Synergistic Effect of Serratia fonticola and Pseudomonas koreensis on Mitigating Salt Stress in Cucumis sativus L.

Beneficial microbes enhance plant growth and development, even under stressful conditions. Serratia fonticola (S1T1) and Pseudomonas koreensis (S4T10) are two multi-trait plant growth-promoting rhizobacteria (PGPRs) that are resistant to saline conditions. This study evaluated the synergistic effect of these PGPRs on mitigating salinity stress (200 mM) in Cucumis sativus. Presently, the synergistic effect of both strains enhances the plant growth-promoting attributes of cucumber, and the growth parameters were significantly higher than those of uninoculated plants. The PGPR-treated plants revealed a significantly higher biomass and improved chlorophyll content. The inoculation of S1T1 and S4T10 and the synergistic effect of both promoted 23, 24, and 28% increases, respectively, in the fresh biomass and 16, 19.8, and 24% increases, respectively, in the dry biomass. Similarly, S1T1 and S4T10 and their synergistic effects led to 16.5, 28.4, and 38% increases, respectively, in the water potential and 18, 22, and 28% decreases, respectively, in abscisic acid (ABA). A reduction in the electrolytic leakage (EL) was additional proof of successful PGPR activities. Similarly, a decrease in the antioxidant levels, including those of malondialdehyde (21-30%), hydrogen peroxide (19-38%), and superoxide anions (24-34%), was observed, alongside an increase in antioxidant enzymes such as catalase (22-29%) and superoxide dismutase (17-27%). Additionally, the synergistic inoculation of the PGPRs enhanced the NaCl stress tolerance by upregulating the expression of the ion transporter genes HKT1 (1-2-fold), NHX (1-3-fold), and SOS1 (2-4-fold). Conclusively, the synergistic effect of the multi-trait PGPRs significantly enhances C. sativus L. growth under salt stress.