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 (HS), 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 HS mediated hexavalent chromium Cr(VI) toxicity mitigation are still not fully understood. Therefore, we investigated the role of NO and HS in sulfur (S)-assimilation and the effect of NO on endogenous HS, 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; O and HO) 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 HS 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 HS scavenger hypotaurine (HT) in growth medium containing NO validated the role of endogenous HS in S-assimilation, HS 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 HS [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 HS 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 HS conferred tolerance of tomato seedlings to Cr(VI) toxicity and maintained better photosynthesis process and plant growth.