Multitrait characterization of salt-tolerant plant growth-promoting bacteria under salinity stress

Abstract

Soil salinity is a major constraint on global crop productivity, making salt-tolerant plant growth-promoting bacteria (PGPB) a promising, eco-friendly solution for mitigating salt stress in crops. This study characterized salt-tolerant PGPB isolated from rhizosphere soil and root/shoot samples of rice grown in salt-affected fields, evaluating their growth-promoting functions under increasing salinity. Fifteen morphologically distinct isolates (IIRRSTB-1 to IIRRSTB-15) were screened for hydrogen cyanide production, phosphate and zinc solubilization, ACC deaminase activity, IAA production, and nitrogenase activity (acetylene reduction assay) under non-saline and saline (5% and 10% NaCl) conditions. All isolates grew luxuriantly at 0% and 5% NaCl, and eight retained this growth at 10% NaCl. At the highest salinity, IIRRSTB-2 and IIRRSTB-12 retained all five qualitative PGP traits, followed by IIRRSTB-1 and IIRRSTB-7 with four traits each. Quantitatively at 10 EC, IIRRSTB-2 recorded the highest values for HCN (35.1), zinc solubilization (19.9), ACC deaminase (84.5), nitrogenase activity, and IAA production (53.4 µg mg⁻¹ protein), while IIRRSTB-1 (156.0) and IIRRSTB-7 (148.1) excelled in phosphate solubilization. IIRRSTB-2 as a strong candidate for further molecular characterization and subsequent pot- and field-scale validation as a bioinoculant to support sustainable rice cultivation in salt-affected agricultural systems.