Harnessing the Potential of Nodule Endophytic Bacteria to Improve Soybean Growth Under Saline Conditions

Soybean [Glycine max (L.) Merrill] is a major leguminous and oilseed crop known for high protein contents, fulfilling the nutritional demand, particularly in regions dominated by staple crops. In recent years, salt stress has heavily limited crop productivity. Salt-tolerant plant growth-promoting bacteria (PGPB) can better serve as biofertilizers for improved crop productivity under saline conditions. In this study, fourteen bacteria were isolated from root nodules of soybean and further tested for salt-tolerance and in vitro plant growth-promoting (PGP) properties under saline stress. Nine bacterial isolates showed salt-tolerance at 2% NaCl. The indole-3-acetic acid (IAA) production of these bacteria ranged from 3.8 to 13.2 µg mL^- 1, wherein HSS-5 showed maximum production. All isolates showed positive results on nitrogen-free minimal (NFM) media and were capable to solubilize phosphorus ranging from 1.08 to 2.44 index. Six isolates exhibited biofilm formation ability. In hydrolytic enzyme activities, five isolates showed potential in cellulase activity and five in pectinase activity. Based on the in vitro testing, five potential isolates were selected for taxonomic identification and further evaluated under controlled and field conditions. Phylogenetic analysis revealed that HSS-2 is closely related to Sinorhizobium psoraleae, HSS-6 to Bradyrhizobium japonicum, HSS-10 to Agrobacterium tumefaciens, HSS-12 to Agrobacterium pusense and HSS-13 to Priestia megaterium. In pot assay, HSS-13, HSS-2, and HSS-6 significantly promoted plant growth, increasing plant biomass by 69.88%, 43.37% and 40.96%, respectively, as compared to the control whereas HSS-6 showed maximum grain yield under field trial. This research highlights the first report of an unexplored diversity of salt-tolerant non-rhizobial endophytic bacterial community associated with soybean nodules. Notably, the identification of salt-tolerant traditional nodule endophyte, Bradyrhizobium japonicum HSS-6 and the unexplored emergence of non-traditional Priestia megaterium HSS-13 as an effective nodule endophyte with strong PGP traits represent a previously unreported finding that opens up new avenues for sustainable production under saline conditions.