Salt stress and the wheat curl mite (Aceria tosichella) infestation reprograms barley nitrogen metabolism via nitric oxide signaling.

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conclusionSalinity (NaCl) and wheat curl mite infestation profoundly alter nitric oxide (NO) metabolism in barley, with NaCl dose-dependent responses indicating that nitrogen metabolism is fine-tuned through NO-signaling pathways. Nitric oxide (NO) serves as a multifaceted regulator in plants' responses to environmental stress, acting as both a signaling molecule and a protective agent, while also exhibiting potential harmful effects. Our study investigated NO metabolism in barley (Hordeum vulgare L.) plants under salinity (50 mM and 100 mM NaCl) and wheat curl mite (WCM) infestation, exposed to either single stressor or both stressors simultaneously. To accomplish our objectives, we adopted an integrated approach that combines biochemical, molecular, and microscopic techniques. We found that these stressors influence the production of NO and its subcellular distribution. Both nitrate reductase (NR) and non-enzymatic processes contribute to the production of NO. Under combined stress (50 mM NaCl + WCM), NO molecules were detected in the cytoplasm, vacuoles, and chloroplasts, along with elevated NR activity. NO fluorescence in cell walls suggests its role in the apoplastic response of barley under dual stress, which induced a probable synergistic effect caused by salinity and WCM effectors. Upregulation of alternative oxidase AOX and phytoglobin Pgb gene expression, along with decreased activity of S-nitrosoglutathione reductase (GSNOR), in all combinations indicates sophisticated regulation of NO metabolism through transcriptional and enzymatic mechanisms. The accumulation of 3-nitrotyrosines under high salinity and dual stress points to increased nitro-oxidative stress. Moreover, an inhibitory effect of a 100 mM NaCl salinity dose was demonstrated on WCM reproduction. Altogether, these findings provide a mechanistic framework for exploiting NO-related pathways as potential targets in breeding or biotechnological strategies aimed at improving barley tolerance to combined abiotic-biotic stress conditions, while simultaneously limiting pest performance under salinity.