Beneficial Microbes in Plant Salinity Stress Responses: Integrating Correlative Evidence With Mechanistic Frameworks

Salinity disrupts ionic balance, osmotic regulation, redox homeostasis, and hormone signaling, thereby constraining plant growth and metabolism. Plants employ adaptive responses to salinity stress, including ion transport regulation, compatible solute accumulation, antioxidant defenses, and signaling reprogramming; however, these mechanisms are often insufficient under high salinity. Beneficial microbes, including plant growth-promoting rhizobacteria, endophytes, fungi, and halotolerant taxa, are increasingly associated with improved plant performance under saline conditions. These associations are consistently linked with changes in ion homeostasis, osmotic adjustments, redox balance, hormone signaling, and root architecture. Importantly, most available evidence derives from transcriptomic, biochemical, and physiological observations, which do not establish direct mechanistic regulation. This review critically evaluates microbial contributions to plant salinity responses by explicitly distinguishing between experimentally validated mechanisms, correlative associations, and hypothesis-driven models. We integrate insights from molecular genetics, biochemistry, and multi-omics approaches while highlighting their limitations in establishing causality. Particular emphasis is placed on experimental strategies required to establish causal mechanisms, including isotope tracing, genetic perturbation, and synthetic community approaches.