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Research
Cite this article: Tikkanen M, Gollan PJ,
Mekala NR, Isojärvi J, Aro E-M. 2014 Lightharvesting mutants show differential gene
expression upon shift to high light as a
consequence of photosynthetic redox and
reactive oxygen species metabolism. Phil.
Trans. R. Soc. B 369: 20130229.
http://dx.doi.org/10.1098/rstb.2013.0229
One contribution of 20 to a Theme Issue
‘Changing the light environment: chloroplast
signalling and response mechanisms’.
Subject Areas:
plant science, molecular biology, biophysics,
bioinformatics
Keywords:
retrograde signalling, light-harvesting,
reactive oxygen species, phosphorylation,
thermal dissipation
Author for correspondence:
Eva-Mari Aro
e-mail: [email protected]
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rstb.2013.0229 or
via http://rstb.royalsocietypublishing.org.
Light-harvesting mutants show
differential gene expression upon shift
to high light as a consequence of
photosynthetic redox and reactive
oxygen species metabolism
Mikko Tikkanen, Peter J. Gollan, Nageswara Rao Mekala, Janne Isojärvi
and Eva-Mari Aro
Molecular Plant Biology, Department of Biochemistry, University of Turku, 20014 Turku, Finland
The amount of light energy that is harvested and directed to the photosynthetic machinery is regulated in order to control the production of
reactive oxygen species (ROS) in leaf tissues. ROS have important roles as
signalling factors that instigate and mediate a range of cellular responses,
suggesting that the mechanisms regulating light-harvesting and photosynthetic energy transduction also affect cell signalling. In this study, we
exposed wild-type (WT) Arabidopsis and mutants impaired in the regulation
of photosynthetic light-harvesting (stn7, tap38 and npq4) to transient high
light (HL) stress in order to study the role of these mechanisms for up- and
downregulation of gene expression under HL stress. The mutants, all of
which have disturbed regulation of excitation energy transfer and distribution,
responded to transient HL treatment with surprising similarity to the WT in
terms of general ‘abiotic stress-regulated’ genes associated with hydrogen peroxide and 12-oxo-phytodienoic acid signalling. However, we identified
distinct expression profiles in each genotype with respect to induction of singlet oxygen and jasmonic acid-dependent responses. The results of this study
suggest that the control of excitation energy transfer interacts with hormonal
regulation. Furthermore, the photosynthetic pigment–protein complexes
appear to operate as receptors that sense the energetic balance between the
photosynthetic light reactions and downstream metabolism.
1. Introduction
The fluency of photosynthetic electron transfer reactions (i.e. the redox state of
the electron transfer chain; ETC) can affect the level of reactive oxygen species
(ROS) produced both by excited chlorophylls and by reduction of molecular
oxygen. Blockage in electron transfer slows the photochemical quenching of
excited chlorophylls, potentially leading to production of singlet oxygen
(1O2). Additionally, inhibition of the free flow of electrons to acceptors (i.e. to
stromal redox enzymes, NADPþ or carbon dioxide) or saturation of electron
acceptors can lead to reduction of molecular oxygen, resulting in the production
of superoxide and hydrogen peroxide (H2O2; for reviews, see [1,2]).
Under excess light, energy conversion efficiency is downregulated in
order to keep photosynthesis within the capacity of energy-consuming metabolism and to minimize the production of ROS. This regulation is based on
two factors; the redox state of the ETC and the proton gradient across the thylakoid membrane (DpH). The distribution of energy from light-harvesting
complex II (LHCII) between photosystem I (PSI) and photosystem II (PSII) is
regulated by reversible phosphorylation of several LHCII proteins by STN7
kinase [3,4] and TAP38/PPH1 phosphatase [5,6], according to the redox state
of the ETC (for reviews, see [7,8]). This distribution mechanism is coupled
& 2014 The Author(s) Published by the Royal Society. All rights reserved.
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relative clustering
tap38
npq4
hormone
regulation
gene
transcription
plant
defence
Figure 1. Functional clustering of HL-responsive genes in WT and redox mutants, showing relative enrichment of genes involved in, or related to, abiotic stress,
hormone regulation, gene transcription and plant defence, according to database annotations.
with the DpH-dependent regulation of light-harvesting
efficiency (for review, see [9]), which relies on the PSBS
protein for rapid thermal dissipation of excess excitation
energy [10].
Interaction between LHCII phosphorylation and thermal
dissipation can be disturbed by knocking out the genes
encoding the responsible proteins [11]. The psbs mutant overexcites photosystems under high light (HL), leading to a high
reduction state of the plastoquinone (PQ) pool [12,13]. The
lack of the PSBS protein also increases the lifetime of excited
chlorophylls in the light-harvesting system [14], increasing the
production of triplet chlorophylls that are able to react with molecular oxygen and produce 1O2. Lack of PSBS also amplifies and
disturbs the light intensity-dependent changes in thylakoid
protein phosphorylation [15], which may be associated with a
breach of the LHCII system [11] and generation of unconnected
and unquenched LHCII fractions in the psbs mutant under HL.
In the tap38/pph1 mutant, PSI excitation is favoured and thus the
PQ pool remains more oxidized than in the wild-type (WT)
[5,6], as opposed to the highly reduced ETC in npq4. Excitation
imbalance in tap38 is emphasized under HL where LHCII is not
dephosphorylated as it is in the WT. Similar to the situation in
npq4, hyperphosphorylation of thylakoid proteins in tap38
under HL leads to the breakdown of PSII–LHCII complexes
and severe problems in the transfer of sufficient excitation
energy to PSII (NR Mekala, M Tikkanen, E-M Aro 2013, unpublished data). It is likely that this abrogates pigment–protein
complex connectivity and enhances the production of 1O2.
The stn7 mutant is missing LHCII phosphorylation that is
required for efficient energy transfer from LHCII to PSI under
low light [3,4], but the lack of PSI excitation is compensated in
stn7 by an increase in the amount of PSI [13,16]. In HL, stn7
plants have similar light-harvesting properties to those in the
WT due to induction of thermal dissipation of excess excitation
energy (qE) in increased light intensity [11], which occurs concomitantly with dephosphorylation of LHCII proteins in the
WT [17]. In stn7, 1O2 production is likely to be greater than in
the WT only under low and moderate lights when the energy
transfer from LHCII to PSI is limited. Additionally, the PQ
pool in stn7 is reduced under low and moderate lights [3,16],
but is more highly oxidized than in the WT upon shift to HL
owing to the higher relative amount of PSI centres [13,16].
The energetic state of the photosynthetic light reactions is
connected to the regulation of gene expression (for review,
[18,19]) through a system known as ‘retrograde signalling’,
which regulates the expression of genes involved in photosynthesis (for reviews, see [7,20 –24]), plant development
and stress responses (for reviews, see [2,25 –27]). The redox
states of the PQ pool [2,28,29] and PSI electron acceptors
play a crucial role in retrograde signalling [30], but the
impact of the mechanisms that regulate photosynthetic light
reactions, as well as the identities of the signalling molecules,
have remained largely unresolved.
In this study, we investigated the effect of light intensity
and the importance of photosynthetic regulation mechanisms
on chloroplast ROS signalling through analysis of gene
expression in Arabidopsis thaliana WT and the stn7, tap38/pph1
and npq4 mutants under HL. Our analysis showed strong
similarity between genotypes in the primary abiotic stress
response, which appeared to be due to similar H2O2-initiated
and 12-oxo-phytodienoic acid (OPDA)-mediated signalling
networks [31], while 1O2-initiated and jasmonic acid (JA)mediated signalling differed between the genotypes [32],
particularly in stn7. Our results demonstrate that strict regulation of photosynthetic light-harvesting and energy
distribution to the photosystems according to light intensity
maintains the hormonal landscape favourable for proper
abiotic stress responses. This is an important factor to be
taken into account when designing crop plants with enhanced
photosynthetic performance.
2. Results
(a) Transcription response to high light in wild-type
and the stn7, tap38/pph1 and npq4 mutants
We prepared microarrays from WT Arabidopsis thaliana and
from the stn7, tap38/pph1 and npq4 mutants that were shifted
from growth light (GL) to HL for 1 h, and analysed the genes
with greater than twofold expression change following the
change in light intensity. The major transcription response
of mutant plants was generally similar to that of the WT,
with around half of the differentially expressed genes appearing in all genotypes. Functional clustering of the 500 most
responsive genes (figure 1) revealed that the largest clusters
in all genotypes comprised genes responding to abiotic
stress, predominantly heat shock proteins (HSPs), which
Phil. Trans. R. Soc. B 369: 20130229
abiotic
stress
2
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WT
stn7
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3
(b)
OPDA
MeJA
H2O2
–5
log(2)-ratio
5
Figure 2. HL-induced expression of 45 genes involved in jasmonate signalling in WT and redox mutants. (a) Shows the expression changes in each genotype in our
arrays after shift from GL to HL, and the comparative levels of expression in each mutant compared to the WT in both GL and HL, with genes expressed more highly
in the mutant shown in red, and more highly in WT shown in green. (b) Shows the expression changes in the same 45 jasmonate-related genes in response to
treatment with methyl jasmonate (MeJA; AT-00110), 12-oxo-phytodienoic acid (OPDA; AT-00293) or hydrogen peroxide (H2O2; AT-00185) accessed from the public
array database.
have a major role in protecting protein stability under stress
conditions (reviewed in [33]). Notably, the mitochondrial and
chloroplastic HSP20 families were enriched in all genotypes,
suggesting a particular need for protein protection within
these organelles under HL. The heat shock transcription factors
(HSFs) that instigate cellular stress response at the transcription
level were likewise strongly upregulated in all genotypes. The
cytosolic H2O2 scavenger ‘ascorbate peroxidase 2’ (APX2) was
one of the most strongly upregulated genes in all genotypes,
and it was expressed at considerably higher levels in tap38
and npq4 compared with the WT and stn7, while another cytosolic isoform, APX1, was upregulated only in npq4. Notably,
chloroplastic H2O2 scavengers such as thylakoid (tAPX) and
stromal APX (sAPX) [34] were not upregulated under HL.
‘Alternative oxidase 1a’ (AOX1a) was strongly upregulated in
all genotypes under HL treatment, while the tap38 mutant
also showed significant increase in expression of AOX1b and
AOX1d that was absent from other genotypes.
(b) Light-harvesting complex II energy capture is
connected to hormone signalling
Functional clustering revealed that genes known to be regulated
by JA, ethylene and auxin were enriched among the differentially expressed genes in the WT and npq4, with the response
being generally stronger in npq4 than the WT. On the other
hand, these hormone-responsive gene groups were much smaller among the HL-responsive genes in tap38 and were largely
missing from stn7, suggesting only small or no change in hormone-regulated gene expression in stn7 and tap38 upon shift
to HL. The majority of the hormone-induced genes responding to HL in the WT were transcription factors or pathogenresponse factors, which explains the weak transcription- and
defence-related clustering of HL-responsive genes seen in stn7
(figure 1). These results highlighted a connection between the
regulation of light-harvesting and the induction and/or propagation of hormone signalling pathways in Arabidopsis. To
explore this further, we analysed the expression profiles of
genes that responded to HL in our arrays from light-harvesting
regulation mutants and WT plants under other experimental
conditions available in GENEVESTIGATOR. The similarity in overall
transcription response among all genotypes was reiterated
using this approach (see electronic supplementary material,
figure S1). However, the total number of genes upregulated
by JA treatment in published arrays was clearly lower in our
stn7 arrays, demonstrating that the transcription response in
stn7 under HL may be more independent of JA than in WT
and the other photosynthetic mutants. Furthermore, many
genes that are upregulated in the conditional fluorescence ( flu)
mutant [35] did not respond to HL in stn7 in our arrays,
suggesting a decrease in the capacity of 1O2 signalling, which
triggers JA-mediated stress response [36].
In order to clarify this apparent deficiency in 1O2 and
JA signalling in light-harvesting regulatory mutants, we
focused on HL-responsive genes in the WT that are described
as ‘hormone-responsive’ in TAIR database annotation. A
clear difference in the expression of JA-responsive genes was
identified in all mutants compared with the WT, and this
difference was especially pronounced in stn7. We found 45
highly responsive JA-related genes in WT, of which only 11
had significant fold change in stn7 upon transfer to HL
(figure 2). Most prominently COI1, which is a central component of many JA-dependent development, stress and
pathogen signalling pathways [37], was not upregulated upon
HL exposure in stn7, although 4.5–5.5-fold changes in COI1
expression occurred in WT and the other mutants. Additionally,
the enzymes DAD1, LOX2 and AOC2, which are involved in
early stages of JA biosynthesis in the chloroplast [38], showed
around twofold upregulation in the WT in response to HL,
but none of these were upregulated in stn7. Furthermore, several
‘jasmonate ZIM-domain’ (JAZ) proteins that are involved
in downstream JA signalling were upregulated in the WT
under HL, but not in stn7, and upregulation of JA-responsive
transcription factors including MYB75 and MYB113, which
Phil. Trans. R. Soc. B 369: 20130229
WT HL/GL
stn7 HL/GL
stn7/WT (GL)
stn7/WT (HL)
tap38 HL/GL
tap38/WT (GL)
tap38/WT (HL)
npq4 HL/GL
npq4/WT (GL)
npq4/WT (HL)
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high light
H2O2
MeJA
OPDA
–2.0
–1.5
down-regulated
–1.0
log(2)-ratio
0.5
–0.5
0
1.0
1.5
2.0
2.5
up-regulated
Figure 3. Expression of HSP genes that were uniformly upregulated in all genotypes in our arrays (as shown in electronic supplementary material, figure S2), in
response to hydrogen peroxide (H2O2; AT-00185), methyl jasmonate (MeJA; AT-00251) and 12-oxo-phytodienoic acid (OPDA; AT-00293) treatments in the public array
database. Expression profiles show overlap between H2O2- and OPDA-induced expression of HSPs, and independence from jasmonate induction.
upregulate anthocyanin production under HL stress [39], was
also missing from stn7.
Considering the impairment of excitation energy transfer
that exists in stn7 under conditions where thermal dissipation
is not induced (i.e. under low and moderate light intensities,
with the latter corresponding to our GL conditions), it is
likely that 1O2 production in stn7 is already high in GL conditions, and upon transfer to HL the capacity for a further
increase in 1O2 may be impaired. In light of the above
results, we suspected that JA signalling pathways in stn7 may
be upregulated under GL, and so we compared the expression
levels of JA-responsive genes in the mutants with those in WT
under both GL and HL conditions. As expected, we found that
many genes were indeed higher than WT levels in stn7 under
GL, and lower under HL, including COI1, LOX2, DAD1 and
several JAZ proteins (figure 2). These results show that JA
signalling of gene expression is overactive in stn7 under
low light conditions, but is not upregulated to WT levels
under light stress, further linking JA pathways with regulation
of light energy-harvesting.
In contrast to the unique profiles of JA-responsive genes in
each of the mutants, the expression of abiotic stress-responsive
genes was similar in all genotypes. This was clear in the strong
upregulation of 26 HSP genes in the WT under HL, which were
expressed at approximately the same levels in all genotypes (see
electronic supplementary material, figure S2). GENEVESTIGATOR
analysis showed that these genes are largely unresponsive to
JA treatment in other microarray experiments, while a subset
is strongly upregulated by OPDA. OPDA sensitivity within
the group of HL-responsive genes clearly overlaps with H2O2
response (figure 3), suggesting that H2O2-induced response to
HL proceeds via OPDA signalling pathways, independently
of JA signalling.
3. Discussion
(a) The role of light in reactive oxygen
species signalling
ROS produced as by-products of energy metabolism, through
chlorophyll excitation (1O2) and reduction of molecular
oxygen (H2O2), can lead to oxidative damage of biomolecules,
but on the other hand both types of ROS play crucial roles in
regulation of cellular stress responses. ROS also play important
roles in regulating growth and development [2], especially
during ageing and senescence [40]. Light-driven production of
ROS can take place only when enough light energy is available.
Under limiting light, the ROS required for signalling responses
against biotic stress are produced by catabolic metabolism
outside the chloroplast [1,41]. Mutants with disturbed coordination between light-dependent and light-independent abiotic
stress signalling have growth, defence and senescence phenotypes that are highly reactive to light intensity [42–44]. Under
natural light conditions, light intensity fluctuates constantly in
time scales from fractions of a second to diurnal and seasonal
variations, and ROS signals emanating from chloroplasts must
be tightly coordinated in order to prevent signalling imbalance
that could encumber growth and lead to premature hypersensitive reactions, senescence, or indeed to uncontrolled growth at
the expense of stress tolerance. As the major source of ROS in
the light, the chloroplast must therefore not only regulate its
own production of ROS, but also send a constant feed of information to the wider cell in order to maintain ROS homeostasis
in the context of the entire cell.
(b) Induction of cell-wide H2O2 response in wild-type
and stn7, tap38 and npq4 mutants
All genotypes tested here showed strong and uniform upregulation of abiotic stress-responsive genes. This suggests that
the H2O2 signalling system is unaffected by impairment
in the ability to regulate light-harvesting and the redox state
of the ETC. A major portion of H2O2 in the plant cell is produced in the chloroplast as a result of reduction of the PSI
acceptor side [45,46]. Strong induction of genes that are
responsive to H2O2 signals upon increase in light intensity
indicates that the source of H2O2 may be the photosynthetic
machinery itself. The induction of cytosolic APX scavengers
was previously found to be induced by ETC energy imbalance
under sudden exposure to HL [47], although expression of
APX1, APX2 and other heat shock factors is independent of
chloroplast-derived H2O2 [34]. This suggests that HL-induced
H2O2, which generated a comparable stress response in all of
our genotypes, evolves from non-chloroplastic sources. Furthermore, strong upregulation of AOX and HSP20 genes
suggests increases in H2O2 in the mitochondria under HL.
Phil. Trans. R. Soc. B 369: 20130229
–2.5
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AT1G52560
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LHCII antennae are the primary receivers and distributors of
light energy, which is transferred from LHCII to PSII and PSI
according to the energetic state of the photosynthetic machinery. Energy transfer from LHCII is fluent when the relative
rate of incoming light energy is balanced with the energyusing reactions. In the fluent state, chlorophyll excitation is
relaxed quickly and the probability of producing 1O2 is low,
but any imbalance interrupts the transfer fluency and increases
the level of 1O2. This system provides a perfect sensor for monitoring the ratio of light energy absorbed to light energy used,
which is the ultimate indicator of the energetic state of the
photosynthetic machinery. A constant flow of information
about the chloroplast energetic state is vital for regulating
growth and development and is especially important for
the H2O2-driven defence responses that depend on the coordinated cooperation between H2O2 produced in different
cellular compartments [44,48,49]. The connection between the
energetic state of the chloroplast and other processes relies
on retrograde signalling to coordinate plant growth, development and stress responses with the capacity for energy
production [20,22,50].
Our results clearly demonstrate that the capacity to regulate
light-harvesting is important for proper hormone signalling
pathways. In stn7, the upregulated JA response under GL can
be attributed to excess 1O2 produced owing to under-excitation
of PSI, and subsequent over-reduction of the ETC and slower
quenching of excited LCHII, while rapid oxidation of the
ETC during HL-induced thermal dissipation by abnormally
large amounts of PSI undermine the capacity for 1O2-induced
JA response in HL. In the npq4 mutant, the response of JAinduced genes was generally stronger than in the WT upon
shift to HL, with several genes clearly upregulated to a greater
degree than in the WT. This agrees with the concept that greater
levels of 1O2 are produced in npq4 owing to damaged, detached
and unquenched LHCII in HL.
There is a wealth of data demonstrating that retrograde
signalling is influenced by the redox states of the PQ pool
[28,29] and PSI acceptors [30], as well as the metabolic state
of PSI electron acceptors in the stroma [2,51] and the intermediates of chlorophyll biosynthesis [35,52,53]. These
factors therefore exert control over the function of the
4. Material and methods
(a) Plant growth
WT Arabidopsis (Arabidopsis thaliana) ecotype Columbia-0 and stn7,
tap38/pph1 and npq4 mutant plants were grown for six weeks in a
phytotron at 238C, relative humidity 60%, 8-h photoperiod under
constant white GL of 130 mmol photons m22 s21. Plants used for
HL samples were shifted to 1000 mmol photons m22 s21 for 1 h.
(b) RNA isolation and microarray preparation
RNA samples contained pooled RNA from three plants of each
genotype under either GL or HL using a Plant RNA Isolation
Mini Kit (Agilent Technologies) according to the manufacturer’s
instructions. RNA concentration and quality were verified spectrophotometrically. Three replicate RNA samples were prepared from
separate plants for each genotype and each light condition. Microarrays were prepared from each RNA sample (i.e. triplicate arrays
for each genotype under each light condition). A total of 100 ng of
RNA was amplified and Cy3-labelled using the Agilent Low Input
Quick Amp Labeling Kit (Agilent Technologies) and hybridized to
Arabidopsis 4 44K chips (Agilent Technologies). Microarrays
were prepared and analysed by the Finnish Microarray and
Sequencing Centre (Turku, Finland).
(c) Bioinformatics methods
Pre-processing of microarrays was performed using the ‘normexp’
background correction method to avoid negative or zero corrected
intensities, followed by between-array normalization using the
quantile method to make all array distributions identical. Control
probes were filtered and then within-array replicate spots were
replaced with their average. Pair-wise comparisons between
groups were conducted using the Linear Models for Microarray
Data (Limma) package from Bioconductor (http://www.
5
Phil. Trans. R. Soc. B 369: 20130229
(c) Regulation of light-harvesting complex II energy
transfer is crucial in retrograde signalling
photosynthetic machinery as a part of the green cell. Here,
we show that dynamic regulation of energy harnessed and
transferred by the LHCII system, through phosphorylation
of LHCII proteins and protonation of PSBS, is crucial for
the proper signalling responses and synchronization of
photosynthesis with cellular metabolism. We propose that
the ROS produced by the photosynthetic machinery is
balanced with that from other cellular compartments by oxylipin signalling (see also [25,27]). As depicted in figure 4, our
results provide evidence that JA signalling is initiated in the
chloroplast by 1O2 according to the fluency of excitation
energy transfer to PSII and PSI from the light-harvesting
system. By contrast, OPDA signalling appears to be controlled by the redox state of the ETC, most probably the PSI
acceptors, and may therefore depend on production of
H2O2. It is conceivable that interaction between these two
ROS-initiated oxylipin-mediated signalling pathways, and
the interaction between cellular ROS and oxylipin networks,
synchronize the photosynthetic machinery with systemic
hormonal regulation of plant growth, development and
defence responses (see figure 4). Further studies are required
to integrate the signalling mechanism proposed here with
previously established retrograde signalling networks, and
to better understand the role of the chloroplast in the wider
networks that regulate plant growth, development and
defence. It is also important to recognize the robust interactions between light-harvesting, hormonal signalling and
stress responses when designing plants with enhanced
photosynthetic performance.
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The reason for a cell-wide stress response to HL is unclear, and
the messenger(s) used to communicate HL stress to the wider
cell remain(s) unidentified; however, our work shows that
H2O2 signalling is functional under HL in all genotypes. Notably, the redox state of the PSI electron acceptors and the
capacity of PSI to produce H2O2 should be quite similar
between the WT and mutants, because the main regulator of
PSI reducing power under HL is photosynthetic control of
Cyt b6f, which should function similarly in all genotypes.
This would corroborate the instigation of HL-induced H2O2
response at the PSI acceptor side. In contrast to PSI, the
redox state of PSII and the PQ pool differs between the genotypes. The excitation energy transfer to PSII is impaired in the
npq4 mutant, leading to a highly reduced PQ pool under HL
and a resistance against electron transfer from PSII, while in
tap38 and stn7 the PQ pool is oxidized. This suggests that
the H2O2 response under HL is independent of the redox
state of the PQ pool.
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(a)
(b)
low/moderate light
P
‘biotic stress response’
‘abiotic stress response’
PSII
JA signalling
WT
Acc
Acc
P P
P P
P
LHCII
PSI
LHCII
PSII
PSI
PQ
PQ
OPDA signalling
Cyt
b6f
Cyt
b6f
PC
PC
?
stn7
?
H2O2 signal
Acc
P
redox signal
PSII
Acc
Acc
P P
PSI
PSII
LHCII
PSI
PSI
PQ
Cyt
b6f
PC
PC
PSII
PQ
Acc
Acc
P
Chl* = excited chlorophyll
PC = plastocyanin pool
PQ = plastoquinone pool
Acc = PSI electron acceptors
tap38
tap38
Cyt
b6f
PC
PSII
P
PSI
PSII
PQ
Cyt
b6f
LHCII
Cyt
b6f
PSII
npq4
Acc
LHCII
PSI
PSII
PQ
LHCII
PQ
Cyt
b6f
PC
P P
1O
2
P P
P P
P P
Cyt
b6f
PSII
PC
Acc
P
PSI
PQ
PC
npq4
P
P P
P P P P P P
P P
LHCII
1O
2
Chl*
Chl*
Chl*
Chl*
PSI
PSI
PSI
stn7
2
PSI
PSII
PQ
PC
P = phosphogroup
?
Figure 4. Hypothetical scheme describing: (a) ROS-mediated interaction of the light-harvesting and electron transfer machinery with hormone-responsive gene
expression: (b) Abnormal production of ROS in various light-harvesting mutants under normal GL (stn7) and under HL (tap38 and npq4) conditions as described
in the text. Fluent excitation energy transfer in the LHCII system minimizes the production of 1O2 from excited chlorophylls (WT). This is dependent on control over
the interactions between LHCII with other LHC molecules, PSII and PSI (through LHCII phosphorylation), and on relaxation of excited chlorophylls via thermal
dissipation of excitation energy (through PSBS protonation). The redox state of PSI depends on the availability of light energy and on regulation of light-harvesting
efficiency and linear electron transfer (reviewed in [54]). Based on results of this study and available literature, we propose that: (i) the redox state of the PSI
acceptor side controls H2O2 production that triggers the abiotic stress pathway proceeding through OPDA signalling; (ii) increased production of 1O2 from excited
chlorophylls upregulates JA synthesis and promotes biotic stress responses. Under increases in light intensity, these two pathways are upregulated in parallel and
they must be balanced by light-harvesting regulation to ensure proper acclimation to biotic and abiotic stresses that overlap in nature. Proper function of H2O2- and
OPDA-based signalling is not dependent on the regulation of excitation energy transfer. By contrast, proper 1O2- and JA-dependent signalling is dependent on fluent
and properly regulated excitation energy transfer and distribution, provided by the STN7 kinase, TAP38/PPH1 phosphatase and the PSBS protein.
bioconductor.org/). The false discovery rate of differentially
expressed genes for treatment/control and between-treatment comparisons was based on the Benjamini and Hochberg (BH) method.
Genes with a score below an adjusted p-value threshold of 0.05
that showed a minimum of twofold change in expression between
conditions were selected as differentially expressed genes. Gene
annotations were taken from the Arabidopsis Information Resource
(TAIR; http://www.arabidopsis.org/). Functional clustering was
performed using the DAVID Bioinformatics Database (http://
david.abcc.ncifcrf.gov/home.jsp). Gene expression in published
microarrays was analysed in GENEVESTIGATOR [13,55]. Heatmaps
were created in GENEVESTIGATOR or using MATRIX2PNG mapping
software (http://www.chibi.ubc.ca/matrix2png/).
Funding statement. This research was supported by the Academy of
Finland (Project 118637 to E.-M.A, Project 260094 to M.T.).
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