Dual efficacy of chromium (Cr) tolerant Pseudomonas aeruginosa and indole-3-acetic acid (IAA) in conferring Cr tolerance to eggplant via improved growth and physiological aspects.

Chromium (Cr) contamination in agricultural soils, similar to other heavy metals (HMs), poses a serious threat to global food security. To mitigate its adverse effects, researchers are increasingly focusing on cost-effective and eco-friendly strategies, such as the use of Cr-tolerant plant growth-promoting rhizobacteria (PGPR). The integration of these beneficial microbes with plant growth regulators (PGRs) like indole-3-acetic acid (IAA) offers a promising approach for reducing metal bioavailability and enhancing plant tolerance. Hence, we used a pre-isolated and pre-characterized Cr-tolerant rhizobacteria 'Pseudomonas aeruginosa' and investigated it along with exogenously applied IAA (10-6 M) in improving Cr tolerance (20 mg kg-1) in eggplant. Cr-tolerance of the P. aeruginosa strain (200 mg L-1) was confirmed in an earlier study by minimum inhibitory concentration (MIC) method. Synergistic application of PGPR and IAA significantly suppressed Cr uptake in roots (33%), shoots (31%) and fruits (75%) of eggplant under soil contaminated conditions and at the same time, exhibited Cr immobilization efficiency (72%). Consequently, the treated plants exhibited marked improvements in growth and physiological attributes, including increases in plant height (174%), root and fruit fresh weights (109 and 89%), chlorophyll a, b, and carotenoids (392, 293 and 223%), SPAD value (124%), photosynthetic and transpiration rates (120 and 218%), vapor pressure deficit (105%), and water use efficiency (193%) relative to the contaminated control. Activities of different antioxidants such as SOD, POD and CAT (35, 63 and 112%) and proline contents (107%) were also found to be significantly enhanced in combined treatment. Overall, the findings demonstrate that the combined use of Cr-tolerant P. aeruginosa and IAA effectively immobilizes Cr in soil and mitigates its toxic effects, thereby enhancing eggplant growth, physiology, and stress resilience under Cr-contaminated conditions.