Halotolerant plant growth promoting Bacillus strains Bm1, E1, and F1 mitigate salt stress in Phaseolus vulgaris

Salinity is a significant issue that affects agricultural productivity, particularly in arid and semi-arid soils. This stress reduces soil fertility and hinders the normal development of plants, thereby affecting crop productivity. To obtain a sustainable management of saline soils, it is necessary to use salt-tolerant symbiotic bacteria with plant growth-promoting (PGP) abilities. In this study, Bacillus strains were isolated from saline soils in Monte San Biagio (Latium, Italy). Bacterial strains were selected based on salt tolerance (up to 5%) and characterized for PGP traits. The consortium formed by the most promising strains was tested in vitro and in planta on Phaseolus vulgaris. The consortium was applied by soaking the seeds for 20 minutes. Under greenhouse, a 5% of salinity stress was applied, testing the consortium ability to induce tolerance in plants by plant growth and development parameters monitoring (shoot and root length, total chlorophyll content, proline accumulation). Most of the tested strains showed good PGP activities. Strains BM1, E1, and F1 were selected based on phosphate solubilization (E1 and F1 =25,53 µg PO₄³− mL−1 in average), indoles (25,2 µg mL-1 IAA equivalents in average), siderophores (F1=1,4%), and ammonia production, and 1-aminocyclopropane-1-carboxylate deaminase (ACC) activity. The consortium presented good association in vitro with P. vulgaris: scanning electron microscopy revealed that Bacillus strains (BM1, E1, and F1) have good adhesion and colonization abilities. Inoculation with the consortium had a positive effect on plant growth parameters. The shoot and root lengths, chlorophyll contents, and proline accumulation in the treated group were higher than those in the control (p < 0.05). These findings suggest that seed priming with halotolerant Bacillus strains can mitigate the negative effects of salt stress and promote the normal growth and development of P. vulgaris plants.