Abstract
Little is known about the interactive effects of exogenous nitric oxide (NO) and abscisic acid (ABA) on nitrogen (N) metabolism and related changes at molecular and biochemical levels under drought stress. The present study highlights the independent and combined effect of NO and ABA (grouped as “nitrate agonists”) on expression profiles of representative key genes known to be involved in N-uptake and assimilation, together with proline metabolism, N–NO metabolism enzyme’s activity and nutrient content in polyethylene glycol (PEG) treated roots of Indian mustard (B. juncea cv. Varuna). Here we report that PEG mediated drought stress negatively inhibited growth performance, as manifested by reduced biomass (fresh and dry weight) production. Total N content and other nitrogenous compounds (NO3−, NO2−) were decreased; however, NH4+, NH4+/ NO3− ratio and total free amino acids content were increased. These results were positively correlated with the PEG induced changes in expression of genes and enzymes involved in N-uptake and assimilation. Also, PEG supply lowered the content of macro- and micro-nutrients but proline level and the activity of ∆1-pyrroline-5-carboxylate synthetase increased indicating increased oxidative stress. However, all these responses were reversed upon the exogenous application of nitrate agonists (PEG + NO, PEG + NO + ABA, and PEG + ABA) where NO containing nitrate agonist treatment i.e. PEG + NO was significantly more effective than PEG + ABA in alleviating drought stress. Further, increases in activities of L-arginine dependent NOS-like enzyme and S-nitrosoglutathione reductase were observed under nitrate agonist treatments. This indicates that the balanced endogenous change in NO and ABA levels together during synthesis and degradation of NO mitigated the oxidative stress in Indian mustard seedlings. Overall, our results reveal that NO independently or together with ABA may contribute to improved crop growth and productivity under drought stress.
Highlights
Drought stress is the most dominant abiotic stress that severely affects plant growth performance and productivity, in arid and semi-arid areas all around the w orld[1,2]
Several studies including the present work have reported that drought stress affects N absorption and inhibits enzymes through alteration in the transcriptional levels of transporters implicated in N-metabolism (e.g., nitrate (NO3−) transporters (NRTs), ammonium (NH4+) transporters (AMTs), nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH))[16,17,18]
Results are discussed by comparing the control plants with polyethylene glycol (PEG) or a group of nitrate agonists (i.e., PEG + abscisic acid (ABA), PEG + nitric oxide (NO) and PEG + NO + ABA) rather than with NO or ABA alone, because ABA or NO treatments were not significantly different to control for all parameters except SNO/GSNO content and some micro-macronutrients studied (Supplementary Materials Table S1)
Summary
Drought stress is the most dominant abiotic stress that severely affects plant growth performance and productivity, in arid and semi-arid areas all around the w orld[1,2]. NO is a small signaling molecule and it plays a crucial role in diverse plant cellular functions including plant defense, stomatal regulation, root development, etc., under stress and non-stress conditions[11]. Cross-talk of NO and ROS modify NO’s benign property into a nitrosative agent, when its endogenous concentration along with duration of cross-talk is higher than appropriate causing nitrosative or nitro-oxidative stress in p lants[12] It is a well-known phenomenon that excess ROS induces oxidative stress in plants by disturbing their physiological. GSNO is metabolized by its conversion into oxidized glutathione (GSSG) and NH4+ by NAD(P)H dependent enzyme GSNO reductase (GSNOR)[20] (Fig. 1) This enzyme is responsible for maintaining NO homeostasis and protecting the plant cell from nitrosative or nitro-oxidative stress.
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