NADPH oxidase in plasma membrane is involved in stomatal closure induced by dehydroascorbate

RtNAC055 promotes drought tolerance via a stomatal closure pathway linked to methyl jasmonate/hydrogen peroxide signaling in Reaumuria trigyna

The stomata regulate CO2 uptake and efficient water usage, thereby promoting drought stress tolerance. NAC proteins (NAM, ATAF1/2, and CUC2) participate in plant reactions following drought stress, but the molecular mechanisms underlying NAC-mediated regulation of stomatal movement are unclear. In this study, a novel NAC gene from Reaumuria trigyna, RtNAC055, was found to enhance drought tolerance via a stomatal closure pathway. It was regulated by RtMYC2 and integrated with jasmonic acid signaling and was predominantly expressed in stomata and root. The suppression of RtNAC055 could improve jasmonic acid and H2O2 production and increase the drought tolerance of transgenic R. trigyna callus. Ectopic expression of RtNAC055 in the Arabidopsis atnac055 mutant rescued its drought-sensitive phenotype by decreasing stomatal aperture. Under drought stress, overexpression of RtNAC055 in poplar promoted ROS (H2O2) accumulation in stomata, which accelerated stomatal closure and maintained a high photosynthetic rate. Drought upregulated the expression of PtRbohD/F, PtP5CS2, and PtDREB1.1, as well as antioxidant enzyme activities in heterologous expression poplars. RtNAC055 promoted H2O2 production in guard cells by directly binding to the promoter of RtRbohE, thus regulating stomatal closure. The stress-related genes RtDREB1.1/P5CS1 were directly regulated by RtNAC055. These results indicate that RtNAC055 regulates stomatal closure by maintaining the balance between the antioxidant system and H2O2 level, reducing the transpiration rate and water loss, and improving photosynthetic efficiency and drought resistance.

Mechanism of Ca2+-mediated NOX modulated in ROS metabolism induced by T-2 toxin in potato tuber

T-2 toxin at low concentrations can induce ROS accumulation and modulate host resistance in plants. NOX plays crucial roles in ROS production and is regulated by Ca²⁺via direct binding to EF-hand motifs. In this study, the effect of EGTA (Ca²⁺ chelating agent) on the expression and enzymatic activity of NOX, as well as the activities and corresponding gene expressions involved in ROS metabolism and cell membrane integrity, were investigated in treated slices. Results indicated that EGTA treatment significantly affected gene expression and activity of NOX, and reduced ROS accumulation and cell membrane integrity and the enzymatic activities and gene expression involved in ROS metabolism when exposed to treatment. The addition of exogenous Ca²⁺ restored the initial relative transcript abundance, ROS accumulation and their activities. Results suggest that Ca²⁺ affected by EGTA plays a crucial role in NOX activity regulation, ultimately affecting ROS metabolism in slices induced by T-2 toxin.

Assessment of Subcellular ROS and NO Metabolism in Higher Plants: Multifunctional Signaling Molecules

Reactive oxygen species (ROS) and nitric oxide (NO) are produced in all aerobic life forms under both physiological and adverse conditions. Unregulated ROS/NO generation causes nitro-oxidative damage, which has a detrimental impact on the function of essential macromolecules. ROS/NO production is also involved in signaling processes as secondary messengers in plant cells under physiological conditions. ROS/NO generation takes place in different subcellular compartments including chloroplasts, mitochondria, peroxisomes, vacuoles, and a diverse range of plant membranes. This compartmentalization has been identified as an additional cellular strategy for regulating these molecules. This assessment of subcellular ROS/NO metabolisms includes the following processes: ROS/NO generation in different plant cell sites; ROS interactions with other signaling molecules, such as mitogen-activated protein kinases (MAPKs), phosphatase, calcium (Ca²⁺), and activator proteins; redox-sensitive genes regulated by the iron-responsive element/iron regulatory protein (IRE-IRP) system and iron regulatory transporter 1(IRT1); and ROS/NO crosstalk during signal transduction. All these processes highlight the complex relationship between ROS and NO metabolism which needs to be evaluated from a broad perspective.

Concentration Does Matter: The Beneficial and Potentially Harmful Effects of Ascorbate in Humans and Plants

SIGNIFICANCE

Ascorbate (Asc) is an essential compound both in animals and plants, mostly due to its reducing properties, thereby playing a role in scavenging reactive oxygen species (ROS) and acting as a cofactor in various enzymatic reactions. Recent Advances: Growing number of evidence shows that excessive Asc accumulation may have negative effects on cellular functions both in humans and plants; inter alia it may negatively affect signaling mechanisms, cellular redox status, and contribute to the production of ROS via the Fenton reaction.

CRITICAL ISSUES

Both plants and humans tightly control cellular Asc levels, possibly via biosynthesis, transport, and degradation, to maintain them in an optimum concentration range, which, among other factors, is essential to minimize the potentially harmful effects of Asc. On the contrary, the Fenton reaction induced by a high-dose Asc treatment in humans enables a potential cancer-selective cell death pathway.

FUTURE DIRECTIONS

The elucidation of Asc induced cancer selective cell death mechanisms may give us a tool to apply Asc in cancer therapy. On the contrary, the regulatory mechanisms controlling cellular Asc levels are also to be considered, for example, when aiming at generating crops with elevated Asc levels.

The plasma membrane NADPH oxidase OsRbohA plays a crucial role in developmental regulation and drought-stress response in rice

Plasma membrane NADPH oxidases are major producers of reactive oxygen species (ROS) in plant cells under normal growth and stress conditions. In the present study the total activity of rice NADPH oxidases and the transcription of OsRbohA, which encodes an Oryza sativa plasma membrane NADPH oxidase, were stimulated by drought. OsRbohA was expressed in all tissues examined throughout development. Its mRNA was upregulated by a number of factors, including heat, drought, salt, oxidative stress and methyl jasmonate treatment. Compared with wild-type (WT), the OsRbohA-knockout mutant osrbohA exhibited upregulated expression of other respiratory burst oxidase homolog genes and multiple abnormal agronomic traits, including reduced biomass, low germination rate and decreased pollen viability and seed fertility. However, OsRbohA-overexpressing transgenic plants showed no differences in these traits compared with WT. Although osrbohA leaves and roots produced more ROS than WT, the mutant had lesser intracellular ROS. In contrast, OsRbohA-overexpressing transgenic plants exhibited higher ROS production at the intracellular level and in tissues. Ablation of OsRbohA impaired the tolerance of plants to various water stresses, whereas its overexpression enhanced the tolerance. In addition, a number of genes related to energy supply, substrate transport, stress response and transcriptional regulation were differentially expressed in osrbohA plants even under normal growth conditions, suggesting that OsRbohA has fundamental and broad functions in rice. These results indicate that OsRbohA-mediated processes are governed by complex signaling pathways that function during the developmental regulation and drought-stress response in rice.

Peroxisomal NADP-isocitrate dehydrogenase is required for Arabidopsis stomatal movement

Peroxisomes are subcellular organelles characterized by a simple morphological structure but have a complex biochemical machinery involved in signaling processes through molecules such as hydrogen peroxide (H2O2) and nitric oxide (NO). Nicotinamide adenine dinucleotide phosphate (NADPH) is an essential component in cell redox homeostasis, and its regeneration is critical for reductive biosynthesis and detoxification pathways. Plants have several NADPH-generating dehydrogenases, with NADP-isocitrate dehydrogenase (NADP-ICDH) being one of these enzymes. Arabidopsis contains three genes that encode for cytosolic, mitochondrial/chloroplastic, and peroxisomal NADP-ICDH isozymes although the specific function of each of these remains largely unknown. Using two T-DNA insertion lines of the peroxisomal NADP-ICDH designated as picdh-1 and picdh-2, the data show that the peroxisomal NADP-ICDH is involved in stomatal movements, suggesting that peroxisomes are a new element in the signaling network of guard cells.

Comprehensive Genomic Analysis and Expression Profiling of the NOX Gene Families under Abiotic Stresses and Hormones in Plants

Plasma membrane NADPH oxidases (NOXs) are key producers of reactive oxygen species under both normal and stress conditions in plants and they form functional subfamilies. Studies of these subfamilies indicated that they show considerable evolutionary selection. We performed a comparative genomic analysis that identified 50 ferric reduction oxidases (FRO) and 77 NOX gene homologs from 20 species representing the eight major plant lineages within the supergroup Plantae: glaucophytes, rhodophytes, chlorophytes, bryophytes, lycophytes, gymnosperms, monocots, and eudicots. Phylogenetic and structural analysis classified these FRO and NOX genes into four well-conserved groups represented as NOX, FRO I, FRO II, and FRO III. Further analysis of NOXs of phylogenetic and exon/intron structures showed that single intron loss and gain had occurred, yielding the diversified gene structures during the evolution of NOXs family genes and which were classified into four conserved subfamilies which are represented as Sub.I, Sub.II, Sub.III, and Sub.IV. Additionally, both available global microarray data analysis and quantitative real-time PCR experiments revealed that the NOX genes in Arabidopsis and rice (Oryza sativa) have different expression patterns in different developmental stages, various abiotic stresses and hormone treatments. Finally, coexpression network analysis of NOX genes in Arabidopsis and rice revealed that NOXs have significantly correlated expression profiles with genes which are involved in plants metabolic and resistance progresses. All these results suggest that NOX family underscores the functional diversity and divergence in plants. This finding will facilitate further studies of the NOX family and provide valuable information for functional validation of this family in plants.

Role of ascorbic acid in photosynthesis

Experimental data concerning the role of ascorbic acid in both the maintenance of photosynthesis and in the protection of the photosynthetic apparatus against reactive oxygen species and photoinhibition are reviewed. The function of ascorbic acid as an electron donor in the "Krasnovsky reaction", as well as its physiological role as a donor to components of the photosynthetic electron transport chain, which was first studied by A. A. Krasnovsky in the 1980s, is discussed. Data on the content and transport of ascorbic acid in plant cells and chloroplasts are presented.

The physiological roles and metabolism of ascorbate in chloroplasts

Ascorbate is a multifunctional metabolite in plants. It is essential for growth control, involving cell division and cell wall synthesis and also involved in redox signaling, in the modulation of gene expression and regulation of enzymatic activities. Ascorbate also fulfills crucial roles in scavenging reactive oxygen species, both enzymatically and nonenzymatically, a well-established phenomenon in the chloroplasts stroma. We give an overview on these important physiological functions and would like to give emphasis to less well-known roles of ascorbate, in the thylakoid lumen, where it also plays multiple roles. It is essential for photoprotection as a cofactor for violaxanthin de-epoxidase, a key enzyme in the formation of nonphotochemical quenching. Lumenal ascorbate has recently also been shown to act as an alternative electron donor of photosystem II once the oxygen-evolving complex is inactivated and to protect the photosynthetic machinery by slowing down donor-side induced photoinactivation; it is yet to be established if ascorbate has a similar role in the case of other stress effects, such as high light and UV-B stress. In bundle sheath cells, deficient in oxygen evolution, ascorbate provides electrons to photosystem II, thereby poising cyclic electron transport around photosystem I. It has also been shown that, by supporting linear electron transport through photosystem II in sulfur-deprived Chlamydomonas reinhardtii cells, in which oxygen evolution is largely inhibited, externally added ascorbate enhances hydrogen production. For fulfilling its multiple roles, Asc has to be transported into the thylakoid lumen and efficiently regenerated; however, very little is known yet about these processes.

Characterization of Rice NADPH oxidase genes and their expression under various environmental conditions

Plasma membrane NADPH oxidases (Noxs) are key producers of reactive oxygen species under both normal and stress conditions in plants. We demonstrate that at least eleven genes in the genome of rice (Oryza sativa L.) were predicted to encode Nox proteins, including nine genes (OsNox1–9) that encode typical Noxs and two that encode ancient Nox forms (ferric reduction oxidase 1 and 7, OsFRO1 and OsFRO7). Phylogenetic analysis divided the Noxs from nine plant species into six subfamilies, with rice Nox genes distributed among subfamilies I to V. Gene expression analysis using semi-quantitative RT-PCR and real-time qRT-PCR indicated that the expression of rice Nox genes depends on organs and environmental conditions. Exogenous calcium strongly stimulated the expression of OsNox3, OsNox5, OsNox7, and OsNox8, but depressed the expression of OsFRO1. Drought stress substantially upregulated the expression of OsNox1–3, OsNox5, OsNox9, and OsFRO1, but downregulated OsNox6. High temperature upregulated OsNox5–9, but significantly downregulated OsNox1–3 and OsFRO1. NaCl treatment increased the expression of OsNox2, OsNox8, OsFRO1, and OsFRO7, but decreased that of OsNox1, OsNox3, OsNox5, and OsNox6. These results suggest that the expression profiles of rice Nox genes have unique stress-response characteristics, reflecting their related but distinct functions in response to different environmental stresses.

Saccharomyces cerevisiae-induced stomatal closure mainly mediated by salicylhydroxamic acid-sensitive peroxidases in Vicia faba

Saccharomyces cerevisiae induced stomatal closure in a dose-dependent manner on Vicia faba L. (cv. Daqingpi). Using pharmacological inhibitors in this study, we found that stomatal closure was completely inhibited by salicylhydroxamic acid (SHAM) and reduced glutathione (GSH), whereas slightly inhibited by diphenyleneiodonium chloride (DPI), suggesting that H2O2 was mostly produced by cell wall peroxidases. The specific NO scavenger (cPTIO), NO synthase (NOS) inhibitor NG-nitro-l-arginine methyl ester (l-NAME) and sodium azide (NaN3; inhibitor of nitrate reductase) prevented yeast-induced stomatal closure, suggesting that NO in guard cells of V. faba is derived from both NOS-like enzyme and nitrate reductase. Results of HgCl2 and β-mercaptoethanol (ME) treatment (as a functional inhibitor of water channels and its reversing agent, respectively) suggest that water channels are involved in yeast-induced stomatal movements. CoCl2 (the blocker of calcium channel), LaCl3 (Ca(2+) antagonist) and EGTA (Ca(2+) chelator) also impaired yeast-induced stomatal closure. Thus, it is concluded that H2O2, NO, water channels and Ca(2+) are involved in yeast-induced stomatal closure.