Holistic phytoanalysis of salt-stressed Solanum lycopersicum L. enhanced by synergistic hexagonal crystalline ZnO nanoparticles and diazotrophic PGP rhizobacterial consortium

The findings demonstrate a cutting-edge approach to developing elevated salt stress resilience at 900 mM NaCl in Solanum lycopersicum L through synergistic co-inoculation of hexagonal crystalline ZnO nanoparticles (ZnONPs) and a diazotrophic plant growth-promoting rhizobacterial (PGPR) consortium comprising Pseudomonas aeruginosa PM36, Bacillus Sp. PM37, and Bacillus cereus PM38. The co-inoculation significantly enhanced the synthesis of vital phytohormones, including indole-3-acetic acid, gibberellic acid, and kinetin, under salt stress. These phytohormones are associated with enhanced production of 2,3 DHBA, phosphate solubilization, exopolysaccharide, and ACC deaminase activity. The amplification of genes related to plant growth-promoting (PGP) and stress response, particularly nifH, acdS, pqqE, ituC, and epsA, validates the efficacy of co-inoculating PGPR consortium with ZnONPs. This synergistic approach markedly improved plant growth, increasing cumulative biomass (45.9 %) and leaf surface area (35.5 %). It also increased photopigments (chlorophyll: 46.94 %, carotenoids: 45.32 %) and osmoprotectants (proline: 15.45 %, proteins: 39.89 %, soluble sugars: 30.19 %, glycine betaine: 24.1 %), all of which are important for osmotic adjustment and stress tolerance. The co-inoculation significantly increased the expression levels of the antioxidant enzymes catalase (49.6 %) and superoxide dismutase (44.1 %) in response to high salt stress. This co-inoculation modulates the activities of antioxidant enzymes to an increase in the inhibitory concentration value of DPPH (32 %), relative water content (32.10 %), alongside decreases in electrolyte leakage (34.6 %), malondialdehyde (76.9 %), and hydrogen peroxide (42.8 %). The results indicate that this combination is highly effective in enhancing reactive oxygen species (ROS)-neutralizing enzymes, specifically ascorbate peroxidase (27.4 %) and peroxidase (41.5 %), which accompany this molecular response. The combined application of PGPR and ZnONPs improves molecular and enzymatic defense mechanisms while promoting robust growth and physiological performance under high salt stress. This approach presents a promising avenue for developing salt-tolerant crops, thereby contributing to the sustainability of agricultural practices in saline environments.