Modulation of AsA-GSH cycle by exogenous nitric oxide and hydrogen peroxide to minimize the Cd-induced damages in photosynthetic cyanobacteria

Abstract

Cyanobacteria, the prominent nitrogen fixers, are facing many stressful situations throughout their life span, hence in recent years the yield and productivity of paddy crops are highly compromised. To address this problem, this study is structured to investigate how signaling molecules can positively influence the ability of tested cyanobacteria to counteract the adverse effects of metal stress. Thus, the main objective of this study is to elucidate the importance and crosstalk of intracellular signaling molecules, nitric oxide (NO), and hydrogen peroxide (H2O2) during Cd stress tolerance in cyanobacteria. The effect of exogenous gasotransmitter, NO and redox molecule, H2O2, in mitigation of cadmium (Cd, 6 µM) toxicity on different physiological and biochemical activities were well recorded in economically important cyanobacteria Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120. Under Cd stress increased Cd accumulation and oxidative stress in test cyanobacteria, impaired the functioning of the ascorbate-glutathione (AsA-GSH) cycle. Exogenous application of NO (10 µM) and H2O2 (1 µM) ameliorated the negative effects of Cd through boosting-up the activities of enzymatic (ascorbate peroxidase, glutathione reductase and dehydroascorbate reductase) and the contents of non-enzymatic (cysteine, proline, ascorbate, glutathione and non-protein-thiols) antioxidants. The application of NO scavenger (PTIO) or NO synthase enzyme inhibitor (LNAME) to the Cd-stressed cells abolished the recovery of the above parameters, even in the presence of exogenous H2O2. Notwithstanding to this, under similar stress exogenous addition of scavenger of H2O2 (NAC) or inhibitor of NADPH oxidase enzyme (DPI) to the growth medium could not restrict the recovery of the above parameters in the presence of NO, hence pointing towards the major role of NO in tolerance of the test organisms to Cd stress. In conclusion, the finding revealed that NO plays a vital role in H2O2 signaling to attain Cd stress tolerance in cyanobacteria. Moreover, the study has created a scope to search for deeper insight into signaling interactions between other gasotransmitters and redox molecules in cyanobacteria. Further, the current finding recommends using a lower dose of SNP (NO donor, a cost-effective chemical) in paddy fields may ensure better growth of cyanobacteria (biofertilizer) even under prevailing stress conditions for sustainable agriculture to maintain productivity of rice crops.