Nitrate reductase-dependent NO production is involved in H2S-induced nitrate stress tolerance in tomato via activation of antioxidant enzymes

Hydrogen sulfide (H2S) has been proposed as the third gasotransmitter after nitric oxide (NO) and carbon monoxide in animals and plants. In this study, the effect of H2S under excess nitrate stress in tomato root was studied. The results showed that excess nitrate inhibited the growth of tomato seedlings, while application of the H2S donor NaHS efficiently alleviated the growth inhibition, and decreased the lipid peroxidation, hydrogen peroxide (H2O2), and protein carbonyl contents, as well as the reactive oxygen species accumulation, especially 100 μM NaHS. Treatment with NaHS under nitrate stress increased the endogenous H2S and l-cysteine desulfhydrases (LCD) activity. Moreover, treatment with NaHS induced NO production and the synthesis of NO was mediated through nitrate reductase, but not through nitric oxide synthase in the presence or absence of nitrate. Besides, exogenous NaHS increased antioxidant enzyme genes (SOD, POD, CAT, and APX) expression, reducing the oxidative stress caused by excess nitrate. The alleviating effects of lipid peroxidation, H2O2 contents, and enhanced antioxidant enzyme activities by NaHS and NO donor (sodium nitroprusside, SNP), were reversed by 2-(4-carboxyphenyl)-4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide potassium salt (cPTIO), the specific scavenger of NO. Together, above results suggested that NR-dependent NO production by NaHS was involved in the alleviation of nitrate stress by enhancing antioxidant enzyme activities in tomato root.