Ecological assessment of biotoxicity of pesticides towards plant growth promoting activities of pea (Pisum sativum)-specific Rhizobium sp. strain MRP1

This study was planned to assess the impact of pesticides [herbicides (metribuzin and glyphosate), insecticides (imidacloprid and thiamethoxam) and fungicides (hexaconazole, metalaxyl and kitazin)] at the recommended and the higher dose rates on plant growth promoting (PGP) traits of Rhizobium sp. strain MRP1 isolated from pea-nodules. Strain MRP1 was unambiguously selected due to high pesticide-tolerance and substantial production of indole acetic acid, siderophores, exo-polysaccharides, HCN and ammonia. Pesticideconcentration dependent progressive-decline for PGP properties of Rhizobium sp. strain MRP1 was observed except exo-polysaccharides which regularly increased on increasing concentration of each pesticide beyond the recommended dose. For example, hexaconazole at three times the recommended dose decreased salicylic acid and 2, 3-dihydroxy benzoic acid biosynthesis by 37% and 55%, respectively above control. Likewise, glyphosate, imidacloprid, and hexaconazole decreased indole acetic acid secretion by 28%, 19%, and 34%, respectively at three times the recommended dose. Among all tested pesticides, the greatest stimulatory effect on exo-polysaccharides secretion was shown by glyphosate which stimulated Rhizobium sp. strain MRP1 to secrete exo-polysaccharides by 40% higher with respect to untreated control. Generally, the maximum toxicity to PGP traits (excluding exo-polysaccharides) of Rhizobium was shown by glyphosate, imidacloprid and hexaconazole at three times the recommended rate among herbicides, insecticides and fungicides, respectively. The results of this study implied that prior to field-application pesticides must be tested in laboratory for the adverse impact on the physiological activities of plant-beneficial soil microorganisms. This study also revealed a circumlocutory mechanism of pesticide-mediated toxicity to plant growth.