Download 1 Figure 23. The plant vascular system serves as an effective inter

Figure 23. The plant vascular system serves as an
effective inter-organ communication system.
In response to a wide range of environmental and
endogenous inputs.
The xylem (blue lines) transmits root-to-shoot signals
(blue circles), including hormones such as
abscisic acid (ABA),
ACC (ethylene precursor) and
cytokinin, as well as
strigolactones (SLs).
These xylem-borne signaling agents serve to
communicate the prevailing conditions within the soil.
The phloem (pink lines) transports a wide array of
shoot-to-root signaling molecules (pink circles), including
auxin,
cytokinin,
proteins and
RNA species, including mRNA and sRNA.
Journal of Integrative Plant Biology
Volume 55, Issue 4, pages 294-388, 10 APR 2013
http://onlinelibrary.wiley.com/doi/10.1111/jipb.12041/full#f13
These phloem-borne signaling agents complete the longdistance communication circuit that serves to integrate
developmental and physiological events, occurring within
shoot and root tissues, in order to optimize the plant
performance under the existing growth/environmental
conditions.
1
กระแสการคายนํ้ าพาธาตุอาหารไปด ้วย
Xylem
unloading
ใบ
ธาตุอาหารในท่อนํ้ าจะถูก
คัดเลือกผ่านเมมเบรนก่อน
เข ้าสูเ่ ซลล์ของใบ
ราก
Xylem
loading
ภายในท่อนํ้ า ธาตุอาหารถูกพา
ตามนํ้ าทีไ่ หลในท่อนํ้ าไปจนถึงใบ
ธาตุอาหารถูกคัดเลือกผ่านเมมเบรน
เข ้าเซลล์ราก และคัดเลือกอีกครัง้
ก่อนสง่ เข ้าท่อนํ้ า
Yeo and Flowers. 2007
Xylem ท่อนํ้ า
2
In order to act as a finely tuned signal, this fraction of cycling
nutrients obviously exchanges only to a very limited extent with
the corresponding nutrients in the bulk tissue of shoots and roots.
3
แม ้ธาตุอาหารจะถูกพาโดยนํ้ าในท่อนํ้ า
ไปจนถึงใบ แต่เมือ
่ จะถูกนํ าไปใช ้
ภายในเซลล์ใบต ้องผ่านการคัดเลือก
ของเยือ
่ หุ ้มเซลล์ อีกครัง้ หนึง่
้ กหมุนเวียน
ธาตุอาหารทีเ่ หลือใชจะถู
โดยถูกสง่ ผ่านเซลล์พเิ ศษ (transfer
ื่ มกับท่ออาหาร
cell) ทีเ่ ชอ
Xylem-to-phloem
transfer is mediated
by a transfer cell
Model for scavenging solutes
from the xylem sap (xylem
unloading) in leaf cells
(Cat+ = cation, An- = anion,
As- = amino acids)
4
้ อนํ้ าเป็ นระบบพาธาตุอาหาร มีปัญหาคือ
การใชท่
• การไหลเป็ นไปตามความต ้องการคายนํ้ า
• ไม่ได ้เป็ นไปตามความต ้องการธาตุอาหาร เฉพาะเจาะจง
ของสว่ นนัน
้ ๆ
• กระแสคายนํ้ าอาจจะพาธาตุอาหารทีม
่ ากเกินไป ไปยังใบที่
้ อาหาร
ขยายขนาดเรียบร ้อยแล ้ว และไม่จําเป็ นต ้องใชธาตุ
ในขณะทีส
่ ง่ ให ้น ้อยเกินไป สําหรับสว่ นทีก
่ ําลังขยายขนาด
และสว่ นทีไ่ ม่คายนํ้ า
้ ้ไขปั ญหาข ้างต ้น
วิธก
ี าร 2 อย่างทีพ
่ ช
ื ใชแก
• ดึงตัวละลายทีไ่ ด ้จาก xylem เข ้าสะสมภายใน
เซลล์กอ
่ นนํ้ าระเหยไป
• หมุนเวียนตัวละลายจาก xylem ไป phloem
5
เซลล์พเิ ศษทําหน ้าทีด
่ ด
ู
้ หมดจาก
เก็บสารทีใ่ ชไม่
้
ท่อนํ้ า เพือ
่ เวียนใชใหม่
โดยสง่ ไปทีท
่ อ
่ อาหาร ทํา
ให ้ท่อนํ้ ากับท่ออาหารมี
้
เสนทางติ
ดต่อถึงกัน
Xylem parenchyma cells can
also be modified as transfer
cells, probably serving to
retrieve and reroute solutes
moving in the xylem, which is
also part of the apoplast.
6
Translocation rate in the xylem of heavy metal
cations is much enhanced when the ions are
complexed, e.g. Cu, Zn, Cd.
In tomato plants about 20% of K+ flux in the
xylem was cycling.
In young wheat and rye plants, the fraction of the
amino-N flux in the xylem was over 60%.
7
What happens to the transpiration stream?
Map of fluorescence intensity at the end of a filter paper wick,
which is carrying sulphorhodamine (SR) solution from the left.
Water has evaporated, concentrating the SR which has
accumulated at the end of the wick. (Bar, 1 mm)
8
ว่าด ้วยการฉีดให ้สารทางใบ??
ี นํ้ ามากเกินไป และป้ องกันไม่ให ้ฝนชะ
ผิวเคลือบคิวตินปกป้ องไม่ให ้ใบเสย
ี ไปได ้
สารต่างๆภายในใบสูญเสย
่ งขนาดเล็กเกิดระหว่างสารเคลือบ
การเคลือบผิวใบด ้วยสารคิวติน ยังคงมีชอ
ซงึ่ มีขนาดเล็กกว่า 1 นาโนเมตร จํานวน 1010 ต่อตารางซม. ตัวละลายขนาด
่ ยูเรีย ผ่านได ้ แต่โมเลกุลขนาดใหญ่ผา่ นไม่ได ้ เชน
่ Fe EDTA
เล็ก เชน
9
Permeation of low-molecular mass solutes and
evaporation of water through the cuticle takes place in
hydrophilic pores within the cuticle.
Most of the pores have a diameter of less than 1 nm,
and a density of about 1010 pores cm-2 has been
calculated. These pores are readily permeable to smallsized solutes (e.g. urea), but not to larger molecules
(e.g. synthetic chelates: Fe EDTA).
These small pores are lined with fixed negative charges
(polygalacturonic acids) increasing in density from the
outside of the cuticle to the inside.
Permeation of cations along this gradient is enhanced
whereas anions are repulsed from this region. Uptake
of cations by leaves is thus faster than that of anions.
10
Cuticular pore density is higher in cell walls between
guard cells and subsidiary cells.
The pores also seem to have different permeability
characteristics and are most likely the sites where
larger solute molecules penetrate the cuticle and are
taken up by the leaf cells.
It is unlikely that direct penetration of solutes from the
leaf surface through open stomata into the leaf tissue
plays an important role, because a cuticular layer also
covers the surface of the guard cells in stomatal
cavities.
11
Substances
Li
Na
K
Ce
Ca
H 2O
CO2
Urea
Glycerol
Glucose
Sucrose
Raffinose
Pores in cell wall
Pore in cuticle
Diameter, nm
0.76
0.72
0.66
0.62
0.82
0.30
0.33
0.40
0.54
0.88
1.06
1.22
<5.0
<1.0
12
Long-distance transport of nutrients in the plant
The rate-limiting stage in the nutrient translocation chain is the uptake,
conduction and release of ions by the symplast in the roots.
In the xylem, ions are rapidly distributed by the transpiration stream from the
root to the shoot.
Transfer cells promote the transport of ions from the vascular system to the
parenchyma cells.
The main function of the phloem transport is the relocation of mineral
13
substances already incorporated into the plant.
14
15
Partition of nitrogen
16
Ion homeostasis in the cell
a. proton (pH)
b. potassium
c. calcium
d. iron
17
I. Cytoplasmic pH
While cell remains viable, even if its metabolism is severely restricted, it
retains the ability to limit any shift in cytoplasmic pH.
Why does the pH change at all?
The change in pH is caused by increases in the concentration of H+ or OHions in the cytoplasm. Proton and hydroxyl ions are central to almost all
activities occurring within cells and so alteration of any activity could
potentially perturb local pH.
The sources of protons or hydroxyl ions can be separated into those
originating from fluxes across the membranes that bound the cytoplasm
(biophysical loads) and those that are produced or consumed by reactions
occurring within the cytoplasm (biochemical loads)
However, under any given set of conditions in which the cytoplasmic pH
changes, it is often difficult to ascribe the change to just one of these
categories.
Rengel. 2002. Handbook of Plant Growth. pH as the Master Variation.
18
Confocal mapping of
cellular pH via
fluorescence intensity
ratio imaging.
Rengel. 2002. Handbook of Plant
Growth. pH as the Master Variation.
19
pH Stat
The stabilization of cytosolic pH in the range 7.3-7.6
is achieved by the so-called pH stat which consists of two
components:
The biophysical pH stat is characterized by proton
exchange (membrane H+-ATPase) through the plasma
membrane or tonoplast.
The biochemical pH stat involves production and
consumption of protons, achieved by the formation and
removal of carboxylic groups (carboxylationdecarboxylation).
Marschner. 1995. Mineral Nutrition of Higher Plants.
20
Principal transport processes
involved in biophysical pH
regulation in plants.
The central component of the
system is the H+-ATPase that
generates both pH and
electrical differences across
the plasma membrane.
The fluxes of major nutrients,
in particular NO3- and H2PO4(Pi), involves significant
proton influx that needs to be
countered by active extrusion
of protons through the H+ATPase.
Rengel. 2002. Handbook of Plant
Growth. pH as the Master Variation.
21
Biochemical pH stat
22
Biochemical pH-stat
Biochemical control of cytoplasmic pH relies on:
1. metabolites acting as pH buffers and
2. enzymic rearrangements of metabolites to convert weak acids
into strong acids and vice versa.
It should be noted that cytoplasmic pH buffers are not capable of
long-term net consumption or production of protons or hydroxyl ions
and therefore should be considered as short-term mechanisms for
preventing large fluctuations in pH that would otherwise have
adverse consequences for metabolism. The cell is full of natural pH
buffers —amino acids, organic acids, phosphates —that potentially
can resist quite large short-term proton or hydroxyl loads.
Likewise, enzymic rearrangement of metabolites can not cope with
long-term acidification.
Rengel. 2002. Handbook of Plant
Growth. pH as the Master Variation.
23
Intracellular pH change associated with excess cation
uptake
In most cases, uptake of cations occurs as uniport – not associated
with the flux of another ion. The inward transfer of positive charge
needs to be compensated for either by uptake of an anion or by
efflux of another cation, which in plants is nearly always H+.
Therefore, where cation uptake exceeds anion uptake, the net efflux
of H+ will cause the cytoplasmic pH to rise, unless other processes
prevent this.
In the short term, the protons would originate from intracellular
buffers, but in the longer term the protons originate from newly
synthesized organic acids.
The end result is accumulation of inorganic cations and carboxylate
anions (R.COO-; e.g. malate), most of which pass into the vacuole.
Rengel. 2002. Handbook of Plant
Growth. pH as the Master Variation.
24
The Role of the Apoplastic pH
Apoplastic pH is more than just "external pH". As external pH – the pH of the
bulk solution is of limited importance for the regulation of membrane
transport in most cases.
Because of their cell walls, plant cells are able to create a specific
microenvironment close to the plasma membrane, the composition of which
determines the activity of the transporters and the amount of fixed charges.
This environment is a physically protected space and an area where ion
exchange takes place due to the negative charges of the polygalacturonic
acid. These interactions between cations (K+, Ca2+) and proton in the
apoplastic space play an important role in all transport.
This space makes the plasma membrane relatively insensitive to changes in
the bulk solution, but transport of ions allows the possibility of changing local
concentrations of one ion and, in turn, altering the concentration of the
other. That means that regulation of a cation channel need not be done
sterically but could be achieved through altering the pH, which will by
binding or releasing cations from their fixed charges, change the
concentration of ions to be transported.
The apoplastic space is the most relevant area for providing the transporters
and the regulation of the forces to mediate their translocation.
Rengel. 2002. Handbook of Plant Growth. pH as the Master Variation.
25
The vacuolar pH of some
hyperacidifying plant species
The values represent the pH of the juice or expressed sap of each
tissue usually a good indicator of valuolar pH.
a
26
II. Potassium
K+ is absorbed and accumulated in plant cells, and it constitutes 2–10% of
plant dry weight. The cytoplasmic K+ concentration ([K+]cyt) in plant cells is
relatively stable at approximately 100 mM ,which is the optimal K+
concentration for cytoplasmic enzyme activities. The K+ accumulated in
vacuoles is the largest K+ pool in plant cells; the vacuolar K+ concentration is
variable (between 10 mM and 200 mM) depending on the K+ supply and
tissue type.
The typical concentration at the surfaces of roots in the soil is extremely low,
varying from 0.1 to 1 mM.
Plant root cells therefore absorb K+ from the soil against the K+ concentration
gradient, a process conducted by plant K+ transport components such as K+
transporters and channels. K+ translocation from roots to shoots or from
source to sink inside plants is also mediated by K+ transporters and channels.
Wang and Wu. Annu. Rev. Plant Biol. 2013.64:451-476
27
Plants have evolved K+-deficiency-sensing and subsequent adaptive
mechanisms that help them survive under K+-deficiency stress. Plant roots are
the main organs that directly contact the external medium or soils, and
therefore the K+-deficiency sensing is initiated mainly in root cells, especially in
the epidermis and root hairs.
The K+-deficiency signal is first perceived at the PM of root epidermal cells and
then transduced into the cytoplasm. After perceiving the signal, plant cells
undergo a series of biochemical and physiological reactions that include both
short- and long-term responses.
The short-term responses occur within a few hours and involve several signal
components, including the membrane potential, reactive oxygen species (ROS),
and phytohormones.
During the early sensing period, the [K+]cyt may be not affected even though
the [K+]ext is low. As the K+-deficiency stress continues over days or weeks, the
[K+]cyt is significantly reduced and the long-term responses are then initiated,
which results in metabolic and morphological changes.
Wang and Wu. Annu. Rev. Plant Biol. 2013.64:451-476
28
Figure 1
Wang and Wu. Annu. Rev. Plant Biol. 2013.64:451-476
29
Figure 1
K+ transport and signaling pathways in Arabidopsis responses to K+ deficiency. The perception of K+deficiency stress leads to enhanced H+-ATPase (most likely AHA2) activity, which then results in plasma
membrane (PM) hyperpolarization and extracellular acidification. The hyperpolarized membrane potential
(ψ) activates the inward K+ channel AKT1 and the Ca2+-permeable channels.
Meanwhile, the extracellular acidification energizes the K+ uptake mediated by a K+ transporter such as
HAK5. K+ deficiency also induces ROS accumulation in root cells via RHD2 and RCI3, which further
regulates the transcription of K+-deficiency-responsive genes such as HAK5. The elevated ROS also
activates the PM-located Ca2+-permeable ANN1 channels that mediate Ca2+ flux into the cytoplasm. The
activation of Ca2+-permeable channels by membrane hyperpolarization and ROS results in the elevation of
cytoplasmic Ca2+, and this Ca2+ signal can be perceived and transduced downstream by specific Ca2+
sensors such as CBL proteins.
At the PM, CBL1 (and/or CBL9) recruits the cytoplasm-located kinase CIPK23 to the PM, where CIPK23
activates AKT1-mediated K+ uptake via phosphorylation. AKT1 channel activity can also be regulated by
CBL10, AIP1 (a protein phosphatase), AtKC1 (a K+ channel subunit), and SYP121 (a SNARE protein). The
CBL4-CIPK6 complex enhances AKT2-mediated K+ flux by facilitating the translocation of the AKT2
channel from the endoplasmic reticulum to the PM in a kinase-interaction-dependent but phosphorylationindependent way.
At the tonoplast, CBL3 (or CBL2) interacts with CIPK9 and activates an unknown tonoplast-located K+
channel or transporter, facilitating K+ release from the vacuole into the cytoplasm. In addition, the
tonoplast channel TPK1 mediates the K+ efflux from the vacuole into the cytoplasm.
Wang and Wu. Annu. Rev. Plant Biol. 2013.64:451-476
30
Closed
Potassium and
Guard Cell Induction
Closed
K+
Cl-
pH
K+
Cl-
pH
pH
Cl- K+
Open
pH
Cl- K+
Closed
K+
Willmer and Fricker. 1996. Stomata.
Open
Cl-
pH
Open
pH
Cl- K+
31
Stomatal opening -- K+ through plasma membrane
Light stimulates proton efflux from guard cells into the bathing
medium from epidermal strips. The measured rates and the
total mount of H+ efflux were similar to the rate and levels of
K+ accumulated. The H+ efflux is most probably due to the
activity of the H+-ATPase.
The K+ inward rectifier has a conductance of 5-10 pS
(picosiemens), opens at voltages more negative than -100 mV
and allows K+ influx if the apoplastic K+ concentration is
greater than 1 mM. At apoplastic K+ levels lower than 1 mM,
the thermodynamic driving force for K+ is likely be out of the
cell. The channel appears to inactivate at these low apoplastic
concentrations. The K+ inward rectifier is sensitive to cytosolic
Ca, cytosolic pH and external pH.
In cases where the membrane is depolarized or the
extracelular K+ is lower than approximately 1 mM, K+ must be
accumulated by ion-coupled carriers.
32
Stomatal opening -- K+ through tonoplast
During opening, the majority of ions accumulated will be
transferred to the vacuole, with generation of a
compatible solute in the cytoplasm, such as sucrose, to
increase the osmotic pressure in the cytoplasm.
K+ accumulation into the vacuole can not occur passively
against an estimated tonoplast membrane potential of
+20 to +40 mV, with the vacuole more positive than the
cytoplasm. K+ uptake may be mediated by a K+/nH+
antiport system or by a tonoplast pyrophosphatase
which is thought to pump H+ and K+ simultaneously into
the vacuole.
The activity of the tonoplast H+-ATPase would be
expected to acidify the vacuole lumen, causing large pH
changes to occur in the different cell types of the
epidermal layer during stomatal movements.
33
Stomatal opening --in the vacuole
The thermodynamic driving force would be sufficient to
accumulate at least a tenfold excess of K+ if coupled to
H+ exchange.
The predicted tonoplast membrane potential (+20 to
+40 mV) would be sufficient to drive a limited (5-fold)
accumulation of anions passively via channels.
Guard cell vacuoles can accumulate significant
concentrations of malate2-. There is evidence for a 40
pS, voltage-gated malate2- channel in guard cell
vacuoles.
34
Willmer and Fricker. 1996. Stomata.
35
Stomatal closure --K+ through the tonoplast
K+ efflux from the vacuole is favored by the prevailing
membrane potential and concentration gradient, and
could occur passively through appropriate ion channel.
Vacuolar K+ (VK)-channel is voltage independent, with a
high conductance (70 pS) and high selectivity for K+. It
is strongly activated by cytoplasmic Ca2+.
Slow-Vacuolar (SV)-type channel showed little
discrimination between cations and anions.
It was suggested that the mode of action is that
elevated cytoplasmic Ca2+ could activate VK-channels
allowing K+ efflux and depolarizing the tonoplast
membrane potential sufficiently to activate SV-channels.
36
Stomatal closure --K+ through the plasma membrane
K+ outward rectifier opens as the membrane potential
falls more positive than the K+ equilibrium potential (e.g.
more positive than -100 mV, which causes the K+ inward
rectifier to close)
K+ outward rectifier has a higher conductance (10-25
pS) than the K+ inward rectifier. The channel will open
under conditions that favor K+ efflux, even in the face of
increasing external K+ levels.
A drastic increase in cytoplasmic Ca2+ (2 to 200 nM)
caused a decrease in the channel conductance. K+
outward rectifier is sensitive to both cytoplasmic
alkalinization and apoplastic alkalinization.
Other channels are detected: a 4-5 pS channel that was
voltage insensitive; a stretch-activated K+-channel that
showed little voltage dependence.
37
FIGURE 23.14
38
Taiz and Zeiger. 2006.
III. Calcium ions
Calcium is a multifunctional signal transducer in plant cells. Usually, plant cells,
like many other cells, maintain their cytosolic-free Ca2+ concentration between
10-7-10-8 M by actively extruding Ca2+ from the cytosol towards the vacuole or
out of the cell and sequestering it within the ER and mitochondria.
Diverse external stimuli (e.g. application of abscisic acid) mobilize Ca2+ from
different subcellular pools and induce an array of response including darkinduced inhibition of sucrose synthesis, bending responses to gravity and blue
light, turgor regulation and stomatal closure. Some of the responses are
mediated by ion channels.
Kin channels are downregulated upon a rise in cytosolic-free Ca2+. It should be
emphasized that under certain conditions, Kin channels appear to be insensitive
to elevated cytosolic Ca2+ concentration. Kout channels in mesophyll cells also
appear to be downregulated by increasing cytosolic Ca2+ concentration.
However, in general, guard cell Kout channels are known to be Ca2+-insensitive.
39
Ann. Plant Reviews. 2004. V15.
[Ca2+]=100-200 nM
Vacuole
[Ca2+]=1 mM
Some of the main transport mechanisms
involved in cellular Ca2+ signaling.
Cytoplasmic Ca2+ is typically maintained at a
resting level of around 100-200 nM, which
requires constant extrusion of Ca2+ into the
apoplast, vacuole and endoplasmic reticulum
(ER). Extrusion is powered by primary Ca2+
pumps that are fuelled by ATP hydrolysis and/or
by H+/Ca2+ antiporter.
During signaling events, cytoplasmic Ca2+ can
increase by opening various voltage- and/or
ligand-gated Ca2+-permeable ion channels
situated in the membranes that separate the
cytosol from internal and external Ca2+ stores.
Ann. Plant Reviews. 2004. V15.
remove large
amount of Ca2+
maintain low resting
state of Ca2+
Dodd et al. 2010. Annu. Rev. Plant Biol. 61:593-620.
40
The phosphoinositide cascade
Binding of the agonist to the surface receptor R activates phospholipase C, either through a G
protein (which uses the energy of GTP hydrolysis to liberate a subunit capable of activating
the next partner in the cascade) or alternatively by activating a tryosine kinase. The activation
of phospholipase C hydrolyses phophatidylinositol-4,5-bisphosphate (PIP2) to InsP3 (IP3 in the
figure) and diacylglycerol (DG). InsP3 stimulates the release of Ca2+, sequestered in the ER,
and this in turn activates numerous cellular processes through Ca2+-binding proteins, such as 41
calmodulin.
Crichton. 2008. Biological Inorganic Chemistry.
Control of hyperpolarisation-activated Ca2+ channels by cytosolic Ca2+
apparently is connected to their physiological function. Stimulation (of
HACC) results from a shift of the activation threshold towards more positive
membrane potentials. Thus, in vivo, elevated cytosolic Ca2+ may form a
negative feedback on Ca2+ influx in guard cells in accordance with its role in
signaling and a positive feedback in root hairs for continued apical Ca2+
influx during tip growth.
The cytoplasmic concentration of Ca2+ is maintained by the activity of
Ca2+/H+ antiporters and Ca2+-ATPases, which transport Ca2+ into cellular
compartments or out of the cell to the apoplasm.
Ca2+/H+ antiporters have a high capacity but low affinity for Ca2+ and are
primarily found in the vacuole membrane, although in some plants they also
seem to be located in the plasma membrane.
Ca2+-ATPases have a low capacity but high affinity for Ca2+ and are found in
the plasma membrane and in several different endomembranes.
It is thus speculated that the physiological role of Ca2+/H+ antiporter is to
remove large amounts of Ca2+ from the cytosol after a Ca2+ signal, while
Ca2+-ATPase maintains the very low resting stage level of Ca2+.
42
Ann. Plant Reviews. 2004. V15.
Ca2+ serves as a secondary messenger in response to a number of stimuli and in
regulating the growth of "tip-growing" systems like pollen tubes and root hairs.
Activation of plasma membrane calcium channels and resulting Ca2+ influx
contributes to the observed increase in cytosolic Ca2+ concentration.
A
B
43
Ann. Plant Reviews. 2004. V15.
Figure 3
Schematic showing how turgor pressure and a selectively pliable cell wall allow for tip growth. Turgor
pressure (small dark gray arrows) is equivalent in all directions. To enable tip growth, the apical wall
must yield more easily than the rest of the cell wall. In the pollen tube, methoxylated pectin (blue
lines) is exocytosed (large green arrow) along with pectin methyl esterase (purple ovals) to the cell
apex. This methoxylated pectin is forced into the cell wall, creating a less viscous zone at the apex.
There, pectin methyl esterase demethoxylates the pectin (pink lines), allowing it to cross-link to Ca2+
ions (brown dots). This creates a more viscous wall as it moves to the side, allowing for less
expansion. Ultimately, cellulose (gold lines) is also added along the shank, strengthening the cell wall.
Rounds and Bezanilla. Annu. Rev. Plant Biol. 2013.64:243-265.
44
IV. Iron
Strategy I. Nongraminaceous plants
Plants reduce ferric iron to ferrous outside of the root cell and
import the iron in the ferrous form.
Strategy II. Graminaceous plants
Plants import the iron in the ferric form and reduce it inside the
cytosol of the root cell.
45
Nongraminaceous plants
Graminaceous plants
Fe acquisition strategies in higher plants: Strategy I in nongraminaceous plants (left) and
Strategy II in graminaceous plants (right).
Ovals represent the transporters and enzymes that play central roles in these strategies,
all of which are induced in response to Fe deficiency.
Abbreviations: DMAS, deoxymugineic acid synthase; FRO, ferric-chelate reductase oxidase; HA, H+ATPase; IRT, iron-regulated transporter; MAs, mugineic acid family phytosiderophores; NA,
nicotianamine; NAAT, nicotianamine aminotransferase; NAS, nicotianamine synthase; PEZ,
PHENOLICS EFFLUX ZERO; SAM, S-adenosyl-L-methionine; TOM1, transporter of mugineic acid family
phytosiderophores 1; YS1/YSL, YELLOW STRIPE 1/YELLOW STRIPE 1–like transporters.
siderophores: iron-complexing molecules of low molecular weight, specifically designed to complex Fe3+.
Kobayashi and Nishizawa. Annu. Rev. Plant Biol. 2012. 63:131–52.
46
IRON TRANSLOCATION
Figure 2
Molecules involved in xylem and phloem Fe loading. Only a few components have been proven to be
responsible for a specific step in transport; the involvement of others (indicated by question marks)
has been suggested by physiological, genetic, or biochemical studies. Abbreviations: ENA, efflux
transporter of nicotianamine; FPN/IREG, ferroportin/iron regulated; FRD3/FRDL, FERRIC REDUCTASE
DEFECTIVE 3/FERRIC REDUCTASE DEFECTIVE 3–like; FRO, ferric-chelate reductase oxidase; IRT, ironregulated transporter; MAs, mugineic acid family phytosiderophores; NA, nicotianamine; NRAMP,
natural resistance-associated macrophage protein; PEZ, PHENOLICS EFFLUX ZERO; TOM1, transporter
of mugineic acid family phytosiderophores 1; YS1/YSL, YELLOW STRIPE 1/YELLOW STRIPE 1–like
transporters.
47
Kobayashi and Nishizawa. Annu. Rev. Plant Biol. 2012. 63:131–52.
IRON TRANSLOCATION
Because of the poor solubility and high reactivity of Fe, its translocation
inside the plant body must be associated with suitable chelating molecules and proper
control of redox states between the ferrous and ferric forms. Fe translocation in plants
involves various steps, including radial transport across the root tissues, which must
include symplastic transport to pass through the Casparian strip; xylem loading,
transport, and unloading; xylem-to-phloem transfer; phloem loading, transport, and
unloading; symplastic movement toward the site of demand; and retranslocation from
source or senescing tissue.
Physiological and molecular studies have indicated some principal chelators
inside the plant body, such as citrate, nicotianamine (NA), and MAs. Recent progress in
identifying transporters has greatly improved our understanding of the components
involved in the metal translocation process, but it is often difficult to determine the
precise contribution of each component in the metal movement flux at each step of
translocation. Figure 2 represents our present understanding and deduction of the
molecules involved in xylem and phloem loading. Because the xylem and phloem
consist of dead and living cells, respectively, xylem loading is assumed to require efflux
transporters, whereas phloem loading would require influx transporters.
Kobayashi and Nishizawa. Annu. Rev. Plant Biol. 2012. 63:131–52.
48
Model for the regulation of iron deficiency responses in Strategy I plants.
In the root to shoot loop, differentiation of
root epidermal cells is controlled by iron
availability, either within the root cell or in the
rhizosphere. Development of extra numbers of
root hairs (which would increase iron uptake)
is repressed by iron in the vicinity of the root.
Iron taken up and translocated to the leaves in
the xylem regulates the uptake of iron into the
leaf cells.
In the shoot to root loop, when the iron levels
in the leaf declines, this is detected by a
sensor(s). A signal molecule is synthesized,
which conveys the information to the root via
the phloem. The effect of this signal is to
increase the transfer of electrons, the net
proton excretion and the activity of the Fe2+transporter, resulting in increased iron uptake
by the root cells.
49
Crichton. 2008. Biological Inorganic Chemistry.
Metal transporters
Fe transporter
Zn transporter
Cu transporter
50
Crichton. 2008. Biological Inorganic Chemistry.