Nitric oxide-induced synthesis of hydrogen sulfide alleviates osmotic stress in wheat seedlings through sustaining antioxidant enzymes, osmolyte accumulation and cysteine homeostasis

Nitric oxide (NO) and hydrogen sulfide (H2S) have been shown to act as signaling molecules in various physiological processes, play significant roles in plant cellular processes, and also mediate responses to both biotic and abiotic stresses in plants. The present investigation was carried out to test the effect of exogenous NO on endogenous synthesis of H2S in osmotic-stressed wheat (Triticum aestivum L.) seedlings. The results show that application of NO to wheat seedlings, suffered from PEG8000-induced osmotic stress, considerably enhanced the activities of H2S-synthesizing enzymes l-cysteine desulfhydrase (LCD) and d-cysteine desulfhydrase (DCD) leading to enhanced level of endogenous H2S content. At the same time exogenous NO also enhanced the activity of cysteine (Cys)-synthesizing enzyme O-acetylserine(thiol)lyase (OAS-TL) and maintained Cys homeostasis under osmotic stress. NO and H2S together markedly improved the activities of antioxidant enzymes viz. ascorbate peroxidase (APX), glutathione reductase (GR), peroxidase (POX), superoxide dismutase (SOD) and catalase (CAT). Furthermore, NO and H2S caused additional accumulation of osmolytes proline (Pro) and glycine betaine (GB), all these collectively resulted in the protection of plants against osmotic stress-induced oxidative stress. On the other hand, NO scavenger cPTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] and H2S scavenger HT (hypotaurine) invalidated the effect of NO on endogenous H2S levels and Cys homeostasis which resulted in weak protection against osmotic stress. Application of N-ethylmaleimide (NEM) suppressed GR activity and caused an increase in oxidative stress. We concluded that NO in association with endogenous H2S activates the defense system to the level required to counter osmotic stress and maintains normal functioning of cellular machinery.