In-Vitro Plant Growth Promotion of Rhizobium Strains Isolated from Lentil Root Nodules under Abiotic Stresses

Plant growth-promoting rhizobia are known to improve crop performance by multiple mechanisms. However, the interaction between host plants andRhizobiumstrains is highly influenced by growing conditions, e.g., heat, cold, drought, soil salinity, nutrient scarcity, etc. The present study was undertaken to assess the use ofRhizobiumas plant growth promoters under abiotic stress conditions. FifteenRhizobiumstrains isolated from lentil root nodules were tested for phosphate solubilization activity (PSA) and phytohormones production under salt and drought conditions. The results showed that 15Rhizobiumstrains were significant phosphate solubilizers, and indole acedic acid (IAA) and gibberellic acid (GA3) producers based on least significant difference (LSD) analysis (p <= 0.05). The highest rate of PSA was attributed to three strains namely, 1145N5, 1159N11, and 1159N32 with a range of 144.6 to 205.6 P2O5(mu g/mL). The highest IAA production was recorded in the strain 686N5 with 57.68 +/- 4.25 mu g/mL as compared to 50.8667 +/- 1.41 mu g/mL and 37.32 +/- 12.59 mu g/mL forRhizobium tropiciCIAT 899 andAzospirillum brasilenseDSM-1690, respectively. Strain 318N2111 produced 329.24 +/- 7.84 mu g/mL of GA3 as against 259.84 +/- 25.55 mu g/mL forA. brasilenseDSM-1690.R. tropici CIAT 899showed tolerance to salt (5% NaCl) and drought (psi = -2.6 MPa) stress, whereas strain 686N5 showed an extremely high level of salt-tolerance (5% NaCl) and moderate level of drought tolerance (psi = -0.75 MPa). These results indicate different pathways for drought and salt tolerance mechanisms. The assessment of plant growth promoting (PGP) activities ofRhizobiumshowed differences between bacterial viability and bacterial PGP activity in terms of abiotic stress tolerance where bacterial PGP activity is interrupted before reaching the bacterial tolerance threshold. These results integrate a new concept of PGPR screening based on PGP activity under abiotic stress.