Meta-Analysis of Antioxidant Mutants Reveals Common Alarm Signals for Shaping Abiotic Stress-Induced Transcriptome in Plants.

Aims: Reactive oxygen species (ROS) are key regulators of plant growth, development, and stress tolerance. Stress-induced changes in ROS levels trigger multilevel signaling. However, the precise mechanisms by which ROS signals are translated into changes in gene expression remain poorly defined. Focusing on six key antioxidant enzymes, we performed a meta-analysis of transcriptome data available in public databases to analyze ROS-mediated control of nuclear gene expression. Results: An information-guided pipeline was developed, which identified 19 putative transcription factors (TFs), as components in a "common alarm signal cascade" pathway following perception of changes in ROS levels. Crucially, 30%-35% of the abiotic stress transcriptome signatures had binding sites for common alarm signal-transcription factors (CAS-TFs) in their promoter regions. Furthermore, Phloem Early Dof 2 (PEAR2), DNA binding with one finger 5.8 (DOF5.8), and Obf-Binding Protein 3 (OBP3) were identified as top-ranked TFs on the basis of a cumulative DAPseq (DNA-affinity purification sequencing) score on the promoters of selected genes regulating core pathways of salt, drought, heat, and cold stress tolerance. Innovation: This study identifies a set of CAS-TFs that may play a major role in shaping the transcriptome of abiotic stress-induced ROS signaling. Ranking analysis identified PEAR2, DOF5.8, and OBP3 as the top-ranked CAS-TFs that regulated known markers of abiotic stress tolerance. Conclusion: The current findings suggest a major role of ROS in the abiotic stress signaling and also identify a set of TFs that take part in the signaling. Taken together, these findings suggested that the common alarm signal cascade underpins broad-range tolerance against multistress conditions. The identification of associated ROS-responsive CAS-TFs may provide novel targets for crop improvement.

Crosstalk between Brassinosteroid and Redox Signaling Contributes to the Activation of CBF Expression during Cold Responses in Tomato.

Brassinosteroids (BRs) play a critical role in plant responses to stress. However, the interplay of BRs and reactive oxygen species signaling in cold stress responses remains unclear. Here, we demonstrate that a partial loss of function in the BR biosynthesis gene DWARF resulted in lower whilst overexpression of DWARF led to increased levels of C-REPEAT BINDING FACTOR (CBF) transcripts. Exposure to cold stress increased BR synthesis and led to an accumulation of brassinazole-resistant 1 (BZR1), a central component of BR signaling. Mutation of BZR1 compromised the cold- and BR-dependent increases in CBFs and RESPIRATORY BURST OXIDASE HOMOLOG 1(RBOH1) transcripts, as well as preventing hydrogen peroxide (H2O2) accumulation in the apoplast. Cold- and BR-induced BZR1 bound to the promoters of CBF1, CBF3 and RBOH1 and promoted their expression. Significantly, suppression of RBOH1 expression compromised cold- and BR-induced accumulation of BZR1 and related increases in CBF transcripts. Moreover, RBOH1-dependent H2O2 production regulated BZR1 accumulation and the levels of CBF transcripts by influencing glutathione homeostasis. Taken together, these results demonstrate that crosstalk between BZR1 and reactive oxygen species mediates cold- and BR-activated CBF expression, leading to cold tolerance in tomato (Solanum lycopersicum).

Persulfidation-based Modification of Cysteine Desulfhydrase and the NADPH Oxidase RBOHD Controls Guard Cell Abscisic Acid Signaling.

Hydrogen sulfide (H2S) is a gaseous signaling molecule that regulates diverse cellular signaling pathways through persulfidation, which involves the post-translational modification of specific Cys residues to form persulfides. However, the mechanisms that underlie this important redox-based modification remain poorly understood in higher plants. We have, therefore, analyzed how protein persulfidation acts as a specific and reversible signaling mechanism during the abscisic acid (ABA) response in Arabidopsis (Arabidopsis thaliana). Here we show that ABA stimulates the persulfidation of l-CYSTEINE DESULFHYDRASE1, an important endogenous H2S enzyme, at Cys44 and Cys205 in a redox-dependent manner. Moreover, sustainable H2S accumulation drives persulfidation of the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG PROTEIN D (RBOHD) at Cys825 and Cys890, enhancing its ability to produce reactive oxygen species. Physiologically, s-persulfidation-induced RBOHD activity is relevant to ABA-induced stomatal closure. Together, these processes form a negative feedback loop that fine-tunes guard cell redox homeostasis and ABA signaling. These findings not only expand our current knowledge of H2S function in the context of guard cell ABA signaling, but also demonstrate the presence of a rapid signal integration mechanism involving specific and reversible redox-based post-translational modifications that occur in response to changing environmental conditions.

l-cysteine desulfhydrase-related H2 S production is involved in OsSE5-promoted ammonium tolerance in roots of Oryza sativa.

Previous studies revealed that rice heme oxygenase PHOTOPERIOD SENSITIVITY 5 (OsSE5) is involved in the regulation of tolerance to excess ammonium by enhancing antioxidant defence. In this study, the relationship between OsSE5 and hydrogen sulfide (H2 S), a well-known signalling molecule, was investigated. Results showed that NH4 Cl triggered the induction of l-cysteine desulfhydrase (l-DES)-related H2 S production in rice seedling roots. A H2 S donor not only alleviated the excess ammonium-triggered inhibition of root growth but also reduced endogenous ammonium, both of which were aggravated by hypotaurine (HT, a H2 S scavenger) or dl-propargylglycine (PAG, a l-DES inhibitor). Nitrogen metabolism-related enzymes were activated by H2 S, thus resulting in the induction of amino acid synthesis and total nitrogen content. Interestingly, the activity of l-DES, as well as the enzymes involved in nitrogen metabolism, was significantly increased in the OsSE5-overexpression line (35S:OsSE5), whereas it impaired in the OsSE5-knockdown mutant (OsSE5-RNAi). The application of the HT/PAG or H2 S donor could differentially block or rescue NH4 Cl-hyposensitivity or hypersensitivity phenotypes in 35S:OsSE5-1 or OsSE5-RNAi-1 plants, with a concomitant modulation of nitrogen assimilation. Taken together, these results illustrated that H2 S function as an indispensable positive regulator participated in OsSE5-promoted ammonium tolerance, in which nitrogen metabolism was facilitated.

Interactions between 2-Cys peroxiredoxins and ascorbate in autophagosome formation during the heat stress response in Solanum lycopersicum.

2-Cys peroxiredoxins (2-CPs) function in the removal of hydrogen peroxide and lipid peroxides but their precise roles in the induction of autophagy have not been characterized. Here we show that heat stress, which is known to induce oxidative stress, leads to the simultaneous accumulation of transcripts encoding 2-CPs and autophagy proteins, as well as autophagosomes, in tomato (Solanum lycopersicum) plants. Virus-induced gene silencing of the tomato peroxiredoxin genes 2-CP1, 2-CP2, and 2-CP1/2 resulted in an increased sensitivity of tomato plants to heat stress. Silencing 2-CP2 or 2-CP1/2 increased the levels of transcripts associated with ascorbate biosynthesis but had no effect on the glutathione pool in the absence of stress. However, the heat-induced accumulation of transcripts associated with the water-water cycle was compromised by the loss of 2-CP1/2 functions. The transcript levels of autophagy-related genes ATG5 and ATG7 were higher in plants with impaired 2-CP1/2 functions, and the formation of autophagosomes increased, together with an accumulation of oxidized and insoluble proteins. Silencing of ATG5 or ATG7 increased the levels of 2-CP transcripts and protein but decreased heat stress tolerance. These results demonstrate that 2-CPs fulfil a pivotal role in heat stress tolerance in tomato, via interactions with ascorbate-dependent pathways and autophagy.

Unravelling how plants benefit from ROS and NO reactions, while resisting oxidative stress.

BACKGROUND AND AIMS: Reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as nitric oxide (NO), play crucial roles in the signal transduction pathways that regulate plant growth, development and defence responses, providing a nexus of reduction/oxidation (redox) control that impacts on nearly every aspect of plant biology. Here we summarize current knowledge and concepts that lay the foundations of a new vision for ROS/RNS functions ­ particularly through signalling hubs ­ for the next decade. SCOPE: Plants have mastered the art of redox control using ROS and RNS as secondary messengers to regulate a diverse range of protein functions through redox-based, post-translational modifications that act as regulators of molecular master-switches. Much current focus concerns the impact of this regulation on local and systemic signalling pathways, as well as understanding how such reactive molecules can be effectively used in the control of plant growth and stress responses. CONCLUSIONS: The spectre of oxidative stress still overshadows much of our current philosophy and understanding of ROS and RNS functions. While many questions remain to be addressed ­ for example regarding inter-organellar regulation and communication, the control of hypoxia and how ROS/RNS signalling is used in plant cells, not only to trigger acclimation responses but also to create molecular memories of stress ­ it is clear that ROS and RNS function as vital signals of living cells.

High atmospheric carbon dioxide-dependent alleviation of salt stress is linked to RESPIRATORY BURST OXIDASE 1 (RBOH1)-dependent H2O2 production in tomato (Solanum lycopersicum).

Plants acclimate rapidly to stressful environmental conditions. Increasing atmospheric CO2 levels are predicted to influence tolerance to stresses such as soil salinity but the mechanisms are poorly understood. To resolve this issue, tomato (Solanum lycopersicum) plants were grown under ambient (380 µmol mol(-1)) or high (760 µmol mol(-1)) CO2 in the absence or presence of sodium chloride (100mM). The higher atmospheric CO2 level induced the expression of RESPIRATORY BURST OXIDASE 1 (SlRBOH1) and enhanced H2O2 accumulation in the vascular cells of roots, stems, leaf petioles, and the leaf apoplast. Plants grown with higher CO2 levels showed improved salt tolerance, together with decreased leaf transpiration rates and lower sodium concentrations in the xylem sap, vascular tissues, and leaves. Silencing SlRBOH1 abolished high CO2 -induced salt tolerance and increased leaf transpiration rates, as well as enhancing Na(+) accumulation in the plants. The higher atmospheric CO2 level increased the abundance of a subset of transcripts involved in Na(+) homeostasis in the controls but not in the SlRBOH1-silenced plants. It is concluded that high atmospheric CO2 concentrations increase salt stress tolerance in an apoplastic H2O2 dependent manner, by suppressing transpiration and hence Na(+) delivery from the roots to the shoots, leading to decreased leaf Na(+) accumulation.

Redox markers for drought-induced nodule senescence, a process occurring after drought-induced senescence of the lowest leaves in soybean (Glycine max).

BACKGROUND AND AIMS: Water is an increasingly scarce resource that limits crop productivity in many parts of the world, and the frequency and severity of drought are predicted to increase as a result of climate change. Improving tolerance to drought stress is therefore important for maximizing future crop yields. The aim of this study was to compare the effects of drought on soybean (Glycine max) leaves and nodules in order to define phenotypic markers and changes in cellular redox state that characterize the stress response in different organs, and to characterize the relationships between leaf and nodule senescence during drought. METHODS: Leaf and crown nodule metabolite pools were measured together with leaf and soil water contents, and leaf chlorophyll, total protein contents and chlorophyll a fluorescence quenching parameters in nodulated soybeans that were grown under either well-watered conditions or deprived of water for up to 21 d. KEY RESULTS: Ureides, ascorbate, protein, chlorophyll and the ratios of variable chlorophyll a fluorescence (Fv') to maximal chlorophyll a fluorescence (Fm') fell to levels below detection in the oldest leaves after 21 d of drought. While these drought-induced responses were not observed in the youngest leaf ranks, the Fv'/Fm' ratios, pyridine nucleotide levels and the reduction state of the ascorbate pool were lower in all leaf ranks after 21 d of drought. In contrast to leaves, total nodule protein, pyridine nucleotides, ureides, ascorbate and glutathione contents increased as a result of the drought treatment. However, the nodule ascorbate pool was significantly less reduced as a result of drought. Higher levels of transcripts encoding two peroxiredoxins were detected in nodules exposed to drought stress but senescence-associated transcripts and other mRNAs encoding redox-related proteins were similar under both conditions. CONCLUSIONS: While the physiological impact of the drought was perceived throughout the shoot, stress-induced senescence occurred only in the oldest leaf ranks. At this stage, a number of drought-induced changes in nodule metabolites were observed but no metabolite or transcript markers of senescence could be detected. It is concluded that stress-induced senescence in the lowest leaf ranks precedes nodule senescence, suggesting that leaves of low photosynthetic capacity are sacrificed in favour of nodule nitrogen metabolism.