Exogenous application of phytohormones modulates pesticide-induced injury in two rice field cyanobacteria: Toxicity alleviation by up-regulation of ascorbate-glutathione cycle

Injudicious usage of insecticides poses a harmful impact on non-targeted biological nitrogen fixers i.e. cyanobacteria, thereby causing substantial loss to paddy productivity. Phytohormones are emerging novel tools to optimize the adverse effect of stress factors. The current study aimed to explore the impact of insecticide cypermethrin (Cyp₁; 2 μg ml⁻¹ and Cyp₂; 4 μg ml⁻¹) on growth, protein, and exopolysaccharides (EPS) contents, oxidative stress, and ascorbate- glutathione cycle in two paddy field cyanobacteria Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120; it also attempted to understand the regulatory role of two phytohormones: kinetin (KN, 20 nM) and indole acetic acid (IAA, 240 nM) on insecticide-induced toxicity. Insecticide at Cyp₁ and Cyp₂ doses diminished the growth by 10% and 30% in Anabaena sp. and by 7% and 23% in N. muscorum, respectively, and a similar reduction was documented in protein (excepts at Cyp₁) and exopolysaccharides contents in both the cyanobacteria. Cypermethrin at tested doses caused cellular accumulation of oxidative biomarkers (SOR, H₂O₂, and MDA equivalents contents) in a concentration-dependent manner. At similar doses, the activity of AsA-GSH cycle enzymes ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR) was stimulated (except at Cyp₂ dose) while the levels of metabolites ascorbate (AsA), glutathione (GSH), AsA+DHA (dehydroascorbate) and GSH+ GSSG (oxidized GSH) and the ratios of AsA/DHA and GSH/GSSG were declined. Exogenous application of KN and IAA alleviated the toxic impact of cypermethrin as evident by a reduction in cellular accumulation of oxidative biomarkers in both the cyanobacteria by further improving the activity of enzymes (APX, MDHAR, DHAR, and GR) and the levels of metabolites (AsA, GSH, AsA+DHA, GSH+GSSH, AsA/DHA, and GSH/GSSG) of AsA-GSH cycle. The current study suggests that enhanced efficiency of the AsA-GSH cycle regulates the level of H₂O₂ under limit due to exogenous application of both the phytohormones, directly favoring the growth of both the cyanobacteria and this effect of KN and IAA was more pronounced in N. muscorum suggesting its resistivity against stress.