+ channel GORK is involved in regulation of stomatal movements and
plant transpiration. Proceedings of the National Academy of Sciences, USA
100: 5549–5554.
Hoth S, Dreyer I, Dietrich P, Becker D, MullerRober B, Hedrich R. 1997.
Molecular basis of plant-specific acid activation of K+ uptake channels.
Proceedings of the National Academy of Sciences, USA 94: 4806 –4810.
Hou XL, Wu P, Jiao FC, Jia QJ, Chen HM, Yu J, Song XW, Yi KK. 2005.
Regulation of the expression of OsIPS1 and OsIPS2 in rice via systemic
and local Pi signalling and hormones. Plant, Cell & Environment
28: 353–364.
Hua BG, Mercier RW, Leng Q, Berkowitz GA. 2003. Plants do
it differently. A new basis for potassium/sodium selectivity in the pore
of an ion channel. Plant Physiology 132: 1353 –1361.
Huber SC, MacKintosh C, Kaiser WM. 2002. Metabolic enzymes as targets
for 14-3-3 proteins. Plant Molecular Biology 50: 1053 –1063.
Hunt L, Mills LN, Pical C, Leckie CP, Aitken FL, Kopka J, Mueller-Roeber
B, McAinsh MR, Hetherington AM, Gray JE. 2003. Phospholipase C
is required for the control of stomatal aperture by ABA. Plant Journal
34: 47–55.
Hwang I, Sze H, Harper JF. 2000. A calcium-dependent protein kinase
can inhibit a calmodulin-stimulated Ca2+ pump (ACA2) located in the
endoplasmic reticulum of Arabidopsis. Proceedings of the National Academy
of Sciences, USA 97: 6224–6229.
Ilan N, Schwartz A, Moran N. 1996. External protons enhance the activity
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
of the hyperpolarization-activated K+ channels in guard cell protoplasts of
Vicia faba. Journal of Membrane Biology 154: 169–181.
Inoue T, Higuchi M, Hashimoto Y, Seki M, Kobayashi M, Kato T, Tabata
S, Shinozaki K, Kakimoto T. 2001. Identification of CRE1 as a cytokinin
receptor from Arabidopsis. Nature 409: 1060 –1063.
Irving HR, Gehring CA, Parish RW. 1992. Changes in cytosolic Ph
and calcium of guard-cells precede stomatal movements. Proceedings
of the National Academy of Sciences, USA 89: 1790–1794.
Ishiki M, Klip A. 2005. Minireview: recent developments in the regulation
of glucose transporter-4 traffic: new signals, locations, and partners.
Endocrinology 146: 5071–5078.
Jahn T, Fuglsang AT, Olsson A, Bruntrup IM, Collinge DB, Volkmann D,
Sommarin M, Palmgren MG, Larsson C. 1997. The 14-3-3 protein
interacts directly with the C-terminal region of the plant plasma
membrane H+-ATPase. Plant Cell 9: 1805–1814.
Jang JC, Leon P, Zhou L, Sheen J. 1997. Hexokinase as a sugar sensor
in higher plants. Plant Cell 9: 5–19.
Jang JC, Sheen J. 1994. Sugar sensing in higher-plants. Plant Cell
6: 1665–1679.
Jelich-Ottmann C, Weiler EW, Oecking C. 2001. Binding of regulatory
14-3-3 proteins to the C terminus of the plant plasma membrane
H+-ATPase involves part of its autoinhibitory region. Journal of Biological
Chemistry 276: 39852–39857.
Johansson F, Sommarin M, Larsson C. 1993. Fusicoccin activates the
plasma-membrane H+-ATPase by a mechanism involving the C-terminal
inhibitory domain. Plant Cell 5: 321–327.
Johansson I, Wulfetange K, Poree F, Michard E, Gajdanowicz P, Lacombe
B, Sentenac H, Thibaud JB, Mueller-Roeber B, Blatt MR et al. 2006.
External K+ modulates the activity of the Arabidopsis potassium channel
SKOR via an unusual mechanism. Plant Journal 46: 269–281.
Kohler B, Hills A, Blatt MR. 2003. Control of guard cell ion channels by
hydrogen peroxide and abscisic acid indicates their action through
alternate signaling pathways. Plant Physiology 131: 385–388.
Korthout HA, de Boer AH. 1994. A fusicoccin binding protein belongs to
the family of 14-3-3 brain protein homologs. Plant Cell 6: 1681–1692.
Krapp A, Fraisier V, Scheible WR, Quesada A, Gojon A, Stitt M, Caboche
M, Daniel-Vedele F. 1998. Expression studies of Nrt2: 1Np, a putative
high-affinity nitrate transporter: evidence for its role in nitrate uptake.
Plant Journal 14: 723–731.
Kwak JM, Mori IC, Pei ZM, Leonhardt N, Torres MA, Dangl JL, Bloom
RE, Bodde S, Jones JD, Schroeder JI. 2003. NADPH oxidase AtrbohD
and AtrbohF genes function in ROS-dependent ABA signaling
in Arabidopsis. EMBO Journal 22: 2623–2633.
Kwak JM, Murata Y, Baizabal-Aguirre VM, Merrill J, Wang M, Kemper A,
Hawke SD, Tallman G, Schroeder JI. 2001. Dominant negative
guard cell K+ channel mutants reduce inward-rectifying K+ currents
and light-induced stomatal opening in Arabidopsis. Plant Physiology 127:
473– 485.
Lalonde S, Ehrhardt DW, Loque D, Chen J, Rhee SY, Frommer WB. 2008.
Molecular and cellular approaches for the detection of protein–protein
interactions: latest techniques and current limitations. Plant Journal
53: 610– 635.
Lanfermeijer FC, Prins HBA. 1994. Modulation of H+-ATPase activity
by fusicoccin in plasma-membrane vesicles from oat (Avena sativa L.)
roots – a comparison of modulation by fusicoccin, trypsin, and
lysophosphatidylcholine. Plant Physiology 104: 1277–1285.
Latz A, Becker D, Hekman M, Mueller T, Beyhl D, Marten I, Eing C,
Fischer A, Dunkel M, Bertl A et al. 2007. TPK1, a Ca2+-regulated
Arabidopsis vacuole two-pore K+ channel, is activated by 14-3 -3 proteins.
Plant Journal 52: 449–459.
Lee SC, Lan WZ, Kim BG, Li LG, Cheong YH, Pandey GK,
Lu GH, Buchanan BB, Luan S. 2007. A protein phosphorylation/
dephosphorylation network regulates a plant potassium channel.
Proceedings of the National Academy of Sciences, USA
104: 15959 –15964.
New Phytologist (2009) 181: 35–52
www.newphytologist.org
49
50 Review
Tansley review
Lee YS, Choi YB, Suh S, Lee J, Assmann SM, Joe CO, Kelleher JF, Crain
RC. 1996. Abscisic acid-induced phosphoinositide turnover in guard
cell protoplasts of Vicia faba. Plant Physiology 110: 987– 996.
Lejay L, Gansel X, Cerezo M, Tillard P, Muller C, Krapp A, von Wiren N,
Daniel-Vedele F, Gojon A. 2003. Regulation of root ion transporters by
photosynthesis: functional importance and relation with hexokinase. Plant
Cell 15: 2218–2232.
Lejay L, Wirth J, Pervent M, Cross JMF, Tillard P, Gojon A. 2008.
Oxidative pentose phosphate pathway-dependent sugar sensing as a
mechanism for regulation of root ion transporters by photosynthesis.
Plant Physiology 146: 2036–2053.
Lemtiri-Chlieh F, MacRobbie EAC, Webb AAR, Manison NF, Brownlee
C, Skepper JN, Chen J, Prestwich GD, Brearley CA. 2003. Inositol
hexakisphosphate mobilizes an endomembrane store of calcium
in guard cells. Proceedings of the National Academy of Sciences, USA
100: 10091–10095.
Leyman B, Geelen D, Quintero FJ, Blatt MR. 1999. A tobacco syntaxin
with a role in hormonal control of guard cell ion channels. Science
283: 537–540.
Li LG, Kim BG, Cheong YH, Pandey GK, Luan S. 2006. A Ca2+
signaling pathway regulates a K+ channel for low-K response in
Arabidopsis. Proceedings of the National Academy of Sciences, USA
103: 12625–12630.
Li LG, Liu K, Hu Y, Li DP, Luan S. 2008. Single mutations convert an
outward K+ channel into an inward K+ channel. Proceedings of the National
Academy of Sciences, USA 105: 2871–2876.
Liu J, Zhu JK. 1998. A calcium sensor homolog required for plant salt
tolerance. Science 280: 1943–1945.
Liu KH, Tsay YF. 2003. Switching between the two action modes of
the dual-affinity nitrate transporter CHL1 by phosphorylation. EMBO
Journal 22: 1005–1013.
Looger LL, Lalonde S, Frommer WB. 2005. Genetically encoded FRET
sensors for visualizing metabolites with subcellular resolution in living
cells. Plant Physiology 138: 555–557.
Loque D, Lalonde S, Looger LL, von Wiren N, Frommer WB. 2007. A
cytosolic trans-activation domain essential for ammonium uptake. Nature
446: 195–198.
Loque D, von Wiren N. 2004. Regulatory levels for the transport of
ammonium in plant roots. Journal of Experimental Botany 55: 1293–1305.
Luan S, Li WW, Rusnak F, Assmann SM, Schreiber SL. 1993.
Immunosuppressants implicate protein phosphatase regulation of K+
channels in guard-cells. Proceedings of the National Academy of Sciences,
USA 90: 2202–2206.
Ludewig U, Wilken S, Wu BH, Jost W, Obrdlik P, El Bakkoury M,
Marini AM, Andre B, Hamacher T, Boles E et al. 2003. Homo- and
hetero-oligomerization of ammonium transporter-1 NH+4 uniporters.
Journal of Biological Chemistry 278: 45603–45610.
Luoni L, Meneghelli S, Bonza MC, DeMichelis MI. 2004. Auto-inhibition
of Arabidopsis thaliana plasma membrane Ca2+-ATPase involves an
interaction of the N-terminus with the small cytoplasmic loop. FEBS
Letters 574: 20–24.
Maathuis FJ, Filatov V, Herzyk P, Krijger GC, Axelsen KB, Chen S, Green
BJ, Li Y, Madagan KL, Sanchez-Fernandez R et al. 2003. Transcriptome
analysis of root transporters reveals participation of multiple gene families
in the response to cation stress. Plant Journal 35: 675–692.
Maathuis FJ, Sanders D. 1995. Contrasting roles in ion transport of two K+channel types in root cells of Arabidopsis thaliana. Planta 197: 456– 464.
MacRobbie EAC. 2000. ABA activates multiple Ca2+ fluxes in stomatal
guard cells, triggering vacuolar K+ (Rb+) release. Proceedings of the National
Academy of Sciences, USA 97: 12361–12368.
Malmstrom S, Askerlund P, Palmgren MG. 1997. A calmodulin-stimulated
Ca2+-ATPase from plant vacuolar membranes with a putative regulatory
domain at its N-terminus. FEBS Letters 400: 324–328.
Marschner H. 1995. Mineral nutrition of higher plants. 2nd edn. London,
UK: Academic Press.
New Phytologist (2009) 181: 35–52
www.newphytologist.org
Marten I, Hoth S, Deeken R, Ache P, Ketchum KA, Hoshi T, Hedrich R.
1999. AKT3, a phloem-localized K+ channel, is blocked by protons.
Proceedings of the National Academy of Sciences, USA 96: 7581–7586.
Martin AC, del Pozo JC, Iglesias J, Rubio V, Solano R, de la Pena A, Leyva
A, Paz-Ares J. 2000. Influence of cytokinins on the expression of
phosphate starvation responsive genes in Arabidopsis. Plant Journal
24: 559– 567.
Maruyama-Nakashita A, Nakamura Y, Watanabe-Takahashi A, Inoue E,
Yamaya T, Takahashi H. 2005. Identification of a novel cis-acting
element conferring sulfur deficiency response in Arabidopsis roots.
Plant Journal 42: 305–314.
Maruyama-Nakashita A, Nakamura Y, Watanabe-Takahashi A, Yamaya T,
Takahashi H. 2004a. Induction of SULTR1;1 sulfate transporter in
Arabidopsis roots involves protein phosphorylation/dephosphorylation
circuit for transcriptional regulation. Plant and Cell Physiology
45: 340– 345.
Maruyama-Nakashita A, Nakamura Y, Yamaya T, Takahashi H. 2004b.
A novel regulatory pathway of sulfate uptake in Arabidopsis roots:
implication of CRE1/WOL/AHK4-mediated cytokinin-dependent
regulation. Plant Journal 38: 779 –789.
Maruyama-Nakashita A, Nakamura Y, Yamaya T, Takahashi H. 2004c.
Regulation of high-affinity sulphate transporters in plants: towards
systematic analysis of sulphur signalling and regulation. Journal
of Experimental Botany 55: 1843 –1849.
Mäser P, Thomine S, Schroeder JI, Ward JM, Hirschi K, Sze H, Talke IN,
Amtmann A, Maathuis FJ, Sanders D et al. 2001. Phylogenetic
relationships within cation transporter families of Arabidopsis.
Plant Physiology 126: 1646–1667.
McAinsh MR, Brownlee C, Hetherington AM. 1997. Calcium ions as
second messengers in guard cell signal transduction. Physiologia Plantarum
100: 16–29.
Mei H, Zhao J, Pittman JK, Lachmansingh J, Park S, Hirschi KD. 2007.
In planta regulation of the Arabidopsis Ca2+/H+ antiporter CAX1. Journal
of Experimental Botany 58: 3419 –3427.
Miedema H, Assmann SM. 1996. A membrane-delimited effect of internal
pH on the K+ outward rectifier of Vicia faba guard cells. Journal of
Membrane Biology 154: 227–237.
Miller AJ, Fan X, Orsel M, Smith SJ, Wells DM. 2007. Nitrate transport
and signalling. Journal of Experimental Botany 58: 2297–2306.
Miura K, Rus A, Sharkhuu A, Yokoi S, Karthikeyan AS, Raghothama KG,
Baek D, Koo YD, Jin JB, Bressan RA et al. 2005. The Arabidopsis SUMO
E3 ligase SIZ1 controls phosphate deficiency responses. Proceedings of the
National Academy of Sciences, USA 102: 7760–7765.
Moore B, Zhou L, Rolland F, Hall Q, Cheng WH, Liu YX, Hwang I,
Jones T, Sheen J. 2003. Role of the Arabidopsis glucose sensor HXK1
in nutrient, light, and hormonal signaling. Science 300: 332–336.
Moorhead G, Douglas P, Morrice N, Scarabel M, Aitken A, MacKintosh
C. 1996. Phosphorylated nitrate reductase from spinach leaves is inhibited
by 14-3-3 proteins and activated by fusicoccin. Current Biology
6: 1104–1113.
Morsomme P, Boutry M. 2000. The plant plasma membrane H+-ATPase:
structure, function and regulation. Biochimica et Biophysica Acta
1465: 1–16.
Morsomme P, Dambly S, Maudoux O, Boutry M. 1998. Single point
mutations distributed in 10 soluble and membrane regions of the
Nicotiana plumbaginifolia plasma membrane PMA2 H+-ATPase activate
the enzyme and modify the structure of the C-terminal region. Journal of
Biological Chemistry 273: 34837–34842.
Morsomme P, d’Exaerde AD, DeMeester S, Thines D, Goffeau A, Boutry
M. 1996. Single point mutations in various domains of a plant plasma
membrane H+-ATPase expressed in Saccharomyces cerevisiae increase
H+-pumping and permit yeast growth at low pH. Embo Journal
15: 5513– 5526.
Neill SJ, Desikan R, Hancock JT. 2003. Nitric oxide signalling in plants.
New Phytologist 159: 11–35.
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Tansley review
Neuhauser B, Dynowski M, Mayer M, Ludewig U. 2007. Regulation of
NH+4 transport by essential cross talk between AMT monomers through
the carboxyl tails. Plant Physiology 143: 1651–1659.
Ng CKY, Mcainsh MR, Gray JE, Hunt L, Leckie CP, Mills L,
Hetherington AM. 2001. Calcium-based signalling systems in guard cells.
New Phytologist 151: 109–120.
Nikiforova V, Freitag J, Kempa S, Adamik M, Hesse H, Hoefgen R. 2003.
Transcriptome analysis of sulfur depletion in Arabidopsis thaliana:
interlacing of biosynthetic pathways provides response specificity. Plant
Journal 33: 633–650.
Nuhse TS, Bottrill AR, Jones AME, Peck SC. 2007. Quantitative
phosphoproteomic analysis of plasma membrane proteins reveals
regulatory mechanisms of plant innate immune responses. Plant Journal
51: 931–940.
Oecking C, Eckerskorn C, Weiler EW. 1994. The fusicoccin receptor
of plants is a member of the 14-3-3 superfamily of eukaryotic regulatory
proteins. FEBS Letters 352: 163–166.
Oecking C, Piotrowski M, Hagemeier J, Hagemann K. 1997. Topology
and target interaction of the fusicoccin-binding 14-3-3 homologs
of Commelina communis. Plant Journal 12: 441– 453.
Orsel M, Chopin F, Leleu O, Smith SJ, Krapp A, Daniel-Vedele F, Miller
AJ. 2006. Characterization of a two-component high-affinity nitrate
uptake system in Arabidopsis. Physiology and protein-protein interaction.
Plant Physiology 142: 1304–1317.
Orsel M, Chopin F, Leleu O, Smith SJ, Krapp A, Daniel-Vedele F, Miller
AJ. 2007. Nitrate signaling and the two component high affinity uptake
system in Arabidopsis. Plant Signaling and Behavior 2: 4.
Palmgren MG, Christensen G. 1993. Complementation in situ of the yeast
plasma-membrane H+-ATPase gene Pma1 by an H+-ATPase gene from a
heterologous species. FEBS Letters 317: 216 –222.
Palmgren MG, Sommarin M, Serrano R, Larsson C. 1991. Identification
of an autoinhibitory domain in the C-terminal region of the plant
plasma-membrane H+-ATPase. Journal of Biological Chemistry
266: 20470–20475.
Pandey GK, Cheong YH, Kim BG, Grant JJ, Li L, Luan S. 2007. CIPK9:
a calcium sensor-interacting protein kinase required for low-potassium
tolerance in Arabidopsis. Cell Research 17: 411–421.
Peiter E, Maathuis FJM, Mills LN, Knight H, Pelloux J, Hetherington AM,
Sanders D. 2005. The vacuolar Ca2+-activated channel TPC1 regulates
germination and stomatal movement. Nature 434: 404 –408.
Peng MS, Hannam C, Gu HL, Bi YM, Rothstein SJ. 2007. A mutation
in NLA, which encodes a RING-type ubiquitin ligase, disrupts the
adaptability of Arabidopsis to nitrogen limitation. Plant Journal
50: 320–337.
Pilot G, Gaymard F, Mouline K, Cherel I, Sentenac H. 2003. Regulated
expression of Arabidopsis shaker K+ channel genes involved in K+ uptake
and distribution in the plant. Plant Molecular Biology 51: 773–787.
Pilot G, Lacombe B, Gaymard F, Cherel I, Boucherez J, Thibaud JB,
Sentenac H. 2001. Guard cell inward K+ channel activity in Arabidopsis
involves expression of the twin channel subunits KAT1 and KAT2. Journal
of Biological Chemistry 276: 3215– 3221.
Pittman JK, Hirschi KD. 2001. Regulation of CAX1, an Arabidopsis
Ca2+/H+ antiporter. Identification of an N-terminal autoinhibitory
domain. Plant Physiology 127: 1020–1029.
Pittman JK, Sreevidya CS, Shigaki T, Ueoka-Nakanishi H, Hirschi KD.
2002. Distinct N-terminal regulatory domains of Ca(2+)/H(+) antiporters.
Plant Physiology 130: 1054–1062.
Qiu QS, Guo Y, Quintero FJ, Pardo JM, Schumaker KS, Zhu JK. 2004.
Regulation of vacuolar Na+/H+ exchange in Arabidopsis thaliana by the
salt-overly-sensitive (SOS) pathway. Journal of Biological Chemistry
279: 207–215.
Quan RD, Lin HX, Mendoza I, Zhang YG, Cao WH, Yang YQ, Shang M,
Chen SY, Pardo JM, Guo Y. 2007. SCABP8/CBL10, a putative calcium
sensor, interacts with the protein kinase SOS2 to protect Arabidopsis
shoots from salt stress. Plant Cell 19: 1415–1431.
© The Authors (2008).
Journal compilation © New Phytologist (2008)
Review
Quintero FJ, Ohta M, Shi HZ, Zhu JK, Pardo JM. 2002. Reconstitution
in yeast of the Arabidopsis SOS signaling pathway for Na+ homeostasis.
Proceedings of the National Academy of Sciences, USA 99: 9061– 9066.
Rawat SR, Silim SN, Kronzucker HJ, Siddiqi MY, Glass ADM. 1999.
AtAMT1 gene expression and NH+4 uptake in roots of Arabidopsis
thaliana: evidence for regulation by root glutamine levels. Plant Journal
19: 143–152.
Reintanz B, Szyroki A, Ivashikina N, Ache P, Godde M, Becker D,
Palme K, Hedrich R. 2002. AtKC1, a silent Arabidopsis potassium channel
alpha-subunit modulates root hair K+ influx. Proceedings of the National
Academy of Sciences, USA 99: 4079– 4084.
Robatzek S, Chinchilla D, Boller T. 2006. Ligand-induced endocytosis
of the pattern recognition receptor FLS2 in Arabidopsis. Genes &
Development 20: 537–542.
Robert S, Bichet A, Grandjean O, Kierzkowski D, Satiat-Jeunemaitre B,
Pelletier S, Hauser MT, Hofte H, Vernhettes S. 2005. An Arabidopsis
endo-1,4-beta-D-glucanase involved in cellulose synthesis undergoes
regulated intracellular cycling. Plant Cell 17: 3378–3389.
Roberts MR. 2003. 14-3-3 Proteins find new partners in plant cell signalling.
Trends in Plant Science 8: 218–223.
Rubio V, Linhares F, Solano R, Martin AC, Iglesias J, Leyva A, Paz-Ares J.
2001. A conserved MYB transcription factor involved in phosphate
starvation signaling both in vascular plants and in unicellular algae.
Genes & Development 15: 2122–2133.
Saalbach G, Schwerdel M, Natura G, Buschmann P, Christov V, Dahse I.
1997. Over-expression of plant 14-3-3 proteins in tobacco: enhancement
of the plasmalemma K+ conductance of mesophyll cells. FEBS Letters
413: 294 –298.
Sanders D, Brownlee C, Harper JF. 1999. Communicating with calcium.
Plant Cell 11: 691–706.
Sanders D, Pelloux J, Brownlee C, Harper JF. 2002. Calcium at the
crossroads of signaling. Plant Cell 14: S401–S417.
Schachtman DP, Shin R. 2007. Nutrient sensing and signaling: NPKS.
Annual Review of Plant Biology 58: 47– 69.
Schiott M, Romanowsky SM, Baekgaard L, Jakobsen MK, Palmgren MG,
Harper JF. 2004. A plant plasma membrane Ca2+ pump is required for
normal pollen tube growth and fertilization. Proceedings of the National
Academy of Sciences, USA 101: 9502– 9507.
Schock I, Gregan J, Steinhauser S, Schweyen R, Brennicke A, Knoop V.
2000. A member of a novel Arabidopsis thaliana gene family of candidate
Mg2+ ion transporters complements a yeast mitochondrial group II
intron-splicing mutant. Plant Journal 24: 489– 501.
Schroeder JI, Allen GJ, Hugouvieux V, Kwak JM, Waner D. 2001.
Guard cell signal transduction. Annual Review of Plant Physiology and Plant
Molecular Biology 52: 627– 658.
Schultz TF, Medina J, Hill A, Quatrano RS. 1998. 14-3-3 proteins
are part of an abscisic acid VIVIPAROUS1 (VP1) response complex
in the Em promoter and interact with VP1 and EmBP1. Plant Cell
10: 837–847.
Seebohm G, Sanguinetti MC, Pusch M. 2003. Tight coupling of rubidium
conductance and inactivation in human KCNQ1 potassium channels.
Journal of Physiology (London) 552: 369–378.
Seebohm G, Strutz-Seebohm N, Ureche ON, Baltaev R, Lampert A,
Kornichuk G, Kamiya K, Wuttke TV, Lerche H, Sanguinetti MC
et al. 2006. Differential roles of S6 domain hinges in the gating
of KCNQ potassium channels. Biophysical Journal 90: 2235–2244.
Shi H, Ishitani M, Kim C, Zhu JK. 2000. The Arabidopsis thaliana
salt tolerance gene SOS1 encodes a putative Na+/H+ antiporter.
Proceedings of the National Academy of Sciences, USA
97: 6896– 6901.
Shin R, Schachtman DP. 2004. Hydrogen peroxide mediates plant root
cell response to nutrient deprivation. Proceedings of the National Academy
of Sciences, USA 101: 8827–8832.
Shin R, Berg RH, Schachtman DP. 2005. Reactive oxygen species and root
hairs in Arabidopsis root response to nitrogen, phosphorus and potassium
New Phytologist (2009) 181: 35–52
www.newphytologist.org
51
52 Review
Tansley review
deficiency. Plant and Cell Physiology 46: 1350–1357.
Sigworth FJ. 2003. Structural biology: life’s transistors. Nature 423: 21–22.
Simpson F, Whitehead JP, James DE. 2001. GLUT4 – At the cross roads
between membrane trafficking and signal transduction. Traffic 2: 2–11.
Smith FW, Hawkesford MJ, Ealing PM, Clarkson DT, VandenBerg PJ,
Belcher AR, Warrilow GS. 1997. Regulation of expression of a cDNA
from barley roots encoding a high affinity sulphate transporter.
Plant Journal 12: 875 –884.
Snedden WA, Fromm H. 2001. Calmodulin as a versatile calcium signal
transducer in plants. New Phytologist 151: 35– 66.
Sohlenkamp C, Shelden M, Howitt S, Udvardi M. 2000. Characterization
of Arabidopsis AtAMT2, a novel ammonium transporter in plants. FEBS
Letters 467: 273–278.
Sokolovski S, Blatt MR. 2007. Nitric oxide and plant ion channel control.
In: Lamattina L, Polacco JC, eds. Nitric oxide in plant growth, development
and stress physiology. Berlin, Germany: Springer, 153 –172.
Sokolovski S, Hills A, Gay R, Garcia-Mata C, Lamattina L, Blatt MR.
2005. Protein phosphorylation is a prerequisite for intracellular Ca2+
release and ion channel control by nitric oxide and abscisic acid in guard
cells. Plant Journal 43: 520 –529.
Sonoda Y, Ikeda A, Saiki S, Yamaya T, Yamaguchi J. 2003. Feedback
regulation of the ammonium transporter gene family AMT1 by glutamine
in rice. Plant and Cell Physiology 44: 1396–1402.
Sutter JU, Campanoni P, Tyrrell M, Blatt MR. 2006. Selective mobility
and sensitivity to SNAREs is exhibited by the Arabidopsis KAT1 K+
channel at the plasma membrane. Plant Cell 18: 935–954.
Sutter JU, Sieben C, Hartel A, Eisenach C, Thiel G, Blatt MR. 2007.
Abscisic acid triggers the endocytosis of the Arabidopsis KAT1 K+ channel
and its recycling to the plasma membrane. Current Biology 17: 1396–
1402.
Szyroki A, Ivashikina N, Dietrich P, Roelfsema MRG, Ache P, Reintanz B,
Deeken R, Godde M, Felle H, Steinmeyer R et al. 2001. KAT1 is not
essential for stomatal opening. Proceedings of the National Academy
of Sciences, USA 98: 2917–2921.
Takano J, Miwa K, von Wiren N, Fujiwara T. 2005. Boron dependent
endocytosis and degradation of boron transporter AtBOR1. Plant and Cell
Physiology 46: S151–S151.
Talke IN, Blaudez D, Maathuis FJ, Sanders D. 2003. CNGCs: prime targets
of plant cyclic nucleotide signalling? Trends in Plant Science 8: 286–293.
Tang RH, Han SC, Zheng HL, Cook CW, Choi CS, Woerner TE, Jackson
RB, Pei ZM. 2007. Coupling diurnal cytosolic Ca2+ oscillations to the
CAS-IP3 pathway in Arabidopsis. Science 315: 1423 –1426.
Thiel G, Blatt MR. 1994. Phosphatase antagonist okadaic acid inhibits
steady-state K+ currents in guard-cells of Vicia faba. Plant Journal
5: 727–733.
Todd CD, Zeng PY, Huete AMR, Hoyos ME, Polacco JC. 2004.
Transcripts of MYB-like genes respond to phosphorous and nitrogen
deprivation in Arabidopsis. Planta 219: 1003–1009.
Tombola F, Pathak MM, Isacoff EY. 2005. How far will you go to sense
voltage? Neuron 48: 719–725.
Vidmar JJ, Zhuo DG, Siddiqi MY, Glass ADM. 2000. Isolation and
characterization of HvNRT2.3 and HvNRT2.4, cDNAs encoding
high-affinity nitrate transporters from roots of barley. Plant Physiology
122: 783–792.
Volchuk A, Wang Q, Bennet MK, Klip A. 1996. Syntaxin 4 participates
in insulin-dependent glucose transport and is phosphorylated in 3T3-L1
adipocytes. Molecular Biology of the Cell 7: 2609–2609.
New Phytologist (2009) 181: 35–52
www.newphytologist.org
Walter M, Chaban C, Schutze K, Batistic O, Weckermann K, Nake C,
Blazevic D, Grefen C, Schumacher K, Oecking C et al. 2004.
Visualization of protein interactions in living plant cells using bimolecular
fluorescence complementation. Plant Journal 40: 428 –438.
Wang YH, Garvin DF, Kochian LV. 2002. Rapid induction of regulatory
and transporter genes in response to phosphorus, potassium, and
iron deficiencies in tomato roots. Evidence for cross talk and root/
rhizosphere-mediated signals. Plant Physiology 130: 1361–1370.
Webb AAR, Larman MG, Montgomery LT, Taylor JE, Hetherington AM.
2001. The role of calcium in ABA-induced gene expression and stomatal
movements. Plant Journal 26: 351– 362.
van den Wijngaard PWJ, Sinnige MP, Roobeek I, Reumer A,
Schoonheim PJ, Mol JNM, Wang M, De Boer AH. 2005. Abscisic acid
and 14-3-3 proteins control K+ channel activity in barley embryonic root.
Plant Journal 41: 43–55.
Williams D, Pessin JE. 2008. Mapping of R-SNARE function at distinct
intracellular GLUT4 trafficking steps in adipocytes. Journal of Cell Biology
180: 375 –387.
Willmer C, Fricker MD. 1996. Stomata 2. London, UK: Chapman and Hall.
Wirth J, Chopin F, Santoni V, Viennois G, Tillard P, Krapp A, Lejay L,
Daniel-Vedele F, Gojon A. 2007. Regulation of root nitrate uptake at the
NRT2.1 protein level in Arabidopsis thaliana. Journal of Biological
Chemistry 282: 23541–23552.
Wu P, Ma L, Hou X, Wang M, Wu Y, Liu F, Deng XW. 2003. Phosphate
starvation triggers distinct alterations of genome expression in Arabidopsis
roots and leaves. Plant Physiology 132: 1260–1271.
Wu SJ, Ding L, Zhu JK. 1996. SOS1, a genetic locus essential for salt
tolerance and potassium acquisition. Plant Cell 8: 617–627.
Xu J, Li HD, Chen LQ, Wang Y, Liu LL, He L, Wu WH. 2006. A protein
kinase, interacting with two calcineurin B-like proteins, regulates K+
transporter AKT1 in Arabidopsis. Cell 125: 1347–1360.
Yamaguchi T, Aharon GS, Sottosanto JB, Blumwald E. 2005. Vacuolar
Na+/H+ antiporter cation selectivity is regulated by calmodulin from
within the vacuole in a Ca2+- and pH-dependent manner. Proceedings of the
National Academy of Sciences, USA 102: 16107–16112.
Yamaguchi T, Apse MP, Shi HZ, Blumwald E. 2003. Topological analysis
of a plant vacuolar Na+/H+ antiporter reveals a luminal C terminus that
regulates antiporter cation selectivity. Proceedings of the National Academy
of Sciences, USA 100: 12510–12515.
Yap KL, Yuan T, Mal TK, Vogel HJ, Ikura M. 2003. Structural basis for
simultaneous binding of two carboxy-terminal peptides of plant glutamate
decarboxylase to calmodulin. Journal of Molecular Biology 328: 193 –204.
Yoshimoto N, Inoue E, Watanabe-Takahashi A, Saito K, Takahashi H.
2007. Post-transcriptional control of SULTR1;1 and SULTR1;2
high-affinity sulfate transporters in Arabidopsis. Plant and Cell Physiology
48: S89–S89.
Yuan L, Loque D, Kojima S, Rauch S, Ishiyama K, Inoue E, Takahashi H,
von Wiren N. 2007. The organization of high-affinity ammonium uptake
in Arabidopsis roots depends on the spatial arrangement and biochemical
properties of AMT1-type transporters. Plant Cell 19: 2636–2652.
Zhuo DG, Okamoto M, Vidmar JJ, Glass ADM. 1999. Regulation of a
putative high-affinity nitrate transporter (Nrt2;1At) in roots of Arabidopsis
thaliana. Plant Journal 17: 563–568.
Zimmermann S, Hartje S, Ehrhardt T, Plesch G, Mueller-Roeber B. 2001.
The K+ channel SKT1 is co-expressed with KST1 in potato guard
cells – both channels can co-assemble via their conserved K-T domains.
Plant Journal 28: 517–527.
© The Authors (2008).
Journal compilation © New Phytologist (2008)