Efficacy of endophytic SB10 and glycine betaine duo in alleviating phytotoxic impact of combined heat and salinity in Glycine max L. via regulation of redox homeostasis and physiological and molecular responses

Adverse environmental stresses occurring simultaneously exhibit a lethal effect on crop productivity at the global level. Here, we investigated the individual and synergistic effects of endophytic T. virens SB10 and glycine betaine (GB) on the physiological and biochemical responses of Glycine max L. to alleviate the devastating effects of combined heat and salinity (HS) stress. Screening against HS stress tolerance showed that SB10 has significant tolerance against heat stress and produces hormones such as gibberellins and indole-3-acetic acid upon GB amendment of the growth medium under HS stress. Moreover, the current findings illustrated that the synergistic application of SB10 and GB was effective in alleviating the negative effects of HS stress on plant growth and physiology. The findings revealed that SB10 + GB led to a reduction in proline accumulation and Na+ uptake. It also maintained a high K+/Na + ratio by regulating GmHKT1 and GmSOS1 expression and enhanced macronutrient uptake (N, Ca, K) in plants. In turn, plants exhibited a higher growth rate and gaseous exchange attributes coupled with the upregulation of APX, SOD, POD, and GSH antioxidant activities and transcript accumulation of GmSOD1 and GmAPX1 to overcome HS-induced oxidative damage. Furthermore, SB10 + GB downregulated DREB2, DREB1B, and GmNCED3 expression and resulted in the reduced accumulation of endogenous ABA while enhancing endogenous SA accumulation via upregulation of PAL genes. In addition, enhanced accumulation of bioactive gibberellins (GA1, GA3, GA4, and GA7) was detected under HS stress in the SB10 + GB treatment group. Moreover, SB10 + GB also significantly regulated GmHsp90A2 and GmHsfA2 expression in tolerance against HS stress. The combination of SB10 and GB was shown to be an effective and alternative approach for growing G. max at high temperature coupled with saline conditions for sustainable agriculture.