Download Redox signals as a language of interorganellar

Cent. Eur. J. Biol. • 8(12) • 2013 • 1153-1163
DOI: 10.2478/s11535-013-0243-4
Central European Journal of Biology
Redox signals as a language of interorganellar
communication in plant cells
Review Article
Tomasz Kopczewski, Elżbieta Kuźniak*
Department of Plant Physiology and Biochemistry,
Faculty of Biology and Environmental Protection,
University of Łódź, 90-237 Łódź, Poland
Received 29 April 2013; Accepted 18 July 2013
Abstract: Plants are redox systems and redox-active compounds control and regulate all aspects of their life. Recent studies have shown that changes in
reactive oxygen species (ROS) concentration mediated by enzymatic and non-enzymatic antioxidants are transferred into redox signals used
by plants to activate various physiological responses. An overview of the main antioxidants and redox signaling in plant cells is presented.
In this review, the biological effects of ROS and related redox signals are discussed in the context of acclimation to changing environmental
conditions. Special attention is paid to the role of thiol/disulfide exchange via thioredoxins (Trxs), glutaredoxins (Grxs) and peroxiredoxins
(Prxs) in the redox regulatory network. In plants, chloroplasts and mitochondria occupying a chloroplasts and mitochondria play key roles
in cellular metabolism as well as in redox regulation and signaling. The integrated redox functions of these organelles are discussed with
emphasis on the importance of the chloroplast and mitochondrion to the nucleus retrograde signaling in acclimatory and stress response.
Keywords: Antioxidants • Chloroplasts • Glutaredoxins • Peroxiredoxins • Plastoquinone • Reactive oxygen species • Redox homeostasis • Thioredoxins
© Versita Sp. z o.o.
1. Introduction
Plants have evolved different mechanisms which are
responsible for acclimation to the constantly fluctuating
environmental conditions. Attacks of pathogens and
herbivores, water deficit and flooding, low and high
temperature – these and many other stress factors induce
defensive responses. However, to respond adequately
to the changing environment, a plant must sense
and process external stimuli and integrate signaling
pathways into a comprehensive signaling network.
Reactive oxygen species (ROS) and modifications
of redox metabolites are integral to stress and stress
defense mechanisms [1]. This review focuses on ROS
and redox signaling in plant cells and on the importance
of the intercompartmental redox communication in the
acclimatory response to environmental stresses.
2. Reactive oxygen species and redox
homeostasis
Molecular oxygen occurs in nature in two main allotropic
varieties: as dioxygen (O2) and trioxygen (ozone, O3).
* E-mail: [email protected]
Oxygen in the ground state, which has two unpaired
electrons in two orbitals π*, is called triplet oxygen
(3O2). This form of oxygen is relatively inactive. ROS are
formed by several different mechanisms (Figure 1) [2].
ROS are synthesized in different parts of a plant cell:
in chloroplasts, mitochondria, peroxisomes, apoplast
and plasma membrane. Generation of ROS during
photosynthesis, cellular respiration and photorespiration
is normally maintained at a safe level. However, under
stress, intensification of ROS production occurs, leading
to potentially harmful effects like lipid peroxidation,
carbonylation of proteins, formation of disulfide and
dityrosine bridges, modification of amino acids in
polypeptide chains, DNA damage as well as modifications
of the structure of pigments [3]. On the other hand, ROS
can also act as signaling molecules involved in many
physiological processes. ROS and related redox signals
make an important contribution to the cell cycle, cell and
organ expansion, cell wall remodeling, photosynthesis,
stomatal opening, senescence, programmed cell death
(PCD) as well as plant responses to abiotic and biotic
stresses [4,5].
Photosynthetic organisms have evolved enzymatic
and non-enzymatic antioxidants and redox buffering
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Redox signals in plant cells
Figure 1.
ROS formation and the antioxidant system in a plant cell. The activity of PS I and PS II in chloroplasts, ETC functioning as well as GO
(in peroxisomes), Ac-CoA O (in glyoxysomes) and NADPH oxidase (in membranes) activities are associated with ROS formation in a
plant cell. Enzymatic (CAT, APX, GPX, SOD) and non-enzymatic (e.g. AA, car, a-toc) antioxidants prevent excessive accumulation of
ROS. AA – ascorbate, Ac-CoA O – acyl-coenzyme A oxidase, APX – ascorbate peroxidase, a-toc – a-tocopherol, car – carotenoids,
CAT – catalase, DHA – dehydroascorbate, ETC – mitochondrial electron transport chain, GO – glycolate oxidase, GPX – glutathione
peroxidase, GSH – glutathione, GSSG – glutathione disulfide, PS I – photosystem I, PS II – photosystem II, ROS – reactive oxygen
species, SOD – superoxide dismutase. According to [1,7-9], modified.
systems to prevent ROS-mediated cellular oxidation
and to maintain cellular redox balance [6-9] (Figure 1).
This review focuses selectively on low molecular weight
antioxidants and redox enzymes that are important for
antioxidant defense and regulate redox signaling.
The total pool of redox-active compounds which are
found in a cell in reduced and oxidized forms creates the
cellular redox buffer and NADPH/NADP+, ascorbate/
dehydroascorbate
(AA/DHA),
glutathione/glutathione
disulfide (GSH/GSSG) and thioredoxin reduced/thioredoxin
oxidized (Trxred/Trxox) pairs are considered the most
important ones [10]. AA and GSH are major components
of the soluble redox buffering system and they contribute
significantly to the redox environment of a cell.
AA is the most abundant antioxidant present in plant
cells at millimolar concentration (10-300 mmol L-1), thus
it is the major contributor to the cellular redox state and
redox signaling [11]. Besides its antioxidant function,
AA regulates cell growth and differentiation and also
takes part in cell wall expansion and cell division [12].
The shift of AA/DHA equilibrium towards the oxidized
state inhibits cell cycle progression [13]. Moreover, the
guard cell AA redox state controls stomatal movement
through H2O2 signaling [14]. AA is also the substrate or
cofactor for enzymes, e.g. prolyl hydroxylase catalyzing
the synthesis of hydroxyproline and violaxanthin
deepoxidase in the xanthophyll cycle [15]. The vitamin
c-1 (vtc1) mutant of Arabidopsis thaliana with a reduced
level of AA is characterized by increased sensitivity
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to abiotic stresses and enhanced basal resistance to
biotrophic pathogens. It has been demonstrated that
in the vtc mutants an abundance of AA modified plant
response to stresses via redox mechanisms [16].
AA is also involved in the xanthophyll cycle that
protects chloroplasts from photooxidatve damage and
contributes to excess excitation energy dissipation
in the antennae complexes of photosystem II (PS II)
[17]. However, it has been shown that in the
non-photochemical quenching 1 (npq1) mutant of
A. thaliana with inactive violaxanthin deepoxidase and
in the wild form of this plant, the levels of photoinhibition
associated with high-light stress were similar. Moreover,
fluctuating light conditions in npq1 mutants give more
negative effects than high light. These facts show
that the role of the xanthophyll cycle in protection of
photosystems is not fully elucidated [18].
AA cooperates tightly with GSH (γ-Glu-Cys-Gly)
in the Halliwell–Asada cycle, comprised of three
interdependent redox couples: AA/DHA, GSH/GSSG
and NADPH/NADP+. They undergo subsequent
reduction/oxidation reactions catalysed by ascorbate
peroxidase (APX), monodehydroascorbate reductase
(MDHAR), dehydroascorbate reductase (DHAR) and
glutathione reductase (GR) that are responsible for H2O2
scavenging and keeping AA and GSH in the reduced
state at the expense of NADPH. The cycle is located in
all cellular compartments in which ROS detoxification is
required. It serves as the main antioxidant pathway in
T. Kopczewski, E. Kuźniak
plant cells and is also involved in redox-regulated plant
defense against environmental stresses [11].
GSH, the main thiol buffer in plant cells, acts as an
antioxidant by scavenging ROS but it also plays a pivotal
role in cellular redox regulation. Similarly to AA, changes
in the GSH pool and its redox status could act as redox
signals integrated into the network that regulates gene
expression, including antioxidant defense genes [11,19].
Protein S-glutathionylation, a process of mixed disulfide
formation between a free thiol of a protein and GSH,
has been suggested to be a mechanism for transducing
GSH-related redox signals under the conditions of
oxidative stress. This process plays an important
regulatory role in controlling metabolism, gene
transcription and protein turnover [20].
One of the best characterized mechanisms of redox
signaling in photosynthetic organisms is that mediated
by thiol/disulfide regulation which is realized by redox
proteins, namely thioredoxins (Trxs), glutaredoxins
(Grxs) and peroxiredoxins (Prxs). The thiol/disulfide
turnover participates in turning on/off the function of
proteins and in modifying the activity of transcription
factors and enzymes [21]. As the Trxs system is linked to
photosynthetic light reactions, its role will be addressed
in the next section.
Grxs are small proteins belonging to the superfamily
of Trxs proteins which utilize the reducing power of
GSH to reduce disulfide bonds of target proteins. They
can reduce disulfide bonds by a monothiol or dithiol
mechanism. Restoration of the reduced form of Grxs is
provided by GSH in the presence of NADPH and GR or
thioredoxin reductase [22,23]. It has been shown that
three classes of Grxs (monothiol, dithiol and CC-type)
occur in plants. Monothiol Grxs contain one cysteine
(Cys) residue occurring most frequently in the CGFS
motif. These Grxs were primarily identified in yeasts but
they also occur in plants. Monothiol Grxs are involved
in the biogenesis of mitochondrial iron-sulfur clusters,
protection of proteins against oxidation damage,
Ca2+ signaling, regulation of iron homeostasis and in
protein kinase C-mediated stress responses [24]. Grxs
recognize glutathionylated substrates, including proteins
and catalyze their reduction, called deglutathionylation
(removal of GSH from glutathionylated proteins). The
reversible protein S-glutathionylation has now emerged
as a redox post-translational mechanism protecting
protein thiols against irreversible oxidation, controlling
protein function and regulating the signaling and
metabolic pathways [20].
Besides regulating protein structure and function,
the glutathionylation-deglutathionylation cycle is
implicated in transducing redox signals [20]. It has been
found that plant cytosolic triose phosphate isomerase
and some Trxs were regulated by glutathionylation.
A similar regulatory mechanism controls the activity
of L-galactono-1,4-lactone dehydrogenase catalyzing
the final step of AA biosynthesis in the mitochondria
[25]. These enzymes are potential targets of Grxs
for deglutathionylation. In chloroplasts, where ROS
production is intense, glutathionylation is a very
important mechanism of redox regulation [26].
Grxs, like Trxs, may operate as disulfide reductases.
However, they differ with respect to specificity versus
target proteins and mode of catalysis. It has been
shown that Grxs reduce glutathionylated proteins
more efficiently than Trxs whereas the reduction of
intramolecular disulfides is more characteristic of Trxs
than of Grxs [26].
Prxs are thiol-dependent non-heme peroxidases
decomposing peroxides which cooperate with
Trxs and Grxs in the antioxidant system and the
thiol-disulfide regulatory network. They are classified
as broad spectrum peroxidases able to detoxify H2O2,
alkyl hydroperoxides and peroxinitrite. Based on the
number of catalytic Cys residues, their position in
amino acid sequence and the mechanism of catalyzed
reaction, four classes of Prxs have been distinguished
in plants, namely 1-Cys Prx, 2-Cys Prx, Prx Q and Prx
II [27]. They are distributed in many isoforms in several
subcellular compartments. Prxs contain one or two
Cys residues at the active site and function as dimers
(2-Cys Prxs) or monomers (1-Cys Prxs, Prxs Q and
Prxs II). In the common catalytic cycle, represented
by 2-Cys Prxs and Prxs Q, the catalytic Cys thiol is
oxidized by peroxides to sulfenic acid and then reduced
by a second Cys thiol forming an intra- or intermolecular
disulfide bond. 1-Cys Prxs have a single catalytic Cys
residue and do not create disulfide bridges (Figure 2A).
Prxs II have an ability to form heterodimers with Grxs
(Figure 2B) [27]. To complete the catalytic cycle Prxs
interact with Trxs, Grxs, GSH, AA and cyclophilins as
well as NADPH thioredoxin reductase C (NTRC) as
electron donors [27]. The biological function of Prxs
in plants goes far beyond the peroxide detoxification
system. Depending on the Prxs type they sense the
redox state, transmit redox information to binding
partners and function as chaperones. In plants, they are
involved in balancing hydroperoxide production during
photosynthesis and respiration as well as in the redox
regulation of cell division and elongation, germination,
organ development and stress adaptation [28].
Interactions of redox-active compounds and the
capacity of redox buffers determine the cellular redox
state. Under optimal conditions the inherent redox
balance in individual organelles is maintained at
different levels compatible with their specific metabolic
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Redox signals in plant cells
Figure 2.
Reaction cycle of peroxiredoxins. Prxs catalyze the reduction of hydroperoxides.
(A) Classical 2-Cys Prx is a homodimer with two reduced Cys residues per subunit. During the catalytic cycle the peroxide substrate is
reduced to the corresponding alcohol and the sulfenic acid derivative of the catalytic Cys of one subunit interacts with the resolving Cys
of the other subunit forming an intermolecular disulfide bridge. The oxidized 2-Cys Prx is regenerated by electron donors such as Grxs.
(B) 1-Cys Prxs possess one catalytic Cys on both subunits. The oxidized 1-Cys Prxs are preferentially regenerated by Trxs. Grx – dithiol
glutaredoxin, GSH – glutathione, GSSG – glutathione disulfide, Prx(s) – peroxiredoxin(s), ROOH – hydrogen peroxide (R=H) or lipid
peroxide (R = hydrocarbon group), ROH – water (R=H) or alcohol/phenol (R=hydrocarbon group), Trx(s) – thioredoxin(s). According
to [27], modified.
requirements. The redox buffering capacity of
compartments such as apoplast, endoplasmic reticulum
and vacuole are considered much weaker than that
of chloroplasts and mitochondria [29]. Redox signals
are key regulators of plant metabolism, morphology,
development and defense responses. As each cellular
compartment has different antioxidants and redox
buffering capacity, at each location the redox state and
redox-based signaling can be controlled independently
[30]. Specific compartment-based redox signaling,
including that involved in chloroplast-to-nucleus and
mitochondria-to-nucleus retrograde communication,
and regulation of gene expression can be achieved by
changes in the redox status of a given compartment via
modifying ROS production and/or antioxidant defenses
[10,30].
The cellular balance between redox buffers and
ROS called redox homeostasis is strongly affected by
environmental cues [31]. Increased ROS production
and changes in redox potential caused by abiotic and
biotic stressors are important factors in cell redox
signaling influencing the specificity of a response.
Amplitude, duration and localization of particular
redox signals specifically indicate what the nature
1156
of stress is and what defense response has to be
activated [32].
3. Redox signaling related to
photosynthesis and mitochondrial
respiration
ROS are produced in many parts of a plant cell, however
their synthesis is mainly associated with activities of
electron transport chains in chloroplasts and mitochondria
[33]. Under stressful conditions an imbalance between
light energy absorbed through PS II and the ultimate
consumption of the photosynthetic electrons through
metabolic pathways leads to increased formation of
ROS and to photooxidative stress. Recently it has been
shown, however, that redox signals generated in the
photosynthetic electron transport chain have a strong
impact on the cellular signaling network. Chloroplasts
act as sensors for changes in the environment which
are communicated via redox signaling. A variation in the
redox status of the photosynthetic apparatus generates
redox signals involved in mediating stress-induced
metabolic changes [19]. It has been proposed that
T. Kopczewski, E. Kuźniak
perturbation of the photosynthetic electron transfer
produces three types of redox signals [34]. Class 1
signaling originates from specific redox pairs in the
photosynthetic electron transport chain, e.g. the reduced
and oxidized plastoquinone (PQ), class 2 signaling
depends on the redox state of stromal Trxs, NAD(P)H,
GSH and AA whereas class 3 signaling is mediated by
ROS [35,36]. Here we focus on the redox signals from
the PQ and Trxs pools.
PQ, 5,6-dimethyl-para-quinone with a hydrophobic
chain attached at position 2 and variable number of
isoprenoid units, carries the electrons from PS II
to the cytochrome b6f (cyt b6f) complex and serves
as an indicator of photosynthetic electron transport
[37]. The redox state of PQ, depending directly on
the photosynthetic electron flux, controls the size of
photosynthetic antennae and regulates chloroplast
protein expression. The PQ redox state regulates
the activity of thylakoid protein kinase STATE
TRANSITION7 (STN7) involved in the reversible
phosphorylation of light harvesting complex II (LHCII)
antenna proteins. STN7 is required for state transition
that optimizes electron transfer under fluctuating light
conditions [38]. It has been shown that the reduction
of PQ into plastoquinol (PQH2) induces expression
of psaA and psaB genes encoding proteins of
photosystem I (PS I) while increased concentration of
PQ activates the transcription of PS II genes (psbA,
psbD). Moreover, changes in the PQ pool affect
the level of nuclear and chloroplast mRNA during
acclimatory response to environmental conditions [39].
Trxs are low molecular redox proteins with enzymatic
activity, discovered both in non-photosynthetic and
photosynthetic organisms. Three types of Trxs (h, f, m)
have been identified in plants. In plant cells, Trxs
were recognized as redox state creators and proteins
affecting the activity of numerous target molecules
via thiol/disulfide exchanges. Thioredoxin h (Trx h)
is present in the cytosol. Thioredoxins f and m are
located in chloroplasts and they are reduced in
the light via ferredoxin and ferredoxin thioredoxin
reductase (FTR) [40]. They were primarily considered
as regulators of several Calvin cycle enzymes, e.g.
fructose-1,6-bisphosphatase,
phosphoribulokinase,
glyceraldehyde-3-phosphate dehydrogenase and
ribulose-1,5-bisphosphate carboxylase oxygenase
(RuBisCO) activase. All these enzymes are activated
in the light by Trxs and remain inactive in the dark.
However, current reports show that Trxs also regulate
the activity of enzymes associated with other metabolic
pathways, e.g. some electron transport chain
components and enzymes of the sulfur assimilation
pathway. Moreover, biosynthesis of amino acids,
lipids and tetrapyrolles is controlled by Trxs. Trxs
also influence the activity of some enzymes involved
in translation and DNA metabolism. They are also
versatile redox regulators interacting with other redox
transmitters, such as GSH and Grxs [40,41].
Trxs may also be part of enzyme complexes.
The Arabidopsis thaliana Tetratricopeptide Domaincontaining thioredoxin (AtTDX) is a Trx-like protein
consisting of a Trx motif and three tetratricopeptide
(TPR) repeat domains. The Trx motif is a disulfide
reductase responsible for redox properties of AtTDX
while TPR exhibits chaperone activity [42].
Moreover, a new Trx-like protein (TrxZ) that is
a subunit of plastid RNA polymerase complex has
been identified recently. It is recognized as redox
active protein linking redox regulations and plastid
transcription [43].
Recent studies highlighted the importance of a
co-operation between PQ and Trxs redox signaling
in tailoring the photosynthetic apparatus to the
illumination conditions. The light-induced changes in
the PQ and Trxs pools affect gene expression and
metabolic enzyme activities indispensable to adapt
plant productivity to the light environment [44].
Although
in
the
photosynthesizing
cells
mitochondria are not regarded as the main source
of ROS, perturbations of the mitochondrial redox
state have important implications for cellular
redox homeostasis. Firstly, electron flow through
the mitochondrial electron transport chain may
control AA biosynthesis as L-galactono-1,4lactone dehydrogenase is associated with NADH
dehydrogenase complex I (complex I) and is required
for its assembly and this enzyme uses oxidized
cytochrome c as the only electron acceptor [45].
Secondly, in Nicotiana sylvestris the cytoplasmic malesterile mutant (CMSII) lacking functional complex
I re-adjustment of mitochondrial, chloroplastic and
peroxisomal antioxidant defense as well as enhanced
alternative oxidase-mediated, cyanide insensitive
respiration were observed [46]. Thus, loss of complex I
function in mitochondria changed the whole cell redox
homeostasis. Moreover, the malate valve also serves
as a redox-based mechanism linking mitochondria
and chloroplasts. The chloroplastic NADP-malate
dehydrogenase, which is Trx-controlled, uses excess
NADPH to convert oxaloacetate to malate in order
to regenerate the electron acceptor NADP and to
prevent photoinhibition. However, the effectiveness
of this mechanism depends on the malate gradient
between chloroplasts and cytosol, and the increase
in the cytosolic malate pool is coupled with a higher
mitochondrial malate level [47].
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Redox signals in plant cells
4. Chloroplasts
–
mitochondria
communication and retrograde
signaling
In plant cells, the main cellular compartments, e.g.
chloroplasts, mitochondria, peroxisomes and cytosol,
exist in a subtle metabolic equilibrium and disturbances
in any of these locations generate perturbations in
the whole cellular metabolism. Special attention has
been paid to the interaction between mitochondrial
metabolism and photosynthetic carbon assimilation.
In the photosynthesizing cells, chloroplasts and
mitochondria, both involved in the production of ATP,
are also connected by metabolite exchange and the
photorespiration pathway (Figure 3). The exchange of
metabolites between chloroplasts and mitochondria
requires the presence of translocators in their inner
envelope membrane and is connected with the
formation of inter-organelle communication channels
[48,49]. Numerous studies showed that the transfer of
photosynthetically active cells from darkness to light is
associated with an increase in photosynthetic activity
Figure 3.
1158
in comparison with respiration which emphasizes the
interaction between chloroplasts and mitochondria
[50]. Moreover, the balance between triose-phosphate
and 3-phosphoglycerate in chloroplasts as well as
between malate and oxaloacetate in mitochondria
reflects the redox status of these organelles and
may modulate photosynthesis and respiration [49].
However, the chloroplasts-mitochondria interaction is
still not known well and the current knowledge refers
mainly to communication based on carbon signals
[51]. Other signals, e.g. AA, nitric oxide (NO) and the
cytosolic pH have also been proposed to be involved
in the biochemical cross-talk between chloroplasts
and mitochondria. It has been suggested that AA, an
important antioxidant synthesized in mitochondria,
influences the redox conditions and ROS scavenging in
chloroplasts [12,52]. Moreover, alkalinization of cytosol
in mesophyll cells resulting from the activity of illuminated
chloroplasts may modulate the cytosolic, chloroplast
and mitochondrial metabolism [49]. In the context of
photorespiration, mitochondria help to dissipate excess
redox equivalents exported from chloroplasts in the
Interactions between chloroplasts, mitochondria and the nucleus. Chloroplasts and mitochondria act as sensors of environmental
changes. Extensive metabolic communication is established between chloroplasts and mitochondria to maintain energy flow through
these organelles and to optimize different cell functions. Moreover, redox signals generated in chloroplasts and mitochondria within
the electron transport chains or by generation of ROS and ROS-scavenging systems initiate signaling pathways that activate nuclear
gene expression. This retrograde signaling coordinates the nuclear gene expression with the functional state of chloroplasts and
mitochondria but also with the acclimatory response that helps the plant to respond optimally to environmental stress.
Cyt b6f – cytochrome b6f, EX1 – EXECUTER 1, EX2 – EXECUTER 2, Fd – ferredoxin, Gc – glycolate, Mg-protoIX – Mg-protoporphyrin
IX, mitETC – mitochondrial electron transport chain, PC – plastocyanin, PGc – 2-phosphoglycolate, PQ – plastoquinone,
PQH2 – plastoquinol, PS I – photosystem I, PS II – photosytem II, RuBP – ribulose-1,5-bisphosphate, STN7 – STATE TRANSITION7
protein kinase, Trxox – thioredoxin oxidized, Trxred – thioredoxin reduced. According to [69], modified.
T. Kopczewski, E. Kuźniak
light via the malate valve, as described earlier [47]. The
photorespiratory pathway is also an internal source
of CO2 which can help to sustain carbon assimilation
under limiting CO2 conditions [49]. However, further
experimental evidence is necessary to establish the
roles of these signals in inter-organelle communication.
Chloroplasts and mitochondria also cooperate in
retrograde signaling, which serves to communicate
the developmental and functional state of chloroplasts
and mitochondria to the nucleus to regulate nuclear
gene expression [13,53-55]. While the expression of
nuclear-encoded proteins and their import to organelles
is called anterograde signaling, the information transfer
from organelles to a nucleus that affects nuclear
gene expression is defined as retrograde signaling.
Chloroplasts and mitochondria produce retrograde
signals to alter nuclear gene expression in order to
coordinate their biogenesis and function under stressful
conditions [56]. There are four retrograde signaling
pathways: tetrapyrrole biosynthesis, organellar protein
synthesis, redox state and ROS [51].
A broad range of ROS/redox signals generated in
chloroplasts and mitochondria is targeted to the nucleus
(retrograde signaling) to induce gene expression.
Singlet oxygen (1O2) produced in chloroplasts in light via
chlorophylls that act as photosensitizers is a signaling
molecule in plants [51]. The primary effect of 1O2
generation is not the control of chloroplast biogenesis
as only about 15% of the 1O2-responsive genes encode
plastid proteins. It was discovered that 1O2 is responsible
for the activation of PCD genes [57]. 1O2 has a relatively
short half-life (t = 200 ns) and its scavenging or
transformation often outruns its physiological actions
[58]. Therefore, it has been suggested that other
molecules must be involved in the transduction of
1
O2-dependent signals. Two proteins associated with
the thylakoid membranes, namely EXECUTER1 (EX1)
and EXECUTER2 (EX2) are mediators of 1O2 signals
[17]. EX1 is responsible for the perception of 1O2
produced in chloroplasts and EX2 controls the activity of
EX1. Evidence for the concerted action of EX1 and EX2
comes from EXECUTER1 EXECUTER2 fluorescent
(ex1 ex2 flu) mutant of A. thaliana. Inactivation of genes
encoding EX1 and EX2 suppressed the 1O2-induced
genes completely [17]. H2O2, which is the product of
further reduction of O2•-, also plays an important role
in retrograde signaling. This molecule is implicated in
modulation of nuclear gene expression via mitogenactivated protein kinase (MAPK) signaling cascades
[59]. In Arabidopsis, transcriptome analysis has
revealed that 1-2% of the transcriptome and one third
of the transcription factor mRNA are altered after H2O2
treatment [60,61]. Moreover, analysis of the flu mutant
of A. thaliana showed that H2O2 activates a different set
of genes in comparison with 1O2 and it antagonizes the
1
O2-mediated stress responses [57].
Mg-protoporphyrin IX (Mg-protoIX), an intermediate
in chlorophyll biosynthesis, is also responsible for
retrograde signaling [62,63]. This compound is
identified as a negative regulator of photosynthetic
gene expression in the nucleus and chloroplasts.
Mg-protoIX is accumulated under stress and after
norflurazon (inhibitor of the last stage of chlorophyll
biosynthesis) treatment and therefore signals from
Mg-protoIX are considered to be associated with inhibition
of photosynthesis and generation of responses against
stress [51,58,64,65]. Studies with genome uncoupled
(gun) mutants failing to repress the expression of
photosynthesis-associated nuclear genes when treated
with norflurazon indicated that the accumulation of
Mg-protoIX triggers plastid-to-nucleus signaling
mediated by ABA INSENSITIVE4 (ABI4) transcription
factor. GUN1 protein integrates signals related to
Mg-protoIX, photosynthetic electron flow and oxidative
stress which are generated in chloroplasts and is
implicated in their transmission to the nucleus. In
response to GUN1-derived signals, ABI4 transcription
factor represses photosynthetic nuclear genes by
preventing DNA binding of factors needed for their
expression [51,66,67]. However, the role of Mg-protoIX
in retrograde signaling needs further elucidation as
tetrapyrrole profiling in Arabidopsis seedlings showed
no correlation between Mg-protoIX accumulation and
the expression of nuclear genes [68].
Photosynthetic electron transport chain components
have also been postulated to be redox-dependent
retrograde signaling molecules that communicate the
chloroplast status to the nucleus. As discussed above,
redox signals generated in chloroplasts are transmitted
by PQH2 and by the elements on the reducing side of
PS I, particularly by ferredoxin (Fd) (Figure 3). The
thylakoid protein kinase STN7, playing a role in the
reversible phosphorylation of LHCII and its temporary
migration to PS I as well as in long-term adaptation of
plants to changes in light quality or intensity [39,70], has
been proposed to transfer the changes in chloroplast
redox status to the nucleus [58,71]. Moreover, electrons
from Fd may be transported to Trxs and GSSG.
Together with other compounds such as Grxs and Prxs,
Trxs can participate in the modification of functions
of target proteins. Especially chloroplast and nuclear
transcription factors may be activated or deactivated
with the involvement of Trxs, Grxs and Prxs [63].
The
latest
data
showed
the
role
of
3’-phosphoadenosine-5’-phosphate (PAP) in the
regulation of nuclear-gene expression associated with
1159
Redox signals in plant cells
stress response in plants. The level of chloroplastic PAP
is controlled by SAL1 – a phosphatase that catalyzes
the conversion of PAP into AMP [72]. In high light- or
drought-stressed Arabidopsis plants, PAP accumulates
in chloroplasts and it can migrate into the nucleus
where it inhibits some 5’ to 3’ exoribonucleases (XRNs).
This leads to the activation of transcription factors
(ZAT12, DREB2A and others) and to the regulation of
stress-responsive genes (e.g. APX2, ELIP2) [56].
Interestingly, SAL1 is dual targeted to chloroplasts and
mitochondria. This suggests that the PAP-mediated
chloroplast retrograde signaling can be linked to the
mitochondrial pathway [73]. Another recent study has
suggested that methylerythritol cyclodiphosphate
(MEcPP), a precursor of isoprenoids synthesized in
the chloroplastic methylerythritol phosphate (MEP)
pathway, also has a function in plastid retrograde
signaling during stress [74]. In chloroplasts, MEcPP is
normally converted into hydroxy-2-methyl-2-(E)-butenyl4-diphosphate (HMBPP) and hydroxymethylbutenyl
diphosphate synthase (HDS) catalyzes this process
[75]. It has been shown that constitutively expressed
hydroperoxide lyase (ceh1) mutant of A. thaliana that
has a mutation in the HDS gene responsible for the
conversion of MEcPP into HMBPP in the MEP pathway
accumulates MEcPP. Ceh1 displays changes in
stress-related gene expression associated with salicylic
acid-mediated defense leading to increased resistance
to Pseudomonas syringae. These data clearly
indicate that accumulation of MEcPP in chloroplasts is
associated with regulation of stress-responsive nuclear
gene expression and MEcPP can be considered as an
important compound involved in retrograde signaling.
Chloroplastic retrograde regulations are, to date,
the best studied retrograde signaling pathways in
plants. The mitochondrial retrograde regulations are
well known in fungi and animals whereas in plants
they remain elusive. However, it has been evidenced
that changes in mitochondrial functions, especially the
dysfunction of mitochondrial electron transport chain,
trigger altered nuclear gene expression. These genes
encode proteins, e.g. alternative oxidase, alternative
NAD(P)H dehydrogenase, indispensible for the recovery
of mitochondrial function, and enzymes involved in
maintaining the prooxidant/antioxidant homeostasis,
namely catalase (CAT), superoxide dismutase (SOD)
and glutathione transferase (GST) [55]. Recently, three
functional targets of mitochondrial retrograde signaling,
i.e. protein synthesis, photosynthesis light reactions
and plant-pathogen interactions, have been identified
based on a comprehensive transcriptional analysis [76].
Although the identity of signals in the mitochondrial
retrograde signaling pathway remains largely unknown,
these results further support the concept of the role
of mitochondria in retrograde signaling in plants and
chloroplast-mitochondria cooperation in this pathway.
In addition, it has been shown that ABI4 transcription
factor is a target of retrograde signaling from
mitochondria and chloroplasts [77].
5. Conclusions
Plants can sense and respond to changes in the
environment. To acclimate to the current environmental
conditions they activate a diverse set of responses
including changes in gene expression and regulations
at the metabolic and physiological levels involving redox
signaling. ROS and ROS-antioxidant interactions are
sources of different types of redox signals which regulate
acclimation processes. In plants, chloroplasts and
mitochondria are the main intracellular ROS producers
and generators of redox signals. Recent studies have
revealed the central role of chloroplasts as a source
and target of redox regulation. A lot of chloroplastic
metabolites, both antioxidants and components of the
photosynthetic electron transport chain, were defined
as important sensors creating the cellular redox status.
NADPH, AA, GSH, PQ and Fd form a specific redox
system that, together with antioxidant enzymes and
thiol/disulfide exchange systems, e.g. Trxs, Grxs and
Prxs, regulates gene expression and allows a plant
to respond adequately to fluctuating environmental
conditions. Signals from both mitochondria and
chloroplasts cooperate in the regulation of nuclear gene
expression. The information transfer from organelles
to the nucleus that affects nuclear gene expression
is defined as retrograde signaling. Although our
understanding of the chloroplast-mitochondrion-nucleus
cross-talk is far from complete, substantial evidence
indicates that this signaling pathway is crucial for plant
response to the changing environment.
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