Nitric oxide improves S-assimilation and GSH production to prevent inhibitory effects of cadmium stress on photosynthesis in mustard (Brassica juncea L.)

Nitric oxide (NO) is an important gaseous signalling molecule that participates in many developmental and physiological processes, including defense responses against toxic metals in plants. The role of NO in cadmium (Cd)-induced toxic effects on photosynthesis was examined in mustard (Brassica juncea L.) plants. Exposure of plants to 50 μM Cd significantly enhanced oxidative stress (H2O2 content and lipid peroxidation) and impaired plant growth and photosynthesis, chlorophyll fluorescence and reduced chlorophyll content and stomatal conductance. However, the exogenous application of 100 μM sodium nitroprusside (SNP, a donor of NO) reversed the effects of Cd through its stimulation of ROS-scavenging compounds (superoxide dismutase, ascorbate peroxidase, glutathione reductase and reduced glutathione). Exogenous SNP significantly increased plant growth, photosynthesis and chlorophyll content and diminished the accumulation of reactive oxygen species (H2O2) and TBARS content. It also reduced the effects of Cd on thylakoid membrane of the chloroplasts. Application of SNP together with 1.0 mM SO42- showed better responses than SNP alone. The protective effect of NO was achieved through enhanced production of reduced glutathione (GSH). GSH biosynthesis in plants treated with buthionine sulfoximine (BSO), a GSH biosynthetic inhibitor, was not completely inhibited in presence of NO and S, suggesting that NO stimulated S-assimilation and GSH production of Cd exposed plants. This study concludes that NO counteracts Cd toxicity in B. juncea strongly by regulating S-assimilation and GSH production.