Assessing the impact of arsenic on symbiotic and free-living PGPB: plant growth promoting traits, bacterial compatibility and adhesion on soybean seed.

Arsenic (As) contamination in agricultural groundwater and soil is a significant economic and health problem worldwide. It inhibits soybean (Glycine max (L.) Merr.) nodulation and biological nitrogen fixation in symbiosis with Bradyrhizobium japonicum E109 (E109), a commonly used rhizobial strain for commercial biofertiliser formulation in Argentina. In the context of sustainable and climate-smart agriculture promoted by FAO, co-inoculating legumes with As-tolerant plant growth-promoting bacteria (PGPB) is suggested as a superior alternative to single inoculation. This study aimed to evaluate the impact of As on plant growth-promoting (PGP) traits -siderophore and indole acetic acid production, phosphate solubilisation, diazotrophic activity and hydrolytic enzymes activity- in E109 and three other PGPB strains: Pseudomonas sp. AW4 (AW4), Bacillus pumilus SF5 (SF5) and Bacillus toyonensis SFC 500-1E (Bt). In addition, bacterial compatibility and adhesion on soybean seed were evaluated. Arsenic significantly reduced PGP traits of E109 even at low concentrations, AW4's traits remained unchanged, while those of SF5 and Bt traits were affected (positively or negatively) only at the highest concentrations tested (500 µM arsenate, 250 µM arsenite). All PGPB strains were compatible with E109 under both control and As-stress conditions. Soybean seed adhesion was reduced for E109, only under As stress. Findings suggest that the effect of As on PGP traits is highly strain-dependent and influenced by As concentration and speciation. AW4, SF5, and Bt strains show promise for co-inoculation with E109 in soybean cultivation.