Regulation of anaplerotic enzymes by melatonin enhances resilience to cadmium toxicity in Vigna radiata (L.) R. Wilczek.

Melatonin (Mel) is a tryptophan-derived (N-acetyl-5-methoxytryptamine) molecule. In the present study, role of Mel in the regulation of various anaplerotic enzymes is discussed in relation to N metabolism and H-ATPase activity in mung bean under Cd stress. The application of Mel to the Cd-stressed mung bean seedlings was remarkable in improving the activity of hexokinase (35.7%), pyruvate kinase (79.2%), phosphoenolpyruvate carboxylase (38.9%) pyruvate dehydrogenase (41.5%), malate dehydrogenase (49.2%), citrate synthase (37.7%), isocitrate dehydrogenase (33.1%), ATP synthase (63.6%), and ATPase (38.6%). Incubation of Cd-stressed seedlings with Mel also improved the activity of nitrate reductase by 89.4%, nitrite reductase by 78.2%, and glutamine synthetase by 35.3% that resulted in higher level of ammonium and their subsequent assimilation to amino acids and proteins. Activation of these enzymes was strongly associated with Mel-induced regulation of H-ATPase activity that improved K retention and N assimilation capacity of the Cd-stressed seedlings of mung bean. The coordinated mechanism of action of tricarboxylic acid (TCA) cycle, N metabolism, and higher K levels were helpful in providing protection against detrimental effects of Cd toxicity through improving the defense system and energy level of the plants. However, inclusion of sodium orthovanadate (PM H-ATPase inhibitor) to the incubation medium reversed the positive effect of Mel and suppressed the performance of plants under Cd-stress. The findings of the study indicate that under Cd stress, the regulatory mechanisms of anaplerotic enzymes and antioxidant defense are mediated by Mel, and this process is facilitated by the retention of K induced by H-ATPase.