Ascorbate peroxidase 1 plays a key role in the response of Arabidopsis thaliana to stress combination

Enhancing crop resilience to combined abiotic and biotic stress through the dissection of physiological and molecular crosstalk

Plants growing in their natural habitats are often challenged simultaneously by multiple stress factors, both abiotic and biotic. Research has so far been limited to responses to individual stresses, and understanding of adaptation to combinatorial stress is limited, but indicative of non-additive interactions. Omics data analysis and functional characterization of individual genes has revealed a convergence of signaling pathways for abiotic and biotic stress adaptation. Taking into account that most data originate from imposition of individual stress factors, this review summarizes these findings in a physiological context, following the pathogenesis timeline and highlighting potential differential interactions occurring between abiotic and biotic stress signaling across the different cellular compartments and at the whole plant level. Potential effects of abiotic stress on resistance components such as extracellular receptor proteins, R-genes and systemic acquired resistance will be elaborated, as well as crosstalk at the levels of hormone, reactive oxygen species, and redox signaling. Breeding targets and strategies are proposed focusing on either manipulation and deployment of individual common regulators such as transcription factors or pyramiding of non- (negatively) interacting components such as R-genes with abiotic stress resistance genes. We propose that dissection of broad spectrum stress tolerance conferred by priming chemicals may provide an insight on stress cross regulation and additional candidate genes for improving crop performance under combined stress. Validation of the proposed strategies in lab and field experiments is a first step toward the goal of achieving tolerance to combinatorial stress in crops.

Cytosolic APX knockdown rice plants sustain photosynthesis by regulation of protein expression related to photochemistry, Calvin cycle and photorespiration

The biochemical mechanisms underlying the involvement of cytosolic ascorbate peroxidases (cAPXs) in photosynthesis are still unknown. In this study, rice plants doubly silenced in these genes (APX1/2) were exposed to moderate light (ML) and high light (HL) to assess the role of cAPXs in photosynthetic efficiency. APX1/2 mutants that were exposed to ML overexpressed seven and five proteins involved in photochemical activity and photorespiration, respectively. These plants also increased the pheophytin and chlorophyll levels, but the amount of five proteins that are important for Calvin cycle did not change. These responses in mutants were associated with Rubisco carboxylation rate, photosystem II (PSII) activity and potential photosynthesis, which were similar to non-transformed plants. The upregulation of photochemical proteins may be part of a compensatory mechanism for APX1/2 deficiency but apparently the finer-control for photosynthesis efficiency is dependent on Calvin cycle proteins. Conversely, under HL the mutants employed a different strategy, triggering downregulation of proteins related to photochemical activity, Calvin cycle and decreasing the levels of photosynthetic pigments. These changes were associated to strong impairment in PSII activity and Rubisco carboxylation. The upregulation of some photorespiratory proteins was maintained under that stressful condition and this response may have contributed to photoprotection in rice plants deficient in cAPXs. The data reveal that the two cAPXs are not essential for photosynthesis in rice or, alternatively, the deficient plants are able to trigger compensatory mechanisms to photosynthetic acclimation under ML and HL conditions. These mechanisms involve differential regulation in protein expression related to photochemistry, Calvin cycle and photorespiration.

Cryptochrome 1 regulates growth and development in Brassica through alteration in the expression of genes involved in light, phytohormone and stress signalling

The blue light photoreceptors cryptochromes are ubiquitous in higher plants and are vital for regulating plant growth and development. In spite of being involved in controlling agronomically important traits like plant height and flowering time, cryptochromes have not been extensively characterized from agriculturally important crops. Here we show that overexpression of CRY1 from Brassica napus (BnCRY1), an oilseed crop, results in short-statured Brassica transgenics, likely to be less prone to wind and water lodging. The overexpression of BnCRY1 accentuates the inhibition of cell elongation in hypocotyls of transgenic seedlings. The analysis of hypocotyl growth inhibition and anthocyanin accumulation responses in BnCRY1 overexpressors substantiates that regulation of seedling photomorphogenesis by cry1 is dependent on light intensity. This study highlights that the photoactivated cry1 acts through coordinated induction and suppression of specific downstream genes involved in phytohormone synthesis or signalling, and those involved in cell wall modification, during de-etiolation of Brassica seedlings. The microarray-based transcriptome profiling also suggests that the overexpression of BnCRY1 alters abiotic/biotic stress signalling pathways; the transgenic seedlings were apparently oversensitive to abscisic acid (ABA) and mannitol.

Differential activity of autochthonous bacteria in controlling drought stress in native Lavandula and Salvia plants species under drought conditions in natural arid soil

The effectiveness of autochthonous plant growth-promoting rhizobacteria was studied in Lavandula dentata and Salvia officinalis growing in a natural arid Mediterranean soil under drought conditions. These bacteria identified as Bacillus megaterium (Bm), Enterobacter sp. (E), Bacillus thuringiensis (Bt), and Bacillus sp. (Bsp). Each bacteria has different potential to meliorate water limitation and alleviating drought stress in these two plant species. B. thuringiensis promoted growth and drought avoidance in Lavandula by increasing K content, by depressing stomatal conductance, and it controlled shoot proline accumulation. This bacterial effect on increasing drought tolerance was related to the decrease of glutathione reductase (GR) and ascorbate peroxidase (APX) that resulted sensitive indexes of lower cellular oxidative damage involved in the adaptative drought response in B. thuringiensis-inoculated Lavandula plants. In contrast, in Salvia, having intrinsic lower shoot/root ratio, higher stomatal conductance and lower APX and GR activities than Lavandula, the bacterial effects on nutritional, physiological and antioxidant enzymatic systems were lower. The benefit of bacteria depended on intrinsic stress tolerance of plant involved. Lavadula demonstrated a greater benefit than Salvia to control drought stress when inoculated with B. thuringiensis. The bacterial drought tolerance assessed as survival, proline, and indolacetic acid production showed the potential of this bacteria to help plants to grow under drought conditions. B. thuringiensis may be used for Lavandula plant establishment in arid environments. Particular characteristic of the plant species as low shoot/root ratio and high stomatal conductance are important factors controlling the bacterial effectiveness improving nutritional, physiological, and metabolic plant activities.

Alleviation of salt stress by enterobacter sp. EJ01 in tomato and Arabidopsis is accompanied by up-regulation of conserved salinity responsive factors in plants

Microbiota in the niches of the rhizosphere zones can affect plant growth and responses to environmental stress conditions via mutualistic interactions with host plants. Specifically, some beneficial bacteria, collectively referred to as Plant Growth Promoting Rhizobacteria (PGPRs), increase plant biomass and innate immunity potential. Here, we report that Enterobacter sp. EJ01, a bacterium isolated from sea china pink (Dianthus japonicus thunb) in reclaimed land of Gyehwa-do in Korea, improved the vegetative growth and alleviated salt stress in tomato and Arabidopsis. EJ01 was capable of producing 1-aminocy-clopropane-1-carboxylate (ACC) deaminase and also exhibited indole-3-acetic acid (IAA) production. The isolate EJ01 conferred increases in fresh weight, dry weight, and plant height of tomato and Arabidopsis under both normal and high salinity conditions. At the molecular level, short-term treatment with EJ01 increased the expression of salt stress responsive genes such as DREB2b, RD29A, RD29B, and RAB18 in Arabidopsis. The expression of proline biosynthetic genes (i.e. P5CS1 and P5CS2) and of genes related to priming processes (i.e. MPK3 and MPK6) were also up-regulated. In addition, reactive oxygen species scavenging activities were enhanced in tomatoes treated with EJ01 in stressed conditions. GFP-tagged EJ01 displayed colonization in the rhizosphere and endosphere in the roots of Arabidopsis. In conclusion, the newly isolated Enterobacter sp. EJ01 is a likely PGPR and alleviates salt stress in host plants through multiple mechanisms, including the rapid up-regulation of conserved plant salt stress responsive signaling pathways.

Dual regulation of cytosolic ascorbate peroxidase (APX) by tyrosine nitration and S-nitrosylation

Post-translational modifications (PTMs) mediated by nitric oxide (NO)-derived molecules have become a new area of research, as they can modulate the function of target proteins. Proteomic data have shown that ascorbate peroxidase (APX) is one of the potential targets of PTMs mediated by NO-derived molecules. Using recombinant pea cytosolic APX, the impact of peroxynitrite (ONOO-) and S-nitrosoglutathione (GSNO), which are known to mediate protein nitration and S-nitrosylation processes, respectively, was analysed. While peroxynitrite inhibits APX activity, GSNO enhances its enzymatic activity. Mass spectrometric analysis of the nitrated APX enabled the determination that Tyr5 and Tyr235 were exclusively nitrated to 3-nitrotyrosine by peroxynitrite. Residue Cys32 was identified by the biotin switch method as S-nitrosylated. The location of these residues on the structure of pea APX reveals that Tyr235 is found at the bottom of the pocket where the haem group is enclosed, whereas Cys32 is at the ascorbate binding site. Pea plants grown under saline (150 mM NaCl) stress showed an enhancement of both APX activity and S-nitrosylated APX, as well as an increase of H2O2, NO, and S-nitrosothiol (SNO) content that can justify the induction of the APX activity. The results provide new insight into the molecular mechanism of the regulation of APX which can be both inactivated by irreversible nitration and activated by reversible S-nitrosylation.

Cloning and functional analysis of a novel ascorbate peroxidase (APX) gene from Anthurium andraeanum

An 888-bp full-length ascorbate peroxidase (APX) complementary DNA (cDNA) gene was cloned from Anthurium andraeanum, and designated as AnAPX. It contains a 110-bp 5'-noncoding region, a 28-bp 3'-noncoding region, and a 750-bp open reading frame (ORF). This protein is hydrophilic with an aliphatic index of 81.64 and its structure consisting of α-helixes, β-turns, and random coils. The AnAPX protein showed 93%, 87%, 87%, 87%, and 86% similarities to the APX homologs from Zantedeschia aethiopica, Vitis pseudoreticulata, Gossypium hirsutum, Elaeis guineensis, and Zea mays, respectively. AnAPX gene transcript was measured non-significantly in roots, stems, leaves, spathes, and spadices by real-time polymerase chain reaction (RT-PCR) analysis. Interestingly, this gene expression was remarkably up-regulated in response to a cold stress under 6 °C, implying that AnAPX might play an important role in A. andraeanum tolerance to cold stress. To confirm this function we overexpressed AnAPX in tobacco plants by transformation with an AnAPX expression construct driven by CaMV 35S promoter. The transformed tobacco seedlings under 4 °C showed less electrolyte leakage (EL) and malondialdehyde (MDA) content than the control. The content of MDA was correlated with chilling tolerance in these transgenic plants. These results show that AnAPX can prevent the chilling challenged plant from cell membrane damage and ultimately enhance the plant cold tolerance.

Tc-cAPX, a cytosolic ascorbate peroxidase of Theobroma cacao L. engaged in the interaction with Moniliophthora perniciosa, the causing agent of witches' broom disease

The level of hydrogen peroxide (H2O2) in plants signalizes the induction of several genes, including that of ascorbate peroxidase (APX-EC 1.11.1.11). APX isoenzymes play a central role in the elimination of intracellular H2O2 and contribute to plant responses to diverse stresses. During the infection process in Theobroma cacao by Moniliophthora perniciosa oxidative stress is generated and the APX action recruited from the plant. The present work aimed to characterize the T. cacao APX involved in the molecular interaction of T. cacao-M. perniciosa. The peroxidase activity was analyzed in protein extracts from cocoa plants infected by M. perniciosa and showed the induction of peroxidases like APX in resistant cocoa plants. The cytosolic protein of T. cacao (GenBank: ABR68691.2) was phylogenetically analyzed in relation to other peroxidases from the cocoa genome and eight genes encoding APX proteins with conserved domains were also analyzed. The cDNA from cytosolic APX was cloned in pET28a and the recombinant protein expressed and purified (rTc-cAPX). The secondary structure of the protein was analyzed by Circular Dichroism (CD) displaying high proportion of α-helices when folded. The enzymatic assay shows stable activity using ascorbate and guaiacol as an electron donor for H2O2 reduction. The pH 7.5 is the optimum for enzyme activity. Chromatographic analysis suggests that rTc-cAPX is a homodimer in solution. Results indicate that the rTc-cAPX is correctly folded, stable and biochemically active. The purified rTc-cAPX presented biotechnological potential and is adequate for future structural and functional studies.

Antioxidant Defenses in Plants with Attention to Prunus and Citrus spp

This short review briefly introduces the formation of reactive oxygen species (ROS) as by-products of oxidation/reduction (redox) reactions, and the ways in which the antioxidant defense machinery is involved directly or indirectly in ROS scavenging. Major antioxidants, both enzymatic and non enzymatic, that protect higher plant cells from oxidative stress damage are described. Biochemical and molecular features of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) are discussed because they play crucial roles in scavenging ROS in the different cell compartments and in response to stress conditions. Among the non enzymatic defenses, particular attention is paid to ascorbic acid, glutathione, flavonoids, carotenoids, and tocopherols. The operation of ROS scavenging systems during the seasonal cycle and specific developmental events, such as fruit ripening and senescence, are discussed in relation to the intense ROS formation during these processes that impact fruit quality. Particular attention is paid to Prunus and Citrus species because of the nutritional and antioxidant properties contained in these commonly consumed fruits.

Effect of long-term salinity on cellular antioxidants, compatible solute and fatty acid profile of Sweet Annie (Artemisia annua L.)

Impact of long-term salinity and subsequent oxidative stress was studied on cellular antioxidants, proline accumulation and lipid profile of Artemisia annua L. (Sweet Annie or Qinghao) which yields artemisinin (Qinghaosu), effective against cerebral malaria-causing strains of Plasmodium falciparum. Under salinity (0.0-160 mM NaCl), in A. annua, proline accumulation, contents of ascorbate and glutathione and activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR) and catalase (CAT) increased, but the contents of reduced forms of glutathione (GSH) and ascorbate declined. The fatty-acid profiling revealed a major salinity-induced shift towards long-chain and mono-saturated fatty acids. Myristic acid (14:0), palmitoleic acid (16:1), linoleic acid (18:2) and erucic acid (22:1) increased by 141%, 186%, 34% and 908%, respectively, in comparison with the control. Contents of oleic acid (18:1), linolenic acid (18:3), arachidonic acid (22:0) and lignoceric acid (24:0) decreased by 50%, 17%, 44% and 78%, respectively. Thus, in A. annua, salinity declines ascorbate and GSH contents. However, increased levels of proline and total glutathione (GSH+GSSG), and activities of antioxidant enzymes might provide a certain level of tolerance. Modification in fatty-acid composition might be a membrane adaptation to long-term salinity and oxidative stress.

Conserved versatile master regulators in signalling pathways in response to stress in plants

From the first land plants to the complex gymnosperms and angiosperms of today, environmental conditions have forced plants to develop molecular strategies to surpass natural obstacles to growth and proliferation, and these genetic gains have been transmitted to the following generations. In this long natural process, novel and elaborate mechanisms have evolved to enable plants to cope with environmental limitations. Elements in many signalling cascades enable plants to sense different, multiple and simultaneous ambient cues. A group of versatile master regulators of gene expression control plant responses to stressing conditions. For crop breeding purposes, the task is to determine how to activate these key regulators to enable accurate and optimal reactions to common stresses. In this review, we discuss how plants sense biotic and abiotic stresses, how and which master regulators are implied in the responses to these stresses, their evolution in the life kingdoms, and the domains in these proteins that interact with other factors to lead to a proper and efficient plant response.

The importance of cardiolipin synthase for mitochondrial ultrastructure, respiratory function, plant development, and stress responses in Arabidopsis

Cardiolipin (CL) is the signature phospholipid of the mitochondrial inner membrane. In animals and yeast (Saccharomyces cerevisiae), CL depletion affects the stability of respiratory supercomplexes and is thus crucial to the energy metabolism of obligate aerobes. In eukaryotes, the last step of CL synthesis is catalyzed by CARDIOLIPIN SYNTHASE (CLS), encoded by a single-copy gene. Here, we characterize a cls mutant in Arabidopsis thaliana, which is devoid of CL. In contrast to yeast cls, where development is little affected, Arabidopsis cls seedlings are slow developing under short-day conditions in vitro and die if they are transferred to long-day (LD) conditions. However, when transferred to soil under LD conditions under low light, cls plants can reach the flowering stage, but they are not fertile. The cls mitochondria display abnormal ultrastructure and reduced content of respiratory complex I/complex III supercomplexes. The marked accumulation of tricarboxylic acid cycle derivatives and amino acids demonstrates mitochondrial dysfunction. Mitochondrial and chloroplastic antioxidant transcripts are overexpressed in cls leaves, and cls protoplasts are more sensitive to programmed cell death effectors, UV light, and heat shock. Our results show that CLS is crucial for correct mitochondrial function and development in Arabidopsis under both optimal and stress conditions.

Coping with abiotic stress: proteome changes for crop improvement

Plant breeders need new and more precise tools to accelerate breeding programs that address the increasing needs for food, feed, energy and raw materials, while facing a changing environment in which high salinity and drought have major impacts on crop losses worldwide. This review covers the achievements and bottlenecks in the identification and validation of proteins with relevance in abiotic stress tolerance, also mentioning the unexpected consequences of the stress in allergen expression. While addressing the key pathways regulating abiotic stress plant adaptation, comprehensive data is presented on the proteins confirmed as relevant to confer tolerance. Promising candidates still to be confirmed are also highlighted, as well as the specific protein families and protein modifications for which detection and characterization is still a challenge. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Short-term chromium-stress-induced alterations in the maize leaf proteome

Soil contamination by chromium (Cr) has become an increasing problem worldwide as a result of extensive industrial activities. Chromium, especially hexavalent Cr, impairs the growth and productivity of plants. Although it has been proposed that plants could modify their metabolism to adapt to Cr stress by reprogramming the expression of genes, especially those related to the antioxidant system, damage response, and electron transport chain, evidence at the protein expression level is lacking. To better understand the precise mechanisms underlying Cr phytoxicity and the plant response to Cr exposure, the time-course of changes in the protein expression profile induced by short-term hexavalent Cr exposure (1, 6 and 24 h) were analyzed in maize leaves. Among the over 1200 protein spots detected reproducibly by two-dimensional electrophoresis (2-DE), 60 were found to be differentially accumulated during Cr stress treatment. Of the Cr-regulated proteins, 58 were identified using tandem mass spectrometry (MS/MS). The Cr-regulated proteins identified were mainly involved in ROS detoxification and defense responses (26%), photosynthesis and chloroplast organization (22%), post-transcriptional processing of mRNA and rRNA (12%), protein synthesis and folding (10%), the DNA damage response (5%), and the cytoskeleton (3%). The possible involvement of these Cr stress-responsive proteins in Cr phytoxicity and the plant response to Cr exposure in maize is discussed, taking into consideration the information available from other plant models. Our results provide preliminary evidence that will facilitate understanding the molecular mechanisms underlying Cr toxicity in maize.

The influence of metal stress on the availability and redox state of ascorbate, and possible interference with its cellular functions

Worldwide, metals have been distributed to excessive levels in the environment due to industrial and agricultural activities. Plants growing on soils contaminated with excess levels of metals experience a disturbance of the cellular redox balance, which leads to an augmentation of reactive oxygen species (ROS). Even though the increased ROS levels can cause cellular damage, controlled levels play an important role in modulating signaling networks that control physiological processes and stress responses. Plants control ROS levels using their antioxidative defense system both under non-stress conditions, as well as under stress conditions such as exposure to excess metals. Ascorbate (AsA) is a well-known and important component of the plant's antioxidative system. As primary antioxidant, it can reduce ROS directly and indirectly via ascorbate peroxidase in the ascorbate-glutathione cycle. Furthermore, AsA fulfills an essential role in physiological processes, some of which are disturbed by excess metals. In this review, known direct effects of excess metals on AsA biosynthesis and functioning will be discussed, as well as the possible interference of metals with the role of AsA in physiological and biochemical processes.

Differential Protein Expression in Response to Abiotic Stress in Two Potato Species: Solanum commersonii Dun and Solanum tuberosum L

Better knowledge on responses to dehydration stress could help to improve the existing cryopreservation protocols for potato, since plant tissues processed for cryopreservation are often submitted to similar in vitro stress conditions. Cryopreservation (the best method of conservation for vegetatively propagated plants) of potato still needs to be standardized to make it available and to conserve the wide diversity of this crop. In the present work, the response to osmotic stress and chilling temperature was investigated in two potato species, Solanum tuberosum and its relative, frost-tolerant S. commersonii. After 14 days of exposure, different growth parameters, such as shoot length and number of leaves, were measured. Furthermore, differentially abundant proteins were identified after performing 2-fluorescence difference gel electrophoresis (2-DIGE) experiments, and soluble carbohydrates were analyzed by High Performance Anion Exchange Chromatography with Pulsed Amperometric Detection (HPAEC-PAD). The results show different responses in both species depending on the stress treatment. Focusing on the differences in growth parameters during the treatments, Solanum commersonii seems to be more affected than S. tuberosum cv. Désirée. At the molecular level, there are some differences and similarities between the two potato species studied that are dependent on the type of stressor.

Gene knockout study reveals that cytosolic ascorbate peroxidase 2(OsAPX2) plays a critical role in growth and reproduction in rice under drought, salt and cold stresses

Plant ascorbate peroxidases (APXs), enzymes catalyzing the dismutation of H₂O₂ into H₂O and O₂, play an important role in reactive oxygen species homeostasis in plants. The rice genome has eight OsAPXs, but their physiological functions remain to be determined. In this report, we studied the function of OsAPX2 gene using a T-DNA knockout mutant under the treatment of drought, salt and cold stresses. The Osapx2 knockout mutant was isolated by a genetic screening of a rice T-DNA insertion library under 20% PEG-2000 treatment. Loss of function in OsAPX2 affected the growth and development of rice seedlings, resulting in semi-dwarf seedlings, yellow-green leaves, leaf lesion mimic and seed sterility. OsAPX2 expression was developmental- and spatial-regulated, and was induced by drought, salt, and cold stresses. Osapx2 mutants had lower APX activity and were sensitive to abiotic stresses; overexpression of OsAPX2 increased APX activity and enhanced stress tolerance. H₂O₂ and MDA levels were high in Osapx2 mutants but low in OsAPX2-OX transgenic lines relative to wild-type plants after stress treatments. Taken together, the cytosolic ascorbate peroxidase OsAPX2 plays an important role in rice growth and development by protecting the seedlings from abiotic stresses through scavenging reactive oxygen species.

Molecular mechanisms of the plant heat stress response

High temperature has become a global concern, which seriously affects the growth and production of plants, particularly crops. Thus, the molecular mechanism of the heat stress response and breeding of heat-tolerant plants is necessary to protect food production and ensure crop safety. This review elaborates on the response networks of heat stress in plants, including the Hsf and Hsp response pathways, the response of ROS and the network of the hormones. In addition, the production of heat stress response elements during particular physiological periods of the plant is described. We also discuss the existing problems and future prospects concerning the molecular mechanisms of the heat stress response in plants.

A Novel Peroxidase CanPOD Gene of Pepper Is Involved in Defense Responses to Phytophtora capsici Infection as well as Abiotic Stress Tolerance

Peroxidases are involved in many plant processes including plant defense responses to biotic and abiotic stresses. We isolated a novel peroxidase gene CanPOD from leaves of pepper cultivar A3. The full-length gene has a 1353-bp cDNA sequence and contains an open reading frame (ORF) of 975-bp, which encodes a putative polypeptide of 324 amino acids with a theoretical protein size of 34.93 kDa. CanPOD showed diverse expression levels in different tissues of pepper plants. To evaluate the role of CanPOD in plant stress responses, the expression patterns of CanPOD were examined using Real-Time RT-PCR. The results indicated that CanPOD was significantly induced by Phytophtora capsici. Moreover, CanPOD was also up-regulated in leaves after salt and drought stress treatments. In addition, CanPOD expression was strongly induced by signaling hormones salicylic acid (SA). In contrast, CanPOD was not highly expressed after treatment with cold. Meanwhile, in order to further assess the role of gene CanPOD in defense response to P. capsici attack, we performed a loss-of-function experiment using the virus-induced gene silencing (VIGS) technique in pepper plants. In comparison to the control plant, the expression levels of CanPOD were obviously decreased in CanPOD-silenced pepper plants. Furthermore, we analyzed the effect of P. capsici on detached-leaves and found that the CanPOD-silenced plant leaves were highly susceptible to P. capsici infection. Taken together, our results suggested that CanPOD is involved in defense responses to P. capsici infection as well as abiotic stresses in pepper plants.

Gene expression analysis in response to low and high temperature and oxidative stresses in rice: combination of stresses evokes different transcriptional changes as against stresses applied individually

Transcript expression profiles of rice seedlings were analyzed in response to (a) prior exposure with oxidative stress followed by heat or cold stress and (b) simultaneous exposure to oxidative stress along with heat stress or cold stress. The numbers of genes differentially regulated during stress combination of cold and oxidative stress as well as heat and oxidative stress treatments were higher when compared with the number of genes differentially regulated in response to individual stress conditions. A large number of transcript changes were noted unique to the stress combination mode as compared with when individual stresses were applied. Specific differences in the transcript expression profiles of OsHsf and OsClp gene family members were noted during combination of stresses as against individual stresses. For instance, OsHsf26 induction was specific to stress combinations, while OsHsfA2a, OsHsfA2f, and OsHsfA3 transcript levels were additively affected during combination of stresses. Unique promoter models and transcription factor binding sites (i.e. P$KNOX3_01, P$OSBZ8_Q6) were noted in the promoters of differentially regulated genes during combination of stresses. It is proposed that stress combinations represent a novel state of abiotic stresses for rice seedlings that might involve a different type of molecular response.

Plant responses to stresses: Role of ascorbate peroxidase in the antioxidant protection

When plants are exposed to stressful environmental conditions, the production of Reactive Oxygen Species (ROS) increases and can cause significant damage to the cells. Antioxidant defenses, which can detoxify ROS, are present in plants. A major hydrogen peroxide detoxifying system in plant cells is the ascorbate-glutathione cycle, in which, ascorbate peroxidase (APX) enzymes play a key role catalyzing the conversion of H(2)O(2) into H(2)O, using ascorbate as a specific electron donor. Different APX isoforms are present in distinct subcellular compartments, such as chloroplasts, mitochondria, peroxisome, and cytosol. The expression of APX genes is regulated in response to biotic and abiotic stresses as well as during plant development. The APX responses are directly involved in the protection of plant cells against adverse environmental conditions. Furthermore, mutant plants APX genes showed alterations in growth, physiology and antioxidant metabolism revealing those enzymes involvement in the normal plant development.

Transcriptome profiling of genes induced by salicylic acid and methyl jasmonate in Polygonum minus

The importance of plant secondary metabolites for both mankind and the plant itself has long been established. However, despite extensive research on plant secondary metabolites, plant secondary metabolism and its regulation still remained poorly characterized. In this present study, cDNA-amplified fragment length polymorphism (cDNA-AFLP) transcript profiling was applied to generate the expression profiles of Polygonum minus in response to salicylic acid (SA) and methyl jasmonate (MeJA) elicitations. This study reveals two different sets of genes induced by SA and MeJA, respectively where stress-related genes were proved to lead to the expression of genes involved in plant secondary metabolite biosynthetic pathways. A total of 98 transcript-derived fragments (TDFs) were up-regulated, including 46 from SA-treated and 52 from MeJA-treated samples. The cDNA-AFLP transcripts generated using 64 different Mse1/Taq1 primer combinations showed that treatments with SA and MeJA induced genes mostly involved in scavenging reactive oxygen species, including zeaxanthin epoxidase, cytosolic ascorbate peroxidase 1 and peroxidase. Of these stress-related genes, 15 % of other annotated TDFs are involved mainly in secondary metabolic processes where among these, two genes encoding (+)-delta cadinene synthase and cinnamoyl-CoA reductase were highlighted.

Screening for changes in leaf and cambial proteome of Populus tremula × P. alba under different heat constraints

Young poplar plants were exposed to different heat regimes, a rapid heat constraint at 42°C (heat shock HS) alone or preceded by a stepwise increase in temperature (heat gradient HG). Proteomics analyses were carried out on both leaf and cambial tissues. The responses of both tissues were compared and linked to morphological and physiological observations. Both heat treatments negatively affected the photosynthetic rate while increasing the stomatal conductance. In the leaf, the HS impacted some photosynthetic proteins, and particularly induced an increase in abundance of proteins of the oxygen evolving complexes. On the other hand, the HG reduced carbohydrate metabolism and induced mainly an increase in germin-like proteins. In the cambial zone, the HS caused a decrease in sucrose synthase content and in enzymes related to protein synthesis. The main effect of HG was the accumulation of thaumatin-like proteins as well as an increase in the abundance of proteins involved in carbohydrate metabolism. Further, both tissues underwent changes in the content of heat shock proteins, but more importantly, of peroxiredoxins. The results show more sustainable changes in leaf and cambial proteomes in response to HS compared to HG.

Modulation of genes related to specific metabolic pathways in response to cytosolic ascorbate peroxidase knockdown in rice plants

As a central component of the hydrogen peroxide detoxifying system in plant cells, ascorbate peroxidases (APX) play an essential role in the control of intracellular reactive oxygen species (ROS) levels. To characterise the function of cytosolic APX isoforms (OsAPX1 and OsAPX2) in the mechanisms of plant defence, OsAPX1/2 knockdown rice plants were previously obtained. OsAPX1/2 knockdown plants (APx1/2s) exhibited a normal phenotype and development, even though they showed a global reduction of APX activity and increased hydrogen peroxide accumulation. To understand how rice plants compensate for the deficiency of cytosolic APX, expression and proteomic analyses were performed to characterise the global expression pattern of the APx1/2s mutant line compared with non-transformed plants. Our results strongly suggest that deficiencies in cytosolic APX isoforms markedly alter expression of genes associated with several key metabolic pathways, especially of genes involved in photosynthesis and antioxidant defence. These metabolic changes are compensatory because central physiological processes such as photosynthesis and growth were similar to non-transformed rice plants. Our analyses showed modulation of groups of genes and proteins related to specific metabolic pathways. Among the differentially expressed genes, the largest number corresponded to those with catalytic activity. Genes related to oxidative stress, carbohydrate metabolism, photosynthesis and transcription factor-encoding genes were also modulated. These results represent an important step toward understanding of the role played by cytosolic APX isoforms and hydrogen peroxide in the regulation of metabolism by redox modulation in monocots.

Cloning, expression and functional validation of drought inducible ascorbate peroxidase (Ec-apx1) from Eleusine coracana

Eleusine coracana (finger millet) is a stress-hardy but under-utilized cereal crop that possesses an efficient antioxidant defense system. The plant is capable of enduring long durations of water deficit stress. Experiments were conducted to clone a potent stress responsive isoform of ascorbate peroxidase and validate its role under drought stress. Reverse transcriptase PCR was used to obtain the partial cDNA of apx1 gene, from a meticulously screened drought tolerant genotype of E. coracana (PR202). Using RACE strategy, the full length apx1 cDNA was cloned and sequenced. The cDNA length of the E. coracana apx1 (Ec-apx1) gene is 1,047 bp with a 750 bp ORF, encoding a 250 amino acid protein having a molecular weight of 28.5 kDa. The identity of the amino acid sequence, deduced from the cDNA, with the APX family homologs was about 74-97 %. The full-length apx1 ORF was sub-cloned in a prokaryotic expression vector pET23b. The recombinant fusion protein, Ec-apx1, had high expression level in BL21 strain of E. coli and exhibited APX enzyme activity. The structure-function relationship of the protein was deduced by modelling a three-dimensional structure of Ec-apx1, on the basis of comparative homology using SWISS-MODEL. Real time PCR analysis of Ec-apx1 expression at mRNA level showed that the transcript increased under drought stress, with maximum levels attained 5-days after imposition of stress. Our results suggest that Ec-apx1 has a distinct pattern of expression and plays a pivotal role in drought stress tolerance. Therefore, the cloned isoform of ascorbate peroxidase can be used for developing stress tolerant genotypes of important crops, through transgenic approach.

ROS signaling as common element in low oxygen and heat stresses

The activation of the oxidative metabolism in plants under low oxygen conditions has prompted controversial views. The presence of a ROS component in the transcriptome in response to low oxygen has been observed and an overlap with heat stress has been proved. It has been also demonstrated that ROS are produced during both anoxia and heat, but the site of their production remain contentious. Membrane NADPH oxidase and mitochondrial electron transport flow have been indicated as possible ROS generation systems. Both anoxia and heat have been shown to induce the transcription of Heat Shock Factors (HSFs) and Heat Shock Proteins (HSPs), among which HSFA2 and some of its targets. HSFA2 over-expressing plant has been shown to be more tolerant to anoxia, while the knockout hsfa2 lose the capability of wild type plants to cross-acclimate to anoxia through mild heat pre-treatment. The production of ROS seems to be an integral part of the anoxia and heat response, where HSFs likely play a central role in activating the HSP pathway. This mechanism is suggested to result in enhanced plant tolerance to both anoxia and heat.

A novel stress-induced sugarcane gene confers tolerance to drought, salt and oxidative stress in transgenic tobacco plants

BACKGROUND

Drought is a major abiotic stress that affects crop productivity worldwide. Sugarcane can withstand periods of water scarcity during the final stage of culm maturation, during which sucrose accumulation occurs. Meanwhile, prolonged periods of drought can cause severe plant losses.

METHODOLOGY/PRINCIPAL FINDINGS

In a previous study, we evaluated the transcriptome of drought-stressed plants to better understand sugarcane responses to drought. Among the up-regulated genes was Scdr1 (sugarcane drought-responsive 1). The aim of the research reported here was to characterize this gene. Scdr1 encodes a putative protein containing 248 amino acids with a large number of proline (19%) and cysteine (13%) residues. Phylogenetic analysis showed that ScDR1is in a clade with homologs from other monocotyledonous plants, separate from those of dicotyledonous plants. The expression of Scdr1 in different varieties of sugarcane plants has not shown a clear association with drought tolerance.

CONCLUSIONS/SIGNIFICANCE

The overexpression of Scdr1 in transgenic tobacco plants increased their tolerance to drought, salinity and oxidative stress, as demonstrated by increased photosynthesis, water content, biomass, germination rate, chlorophyll content and reduced accumulation of ROS. Physiological parameters, such as transpiration rate (E), net photosynthesis (A), stomatal conductance (gs) and internal leaf CO(2) concentration, were less affected by abiotic stresses in transgenic Scdr1 plants compared with wild-type plants. Overall, our results indicated that Scdr1 conferred tolerance to multiple abiotic stresses, highlighting the potential of this gene for biotechnological applications.

Proteomic analysis revealed nitrogen-mediated metabolic, developmental, and hormonal regulation of maize (Zea mays L.) ear growth

Optimal nitrogen (N) supply is critical for achieving high grain yield of maize. It is well established that N deficiency significantly reduces grain yield and N oversupply reduces N use efficiency without significant yield increase. However, the underlying proteomic mechanism remains poorly understood. The present field study showed that N deficiency significantly reduced ear size and dry matter accumulation in the cob and grain, directly resulting in a significant decrease in grain yield. The N content, biomass accumulation, and proteomic variations were further analysed in young ears at the silking stage under different N regimes. N deficiency significantly reduced N content and biomass accumulation in young ears of maize plants. Proteomic analysis identified 47 proteins with significant differential accumulation in young ears under different N treatments. Eighteen proteins also responded to other abiotic and biotic stresses, suggesting that N nutritional imbalance triggered a general stress response. Importantly, 24 proteins are involved in regulation of hormonal metabolism and functions, ear development, and C/N metabolism in young ears, indicating profound impacts of N nutrition on ear growth and grain yield at the proteomic level.

The use of metabolomics to dissect plant responses to abiotic stresses

Plant metabolism is perturbed by various abiotic stresses. As such the metabolic network of plants must be reconfigured under stress conditions in order to allow both the maintenance of metabolic homeostasis and the production of compounds that ameliorate the stress. The recent development and adoption of metabolomics and systems biology approaches enable us not only to gain a comprehensive overview, but also a detailed analysis of crucial components of the plant metabolic response to abiotic stresses. In this review we introduce the analytical methods used for plant metabolomics and describe their use in studies related to the metabolic response to water, temperature, light, nutrient limitation, ion and oxidative stresses. Both similarity and specificity of the metabolic responses against diverse abiotic stress are evaluated using data available in the literature. Classically discussed stress compounds such as proline, γ-amino butyrate and polyamines are reviewed, and the widespread importance of branched chain amino acid metabolism under stress condition is discussed. Finally, where possible, mechanistic insights into metabolic regulatory processes are discussed.

Methylglyoxal inhibition of cytosolic ascorbate peroxidase from Nicotiana tabacum

Methylglyoxal (MG) is one of the aldehydes accumulated in plants under environmental stress. Cytosolic ascorbate peroxidase (cAPX) plays a key role in the protection of cells from oxidative damage by scavenging reactive oxygen species in higher plants. A cDNA encoding cAPX, named NtcAPX, was isolated from Nicotiana tabacum. We characterized recombinant NtcAPX (rNtcAPX) as a fusion protein with glutathione S-transferase to investigate the effects of MG on APX. NtcAPX consists of 250 amino acids and has a deduced molecular mass of 27.5 kDa. The rNtcAPX showed a higher APX activity. MG treatments resulted in a reduction of APX activity and modifications of amino groups in rNtcAPX with increasing K(m) for ascorbate. On the contrary, neither NaCl nor cadmium reduced the activity of APX. The present study suggests that inhibition of APX is in part due to the modification of amino acids by MG.

Plasma membrane cyclic nucleotide gated calcium channels control land plant thermal sensing and acquired thermotolerance

Typically at dawn on a hot summer day, land plants need precise molecular thermometers to sense harmless increments in the ambient temperature to induce a timely heat shock response (HSR) and accumulate protective heat shock proteins in anticipation of harmful temperatures at mid-day. Here, we found that the cyclic nucleotide gated calcium channel (CNGC) CNGCb gene from Physcomitrella patens and its Arabidopsis thaliana ortholog CNGC2, encode a component of cyclic nucleotide gated Ca(2+) channels that act as the primary thermosensors of land plant cells. Disruption of CNGCb or CNGC2 produced a hyper-thermosensitive phenotype, giving rise to an HSR and acquired thermotolerance at significantly milder heat-priming treatments than in wild-type plants. In an aequorin-expressing moss, CNGCb loss-of-function caused a hyper-thermoresponsive Ca(2+) influx and altered Ca(2+) signaling. Patch clamp recordings on moss protoplasts showed the presence of three distinct thermoresponsive Ca(2+) channels in wild-type cells. Deletion of CNGCb led to a total absence of one and increased the open probability of the remaining two thermoresponsive Ca(2+) channels. Thus, CNGC2 and CNGCb are expected to form heteromeric Ca(2+) channels with other related CNGCs. These channels in the plasma membrane respond to increments in the ambient temperature by triggering an optimal HSR, leading to the onset of plant acquired thermotolerance.

Drought stress has contrasting effects on antioxidant enzymes activity and phenylpropanoid biosynthesis in Fraxinus ornus leaves: an excess light stress affair?

The experiment was conducted using Fraxinus ornus plants grown outside under full sunlight irradiance, and supplied with 100% (well-watered, WW), 40% (mild drought, MD), or 20% (severe drought, SD) of the daily evapotranspiration demand, with the main objective of exploring the effect of excess light stress on the activity of antioxidant enzymes and phenylpropanoid biosynthesis. Net CO₂ assimilation rate at saturating light and daily assimilated CO₂ were significantly smaller in SD than in WW and MD plants. Xanthophyll-cycle pigments supported nonphotochemical quenching to a significantly greater extent in SD than in MD and WW leaves. As a consequence, the actual efficiency of PSII (Φ(PSII)) was smaller, while the excess excitation-energy in the photosynthetic apparatus was greater in SD than in WW or MD plants. The concentrations of violaxanthin-cycle pigments relative to total chlorophyll (Chl(tot)) exceeded 200 mmol mol⁻¹ Chl(tot) in SD leaves at the end of the experiment. This leads to hypothesize for zeaxanthin a role not only as nonphotochemical quencher, but also as chloroplast antioxidant. Reductions in ascorbate peroxidase and catalase activities, as drought-stress progressed, were paralleled by greater accumulations of esculetin and quercetin 3-O-glycosides, both phenylpropanoids having effective capacity to scavenge H₂O₂. The drought-induced accumulation of esculetin and quercetin 3-O-glycosides in the vacuoles of mesophyll cells is consistent with their putative functions as reducing agents for H₂O₂ in excess light-stressed leaves. Nonetheless, the concentration of H₂O₂ and the lipid peroxidation were significantly greater in SD than in MD and WW leaves. It is speculated that vacuolar phenylpropanoids may constitute a secondary antioxidant system, even on a temporal basis, activated upon the depletion of primary antioxidant defences, and aimed at keeping whole-cell H₂O₂ within a sub-lethal concentration range.

A comparison of hydroponic and soil-based screening methods to identify salt tolerance in the field in barley

Success in breeding crops for yield and other quantitative traits depends on the use of methods to evaluate genotypes accurately under field conditions. Although many screening criteria have been suggested to distinguish between genotypes for their salt tolerance under controlled environmental conditions, there is a need to test these criteria in the field. In this study, the salt tolerance, ion concentrations, and accumulation of compatible solutes of genotypes of barley with a range of putative salt tolerance were investigated using three growing conditions (hydroponics, soil in pots, and natural saline field). Initially, 60 genotypes of barley were screened for their salt tolerance and uptake of Na(+), Cl(-), and K(+) at 150 mM NaCl and, based on this, a subset of 15 genotypes was selected for testing in pots and in the field. Expression of salt tolerance in saline solution culture was not a reliable indicator of the differences in salt tolerance between barley plants that were evident in saline soil-based comparisons. Significant correlations were observed in the rankings of genotypes on the basis of their grain yield production at a moderately saline field site and their relative shoot growth in pots at EC(e) 7.2 [Spearman's rank correlation (rs)=0.79] and EC(e) 15.3 (rs=0.82) and the crucial parameter of leaf Na(+) (rs=0.72) and Cl(-) (rs=0.82) concentrations at EC(e) 7.2 dS m(-1). This work has established screening procedures that correlated well with grain yield at sites with moderate levels of soil salinity. This study also showed that both salt exclusion and osmotic tolerance are involved in salt tolerance and that the relative importance of these traits may differ with the severity of the salt stress. In soil, ion exclusion tended to be more important at low to moderate levels of stress but osmotic stress became more important at higher stress levels. Salt exclusion coupled with a synthesis of organic solutes were shown to be important components of salt tolerance in the tolerant genotypes and further field tests of these plants under stress conditions will help to verify their potential utility in crop-improvement programmes.

The interaction of plant biotic and abiotic stresses: from genes to the field

Plant responses to different stresses are highly complex and involve changes at the transcriptome, cellular, and physiological levels. Recent evidence shows that plants respond to multiple stresses differently from how they do to individual stresses, activating a specific programme of gene expression relating to the exact environmental conditions encountered. Rather than being additive, the presence of an abiotic stress can have the effect of reducing or enhancing susceptibility to a biotic pest or pathogen, and vice versa. This interaction between biotic and abiotic stresses is orchestrated by hormone signalling pathways that may induce or antagonize one another, in particular that of abscisic acid. Specificity in multiple stress responses is further controlled by a range of molecular mechanisms that act together in a complex regulatory network. Transcription factors, kinase cascades, and reactive oxygen species are key components of this cross-talk, as are heat shock factors and small RNAs. This review aims to characterize the interaction between biotic and abiotic stress responses at a molecular level, focusing on regulatory mechanisms important to both pathways. Identifying master regulators that connect both biotic and abiotic stress response pathways is fundamental in providing opportunities for developing broad-spectrum stress-tolerant crop plants.

A LEA gene regulates Cadmium tolerance by mediating physiological responses

In this study, the function of a LEA gene (TaLEA1) from Tamrix androssowii in response to heavy metal stress was characterized. Time-course expression analyses showed that NaCl, ZnCl₂, CuSO₄, and CdCl₂ considerably increased the expression levels of the TaLEA1 gene, thereby suggesting that this gene plays a role in the responses to these test stressors. To analyze the heavy metal stress-tolerance mechanism regulated by TaLEA1, TaLEA1-overexpressing transgenic poplar plants (Populus davidiana Dode × P. bollena Lauche) were generated. Significant differences were not observed between the proline content of the transgenic and wild-type (WT) plants before and after CdCl₂ stress. However, in comparison with the WT plants, the TaLEA1-transformed poplar plants had significantly higher superoxide dismutase (SOD) and peroxidase (POD) activities, and lower malondialdehyde (MDA) levels under CdCl₂ stress. Further, the transgenic plants showed better growth than the WT plants did, indicating that TaLEA1 provides tolerance to cadmium stress. These results suggest that TaLEA1 confers tolerance to cadmium stress by enhancing reactive oxygen species (ROS)-scavenging ability and decreasing lipid peroxidation. Subcellular-localization analysis showed that the TaLEA1 protein was distributed in the cytoplasm and nucleus.

Downregulation of chloroplast RPS1 negatively modulates nuclear heat-responsive expression of HsfA2 and its target genes in Arabidopsis

Heat stress commonly leads to inhibition of photosynthesis in higher plants. The transcriptional induction of heat stress-responsive genes represents the first line of inducible defense against imbalances in cellular homeostasis. Although heat stress transcription factor HsfA2 and its downstream target genes are well studied, the regulatory mechanisms by which HsfA2 is activated in response to heat stress remain elusive. Here, we show that chloroplast ribosomal protein S1 (RPS1) is a heat-responsive protein and functions in protein biosynthesis in chloroplast. Knockdown of RPS1 expression in the rps1 mutant nearly eliminates the heat stress-activated expression of HsfA2 and its target genes, leading to a considerable loss of heat tolerance. We further confirm the relationship existed between the downregulation of RPS1 expression and the loss of heat tolerance by generating RNA interference-transgenic lines of RPS1. Consistent with the notion that the inhibited activation of HsfA2 in response to heat stress in the rps1 mutant causes heat-susceptibility, we further demonstrate that overexpression of HsfA2 with a viral promoter leads to constitutive expressions of its target genes in the rps1 mutant, which is sufficient to reestablish lost heat tolerance and recovers heat-susceptible thylakoid stability to wild-type levels. Our findings reveal a heat-responsive retrograde pathway in which chloroplast translation capacity is a critical factor in heat-responsive activation of HsfA2 and its target genes required for cellular homeostasis under heat stress. Thus, RPS1 is an essential yet previously unknown determinant involved in retrograde activation of heat stress responses in higher plants.

Responses of growth and primary metabolism of water-stressed barley roots to rehydration

Barley seedlings were grown in pots in controlled environment chambers and progressive drought treatments were imposed 11 d after sowing. Soil water content decreased from 92 to 10% following 14 d without watering. Increases of biomass in shoots and roots slowed after 4 and 9 d of water stress, respectively. Thirty barley root metabolites were monitored in this study and 85% were significantly altered by drought. Sucrose, raffinose, glucose, fructose, maltose, malate, asparagine and proline increased and myo-inositol, glycerate, alanine, serine, glycine and glutamate decreased during drought. Primary metabolism was likely involved in various crucial processes during water stress including, osmotic adjustment, nitrogen sequestration and ammonia detoxification. Rates of photosynthesis and stomatal conductance recovered in 2 d and shoot growth commenced the 3rd day after rehydration. Root growth also exhibited a lag after rehydration but this was attributed to high nutrient concentrations during water stress. Malate and proline recovered within 1 d but serine was only partially reversed 6 d after rehydration. Malate, aspartate and raffinose decreased below well-watered, control levels following rehydration. Variation in the magnitude and time necessary for individual compounds to fully recover after rehydration suggested the complexity of metabolic processes initiated by re-watering.

Organ-specific proteomic analysis of drought-stressed soybean seedlings

Changes in protein levels in drought-stressed soybean seedlings were analyzed using a proteomics approach. Three-day-old soybean seedlings were subjected to drought stress or treated with 10% polyethylene glycol (PEG) as osmotic stress. After treatment, the proteins were extracted from the leaf, hypocotyl, and root and separated using two-dimensional polyacrylamide gel electrophoresis. The root was the most drought-responsive organ, with the levels of 32, 13, and 12 proteins changing in response to drought stress, PEG treatment, and both, respectively. In the leaves of PEG-treated and drought-stressed seedlings, metabolism-related proteins increased and energy production- and protein synthesis-related proteins decreased. For 3 proteins present in all organs in drought-stressed plants, mRNA was differentially regulated: heat shock protein 70 and actin isoform B were upregulated, and methionine synthase was downregulated. mRNA expression patterns reflected those of protein levels, suggesting transcriptional regulation of these proteins. Western blot analysis confirmed the increase in ascorbate peroxidase in drought-stressed plants. The downregulation of mRNA and decreased protein levels of methionine synthase in the leaves, hypocotyl, and roots of drought-stressed plants, but not in other treatments, indicated that methionine synthase is a drought response protein. These results also suggest that the decreased methionine synthase in response to drought stress can impair the soybean seedling growth.

Homologous NF-YC2 subunit from Arabidopsis and tobacco is activated by photooxidative stress and induces flowering

The transcription factor NF-Y consists of the three subunits A, B and C, which are encoded in Arabidopsis in large gene families. The multiplicity of the genes implies that NF-Y may act in diverse combinations of each subunit for the transcriptional control. We aimed to assign a function in stress response and plant development to NF-YC subunits by analyzing the expression of NF-Y genes and exploitation of nf-y mutants. Among the subunit family, NF-YC2 showed the strongest inducibility towards oxidative stress, e.g. photodynamic, light, oxidative, heat and drought stress. A tobacco NF-YC homologous gene was found to be inducible by photooxidative stress generated by an accumulation of the tetrapyrrole metabolite, coproporphyrin. Despite the stress induction, an Arabidopsis nf-yc2 mutant and NF-YC2 overexpressors did not show phenotypical differences compared to wild-type seedlings in response to photooxidative stress. This can be explained by the compensatory potential of other members of the NF-YC family. However, NF-YC2 overexpression leads to an early flowering phenotype that is correlated with increased FLOWERING LOCUS T-transcript levels. It is proposed that NF-YC2 functions in floral induction and is a candidate gene among the NF-Y family for the transcriptional activation upon oxidative stress.

Impact of carbon dioxide enrichment on the responses of maize leaf transcripts and metabolites to water stress

Maize (Zea mays) was grown in indoor chambers with ambient (38 Pa) and elevated (70 Pa) CO(2) . Drought treatments were imposed 17 days after sowing by withholding nutrient solution. Decreases of soil water content, leaf water potential, net CO(2) assimilation and stomatal conductance as a result of drought were delayed approximately 2 days by CO(2) enrichment. Concentrations of 28 of 33 leaf metabolites were altered by drought. Soluble carbohydrates, aconitate, shikimate, serine, glycine, proline and eight other amino acids increased, and leaf starch, malate, fumarate, 2-oxoglutarate and seven amino acids decreased with drought. Drought-dependent decreases of nitrate, alanine and aspartate were impacted by limiting nitrogen. Transcript levels of 14 stress-related maize genes responded to drought but this was delayed or modified by CO(2) enrichment. Overall, CO(2) enrichment eliminated many early responses of maize metabolites and transcripts to water stress but was less effective when drought was severe. Four metabolite groupings were identified by clustering analysis. These groupings included compounds that decreased with water stress, compounds involved in osmotic adjustment and aromatic compounds that alleviate oxidative stress. Metabolite changes also supported the suggestion that water stress inhibited C(4) photosynthesis and induced photorespiration.

REDOX regulation of mitochondrial function in plants

Mitochondrial components dynamically change in response to environmental and developmental cues. However, the regulatory pathways that underlie these changes are largely unknown. A global analysis of changes in mitochondrial components at the transcript, protein and metabolite levels was undertaken, to gain a greater insight into how mitochondrial functions are regulated and respond to various internal or external cues. At the transcript level, large-scale changes in groups of genes suggest the presence of co-regulatory mechanisms for these components. Furthermore, the pathways that regulate these changes appear to be integrated into regulatory pathways that alter a variety of functions in cells. However, the changes in transcripts are not always observed at the protein or the metabolite level. This is likely to be due to post-transcriptional levels of regulation and also the fact that in-depth profiles, which have been obtained for transcripts from a variety of studies, are currently not available for proteins and metabolites. Thus, while transcripts for genes give us a picture of what the cells are 'thinking' in relation to mitochondrial components, some of these responses may be lost in translation.

Gene families of maize glutathione-ascorbate redox cycle respond differently to abiotic stresses

The glutathione-ascorbate (GSH-ASC) cycle in plants plays an important role in detoxifying reactive oxygen species. Little is known about how the enzymes and antioxidants in the maize GSH-ASC cycle respond to stress. We clarified the genome positions, exon-intron structures and predicted subcellular locations of the ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) families in maize. ABA treatment increased the transcript levels of most of the APX genes except ZmAPX3 and ZmAPX6, upregulated the transcription of ZmMDAR1 and downregulated the transcriptions of ZmMDAR3 and ZmMDAR4. However, it had little effect on the expressions of the ZmDHAR and ZmGR gene families. ABA treatment increased the activities of only 2 enzymes, ZmAPX and ZmDHAR. The PEG treatment led to similar expression patterns as that of ABA. ZmAPX1.1 and ZmAPX2 exhibited the same expression patterns under PEG treatment conditions. Enzyme activities were not affected by the PEG treatment with the exception of a significant decrease in MDAR activity that was observed after 6h. Compared to the ABA and PEG treatments, the NaCl treatment only slightly affected the transcription of the four gene families but significantly increased the activity of ZmGR. The ABA and PEG treatments elevate the ASC levels and decrease the GSSG level. Our results show that the gene families of the maize GSH-ASC redox cycle respond differently to abiotic stresses and suggest that APX and MDAR may play more important roles in stress tolerance in plants.