Drought stress retards plant growth and yield. Melatonin and nitric oxide (NO) have demonstrated their potential role against abiotic stresses; however, the underlying molecular mechanism by which they interact and extend drought stress tolerance has not been fully elucidated. Herein, the current study was performed to establish the optimum beneficial concentration of MT and NO in combating drought stress and later understand its responses at biochemical, and molecular levels. Results showed exogenous MT, and sodium nitroprusside (SNP as NO donor) have counteracted drought-induced growth inhibition of soybean (Glycine max L.) by increasing plant biomass, photosynthesis efficiency and water content and reducing reactive oxygen species accumulation. MT and NO treatments showed reduced lipid peroxidation and improved defense responses via significantly higher antioxidant enzyme activities than control during drought. Surprisingly, endogenous abscisic acid (ABA) contents and gene expression of its synthesis and ABA-responsive proteins and their promoters were significantly decreased in drought by MT + NO. This was coupled with an increase in endogenous MT levels. In endo-NO regulations, S-nitrosoglutathione was increased, but L-NAME (NO inhibitor) and cPTIO (NO scavenger) decreased the S-nitrosothiol (SNO) contents, which was followed by the increased expression of NO-synthesis-related-genes by MT + NO. Interestingly, MT + NO-induced drought stress tolerance was coupled with increased expression of transcription factors such as GmWRKY27 and GmMYB174. Conclusively, the physiological, antioxidant, and molecular analysis showed that MT triggers downregulated NO accumulation, promoting tolerance against drought stress.
WRKY transcription factors are important plant-specific regulatory genes characterized by one or two conserved WRKY domain(s) usually followed by a zinc-finger motif. In this study using Arabidopsis thaliana, the RNA-Seq based transcriptomic analysis showed differential expression of 33 genes encoding WRKY TFs in response to the nitric oxide (NO) donor S-Nitrosocysteine (CySNO). Interestingly, 93.9% of these TFs were up-regulated with at least 2-fold change, suggesting their putative involvement in NO mediated gene regulation. GO- analysis of all the 33 transcriptomic elements showed their putative involvement in biological processes such as abiotic stress tolerance and defense against fungal pathogens (89.39 fold enrichment). Analysis of the NO-responsive AtWRKY TFs promoter region revealed the presence of the cis-acting elements such as ABRE, EIRE, ERE, and MBS involved in osmotic stress response, maximal elicitor-mediated activation, and drought-stress regulation. The analysis of NO-responsive AtWRKY TF motifs and their comparison with rice, soybean, and tomato orthologs suggested that members of the WRKY family belonging to the same group shared similar motifs and phylogenetic tree suggested that these TFs were highly conserved. Validation of transcriptomic data through quantitative real time-PCR showed a high correlation coefficient (0.85) indicating the high reliability and similarity of both types of analysis. Comparison of the NO-responsive and non-responsive WRKYs showed the presence of tyrosine (T) and cysteine (C) residues at a distance of 7 residues from the WRKYGQK motif which may serve as potential targets for modification by NO via tyrosine nitration and S-nitrosylation. We also validated the response of WRKYs through in vivo analysis using atwrky62 loss of function mutant and the results indicated a negative role of AtWRKY62 in plant growth. Furthermore, atwrky62 showed significantly less SNO contents compared to wild type plants indicating putative role of AtWRKY62 in NO metabolism.
Non-photochemical quenching (NPQ) of chlorophyll fluorescence is a valuable feature for the study of photosynthetic organisms’ light utilization and dissipation. However, all too often NPQ is simply equated with the harmless dissipation of excess absorbed light energy as heat. This is not always the case as some processes cause NPQ without thermal dissipation. Photoinhibitory quenching, qI, is sustained NPQ that continuously depresses the commonly used fluorescence parameter “quantum yield of photosystem II (PSII)”, or Fv/Fm, and is often viewed as a result of PSII core inactivation due to D1 damage. Inactivated PSII cores might have a photoprotective role but that is not the topic of the present review. Instead, this review focuses on a sustained photoprotective antenna quenching component, which we have termed qH, and summarizes the recently uncovered molecular players of this sustained form of NPQ.
Plants are exposed to diverse abiotic stresses like drought, heat, salinity, and high-metal concentrations at different stages of their life cycle. As protection against stress, plants release signaling molecules that initiate a cascade of stress-adaptation responses leading either to programmed cell death or plant acclimation. Nitric oxide (NO) is a small but important redox signaling molecule that in plants is involved in a diverse range of physiological processes including germination, development, flowering, senescence, and abiotic stress. Although the exact role of NO in plants remains unclear and is species dependent, various studies have suggested a positive correlation between NO accumulations in stress in plants. In this article, we review and discuss the biosynthesis of NO, sources and exogenous application of NO donors under drought, salt, and heavy metal stress. A review of publications indicated that, in general, application of exogenous NO alleviates the negative stress effects in plants and improves antioxidant activity in most plant species. In addition, S-nitrosylation and tyrosine nitration are two NO-mediated posttranslational modification. All these factors are important in protecting plants from diverse stresses and vary with the species. Furthermore, to determine precise mechanisms of action of NO is expected to help in efficient utilization of crop cultivation under stress conditions.
Short-term (5-120 sec) transport of cadmium (Cd-109) into cells of the unicellular cyanobacterium Synechocystis aquatilis was studied using a rapid filtration technique. The transport was strongly pH-dependent, occurring at pH 7, but not at pH 5.5, and was also observed in long-term (40 min) experiments. For this process the optimum pH was approximately 7.5; uptake was considerably reduced at lower pHs. The entrance of cadmium disturbed photosynthetic activity and related processes. At pH 7, cadmium (8.9 muM) decreased CO2 fixation by about 55%, inhibited carbonic anhydrase activity (completely in intact cells, by 65%, in cell-free extracts) and photosynthetic O2-evolution by about 50%. At pH 5.5 no effects were observed.
The effect of different Ca concentrations in the growth medium on the toxicity of 25 mu M CdSO4 was studied in runner bean plants (var. Piekny Jas) at two different growth stages of primary leaves. In young plants growing in a medium with low level of Ca a treatment with Cd for 12 days resulted in Ca accumulation in roots, a strong reduction of the leaf area, a decreased monogalactosyl diacylglycerol/digalactosyl diacylglycerol ratio and efficiency of the photosynthetic apparatus. In leaves of older plants growing under the same conditions, and surviving Cd treatment, a high accumulation of Ca but a low one of Cd, chlorosis of leaves, a decrease of the ratio monogalactosyl diacylglycerol/digalactosyl diacylglycerol and photosynthetic activity were shown. At a high level of Ca in the nutrient medium plant roots showed a remarkably high specificity to accumulate Cd but the toxic effect of the metal on plant growth parameters and content of pigments was decreased. No changes were observed in the level of galactolipids, but changes in fluorescence quenching were recorded. Calcium deficit enhanced the effect of Cd toxicity, including primary photochemistry, whereas excess Ca reduced toxic effects, while it is increasing the nonphotochemical quenching of excitation energy. (C) 1998 Elsevier Science B.V. All rights reserved.