Moderate modulation by S-nitrosoglutathione of photorespiratory enzymes in pea (Pisum sativum) leaves, compared to the strong effects of high light.

When plants are exposed to water stress, photosynthesis is downregulated due to enhanced reactive oxygen species (ROS) and nitric oxide (NO). In contrast, photorespiratory metabolism protected photosynthesis and sustained yield. Modulation of photorespiration by ROS was established, but the effect of NO on photorespiratory metabolism was unclear. We, therefore, examined the impact of externally added NO by using S-nitrosoglutathione (GSNO), a natural NO donor, in leaf discs of pea (Pisum sativum) under dark or light: moderate or high light (HL). Maximum NO accumulation with GSNO was under high light. The presence of 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO scavenger, prevented the increase in NO, confirming the release of NO in leaves. The increase in S-nitrosothiols and tyrosine-nitrated proteins on exposure to GSNO confirmed the nitrosative stress in leaves. However, the changes by GSNO in the activities and transcripts of five photorespiratory enzymes: glycolate oxidase, hydroxypyruvate reductase, catalase, glycerate kinase, and phosphoglycolate phosphatase activities were marginal. The changes in photorespiratory enzymes caused by GSNO were much less than those with HL. Since GSNO caused only mild oxidative stress, we felt that the key modulator of photorespiration might be ROS, but not NO.