The effect of salinity-resistant biofilm-forming Azotobacter spp. on salt tolerance in maize growth

Increasing soil salinity is among the most detrimental threats restricting crop growth and productivity. In recent years, root inoculation with biofilm-forming plant growth-promoting rhizobacteria (PGPR) has been greatly interested in improving abiotic stress tolerance. This study examined the plant growth-promoting and biofilmforming potential of Azotobacter spp. obtained from maize rhizosphere, tested the salinity effect (up to 300 mM NaCl) on biofilm formation and exopolysaccharide (EPS) production, and evaluated their effect on maize growth at different concentrations (0, 50, and 150 mM) of NaCl under greenhouse conditions. The isolates produced different amounts of indole acetic acid (IAA) (from 0.96 to 7.51 mu g mL-1) and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase (from 2.10 to 19.50 mu mol alpha-ketobutyrate mg-1 h-1). On average, the highest biofilm formation was found in A. chroococcum. Both biofilm formation and EPS production significantly increased (p < 0.05) at 150 and 300 mM NaCl in A. chroococcum SC8, A. beijerinckii SC10, and A. tropicalis SC4. These strains also significantly increased (p < 0.05) root depth by 44.1-55.9%, shoot height by 52.3-58.8%, fresh root weight by 42.8-52.4%, and fresh shoot weight by 44.6-53.6% at 150 mM NaCl compared with the uninoculated salinitystressed (control) treatment. Among plant growth regulators, higher biofilm formation and EPS production under increasing salinity exhibited better maize growth under salinity stress. Therefore, these respective strains might be promising bioinoculants for alleviating salinity stress in field experiments.