Consortium of Endophytic Bacillus australimaris CK11 and Staphylococcus epidermidis CK9 from Commiphora gileadensis Mediates Tomato Resilience to Combined Salinity, Heat, and Drought Stresses

Climate change poses a critical threat to global agriculture. Plant growth-promoting bacteria (PGPB) present a sustainable approach to increase climate resilience. The study focused on isolating and screening abiotic stress-resistant endophytic bacteria from the Arabian balsam tree (Commiphora gileadensis); these bacteria can lessen the phytotoxic impacts of heat, salinity, and drought stress. C. gileadensis is known for its resilience to diverse abiotic stresses and hosts a diverse array of PGPB. Isolated endophytic bacteria were evaluated for their growth-promoting activities, including phosphate and silicate solubilization and indole3-acetic acid production, and screened for tolerance to multiple abiotic stresses. Out of 20 distinct endophytic bacterial isolates exhibiting various plant growth-promoting (PGP) traits, the Staphylococcus epidermidis CK9 strain and the Bacillus australimaris CK11 strain demonstrated remarkable resilience to a range of abiotic stresses, including heat, salinity, and drought. Tomato inoculation with sole or a consortium of CK9 and CK11 under combined abiotic stresses led to significantly enhanced plant growth attributes and photosynthetic pigments (chlorophyll a, b and carotenoids), reduced Na⁺ uptake and maintained a high K⁺/Na⁺ ratio. Combined abiotic stress-induced oxidative stress (lipid peroxidation and superoxide anion) was significantly counteracted by the enhanced accumulation of antioxidant activities (catalase and peroxidase) and upregulated expression of Glutathione reductase and catalase (CAT) genes compared with noninoculated plants. Co-inoculation promoted phytohormones crosstalk by downregulating abscisic acid and jasmonic acid accumulation while stimulating salicylic acid accumulation under stress conditions. This hormonal crosstalk significantly induced abiotic stress-related heat shock protein (HSP) genes (HSP70 and HSP90) compared to noninoculated plants. This study provides valuable insights into the potential use of PGPB from C. gileadensis as a bioinoculant for enhancing tomato growth and yield under combined abiotic stress conditions. Future research will focus on the field assessment of this consortium in hot weather under saline- and drought-induced stresses to determine their effect on crop productivity.