Exogenous nitric oxide requires endogenous hydrogen sulfide to induce the resilience through sulfur assimilation in tomato seedlings under hexavalent chromium toxicity.

Nitric oxide (NO) and hydrogen sulfide (H2S), versatile signaling molecules, play multiple roles in plant growth, physiological and biochemical processes under heavy metal stress. However, the mechanisms through which NO in association with endogenous H2S mediated hexavalent chromium Cr(VI) toxicity mitigation are still not fully understood. Therefore, we investigated the role of NO and H2S in sulfur (S)-assimilation and the effect of NO on endogenous H2S, and cysteine (Cys) biosynthesis and maintenance of cellular glutathione (GSH) pool in tomato seedlings under Cr(VI) stress. Cr(VI) toxicity caused an increase in reactive oxygen species (ROS; O2•- and H2O2) formation and activity of chlorophyll (Chl) degrading enzyme [Chlorophyllase (Chlase)] and decrease in seedlings growth attributes, Chl a and b content, and activity of Chl synthesizing enzyme [δ-aminolevulinic acid dehydratase (δ-ALAD)], gas exchange parameters, S-assimilation, and Cys and H2S metabolism. An increase in the content of glycinebetaine (GB), total soluble carbohydrates (TSCs) and total phenols (TPls), and decrease in DNA damage and ROS in NO treated seedlings conferred Cr(VI) toxicity tolerance. Under Cr(VI) toxicity conditions, the inclusion of H2S scavenger hypotaurine (HT) in growth medium containing NO validated the role of endogenous H2S in S-assimilation, H2S and Cys and GSH metabolism by withdrawing activity of enzymes involved in S-assimilation [adenosine 5-phosphosulfatereductase (APS-R), ATP-sulfurylase (ATP-S)], in the biosynthesis of H2S [L-cysteine desulfhydrase (L-CD) and D-cysteine desulfhydrase (D-CD)], Cys [O-acetylserin (thiol) lyase (OAST-L)], and GSH [glutamylcysteine synthetase (γ-GCS) and glutathione synthetase (GS)], and in antioxidant system. On the other hand, application of cPTIO [2-(4-32 carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide], a NO scavenger and HT diminished the effect of NO on internal H2S levels, Cys and glutathione homeostasis, and S-assimilation, which resulted in poor immunity against oxidative stress induced by Cr(VI) toxicity. The obtained results postulate that NO-induced internal H2S conferred tolerance of tomato seedlings to Cr(VI) toxicity and maintained better photosynthesis process and plant growth.