Multifaceted Rhizobacterial Co-Inoculation Enhances Drought-Stress Tolerance in Tomato: Insights Into Physiological, Biochemical, and Molecular Responses.

Drought-tolerant multifunctional soil bacteria can increase drought tolerance mechanisms in plants. Here, rhizobacteria CRB-4 and SPGPR-11 were isolated and their single and co-inoculation effect was evaluated in drought-stressed tomato plants. Isolates were selected based on their preliminary polyethylene glycol (PEG) screening, and plant growth-promoting properties. Increasing water stress adversely affected growth and physiological attributes of tomato plant. However, plant growth-promoting bacteria (PGPB), particularly their combined inoculation, alleviated drought stress. For instance, CRB-4, SPGPR-11 and their co-inoculation significantly increased root biomass (33.3, 37.5% and 45.4%), total chlorophyll (17.5, 15.6% and 19.2%) and carotenoid content (20, 30.4% and 48.3%) in 3%-PEG-stressed tomatoes. Similarly, co-inoculation of 3%-PEG-treated plants with PGP isolates resulted in a significant increase in Fv/Fm (50%), Fv'/Fm' (29.4%), PS-II (44.4%), Pq (40%), NPQ (40%), and effective electron transfer rate (37.5%). Furthermore, under 5%-PEG stress, CRB-4, SPGPR-11, and their co-inoculation enhanced drought stress resilience in tomato by improving leaf gas exchange attributes. Combined inoculation significantly enhanced gs (19%), Ci (31.2%), transpiration rate (41%), water vapor deficit (38.7%), iWUE (33.7%), and photosynthetic rate (33.3%) in 5%-PEG-stressed tomatoes. Among the treatments, co-inoculations significantly enhanced the antioxidant defense responses in drought-stressed tomatoes. Concurrently, qRT-PCR analysis revealed a significant upregulation in ROS scavenging genes, SOD, CAT, APX, GR, and POD, by 6.53, 14.08, 11.72, 10.12, and 5.95-fold, respectively, in drought-stressed plants co-inoculated with bacterial strains. This study concludes that PGP isolates CRB-4 and SPGPR-11, alone or in combination, offer an effective, eco-friendly solution for improving drought resilience in tomatoes.