Nitric oxide donor, sodium nitroprusside modulates hydrogen sulfide metabolism and cysteine homeostasis to aid the alleviation of chromium toxicity in maize seedlings (Zea mays L.)

The current research aimed to assess the protective role of nitric oxide (NO) against chromium (Cr) toxicity in maize seedlings. Chromium (200 mu M) lowered osmotic potential in epicotyls and mostly in radicles (by 38% and 63%, respectively) as compared to the control. Sodium nitroprusside (SNP, NO donor) restored seedling biomass (+90% for both organs) and water potential, whereas application of N omega-nitro-L-arginine methylester (L-NAME, a NOS inhibitor) increased sensitivity to Cr. SNP suppressed Cr-triggered proline accumulation by inhibiting Delta(1)-pyrroline-5-carboxylate synthetase activity and stimulating proline dehydrogenase activity, leading to glutamate over-accumulation (similar to 30% for both organs). Cr stimulated cysteine metabolism and this was further enhanced by SNP which stimulated serine acetyl-transferase and O-acetylserine (thiol) lyase activities. This was followed by an increase in endogenous hydrogen sulfide (H2S) generation by up-regulating L-cysteine desulfhydrase (+205%), D-cysteine desulfhydrase (+150%) and cyanoalanine synthase (+65%) activities in radicles compared to Cr-treatments plants. These positive effects were reduced in L-NAME compared to control. Combined Cr+SNP affected the levels of compounds involved in glutathione metabolism (gamma-glutamyl-cysteinyl, gamma-glutamyl-cysteinylclycine, gamma-cysteinyl-glycine, and glycine.). All together, our findings indicate that NO and elicited cellular H2S act synergistically to alleviate Cr stress in maize seedlings by influencing a metabolic interplay between cysteine, proline, and glutathione.