Assessing the drought-tolerance and growth-promoting potential of strawberry (Fragaria × ananassa Duch.) rhizobacteria for consortium bioformulation

Abstract

Plant growth-promoting rhizobacteria (PGPR) are known to influence plant root cells to regulate and induce specific traits related to growth promotion and survival. In the present study, rhizobacteria from strawberry plants were screened for stress tolerance under osmotic stress in TSB-mediated PEG 6000 (−0.73 MPa) and tested their ability to produce proline, exopolysaccharide, and free amino acids, all of which induce and regulate stress tolerance. The rhizobacteria were also distinguished for the production of 1-aminocyclopropane-1-carboxylate deaminase (ACCd) for the mitigation of drought stress. Among the 111 rhizobacterial isolates, 41 isolates grow above threshold limit in osmotic stress tolerance, 33 of which exhibited stress tolerance. Further characterization screened 27 isolates as capable growth promoters. Among the experimentally screened rhizobacteria, the isolates SBU4 and SDK8 [Pseudomonas fluorescens (OP627557) (PGPR1) and Pseudomonas glycinae (OP627558) (PGPR2)] exhibited the most promising phyto-beneficial potential. Both the isolates grew exponentially well during the log phase, with increased growth in Log CFU ml⁻¹ under experimentally produced osmotic stress. The drought stress tolerance test results of the SBU4 and SDK8 isolates revealed the presence of proline (1.93 μg mL⁻¹, 2.05 μg mL⁻¹), exopolysaccharide (2.19 mg mg⁻¹ protein, 2.58 mg mg⁻¹ protein), and free amino acid (11.47 μmol g⁻¹, 13.32 μmol g⁻¹) and positive growth in ACCd-enriched DF media, an ACCd assay (α-ketobutyrate (0.56 μmol/ml, 0.64 μmol/ml) and ammonia (0.37 μg mL⁻¹, 0.53 μg mL⁻¹)). The isolates SBU4 and SDK8 performed well in qualitative tests for P solubilization, N fixing ability, siderophore chelation, HCN, and ammonia production, as well as in assays involving P producers (94.57 μg mL⁻¹ and 92.86 μg mL⁻¹), siderophore units (52.14 % SU and 63.12 % SU), and IAA producers (74.63 μg mL⁻¹ and 72.64 μg mL⁻¹). These rhizobacterial isolates were optimized under various growth factors (pH, temperature, incubation) to achieve a relatively high log CFU ml⁻¹. The isolates achieved maximum Log CFU mL⁻¹ when cultured in either a specific range of pH or temperature or growth period (incubation) under standard test conditions. The growth of cultures on cross-streaked nutrient agar plates was tested for an efficient, effective consortium bioformulation that enhances growth and specific traits (drought stress mitigation) in strawberry plants.