Microbial Inoculants as Modulators of Plant Acclimation to Drought, Salinity, and Heat Stress: Hormonal, Redox, Osmotic, Ionic, and Hydraulic Mechanisms

Abiotic stresses such as drought, salinity, or heat restrict crop productivity by disrupting plant water relations, hormonal regulation, redox homeostasis, osmotic balance, ion transport, photosynthesis, and reproductive development. Plant-associated microorganisms offer a biologically based means of supporting crop acclimation, although their effectiveness depends on the microbial strain, inoculum formulation, host genotype, environmental conditions, interaction with the resident microbiome and capacity to colonise and persist in the target system. This review critically examines the physiological, biochemical, and molecular mechanisms through which microbial inoculants, mainly bacterial, influence plant responses to drought, salinity, and heat stress. Particular attention is given to microbial modulation of abscisic acid, auxins, cytokinins, ethylene and 1-aminocyclopropane-1-carboxylate metabolism, together with less extensively characterised interactions involving gibberellins, jasmonates, salicylic acid, brassinosteroids, and strigolactones. Microbial regulation of reactive oxygen and nitrogen species, enzymatic and non-enzymatic antioxidant systems, compatible-solute metabolism, K+ and Na+ homeostasis, root hydraulic conductivity, aquaporins, and rhizosphere hydraulic properties is also evaluated. The evidence indicates that these mechanisms operate as interconnected regulatory networks rather than as independent protective processes. Particular caution is required when interpreting changes in stress markers, hormone concentrations, antioxidant activities, osmolytes, or transporter transcripts because these responses may indicate enhanced acclimation, stress alleviation, or differences in stress intensity experienced by plant tissues. Stronger mechanistic evidence is provided by microbial biosynthetic mutants, complemented strains, hormone- or signalling-impaired plants, direct hydraulic measurements, protein localisation, ion-flux analysis, and isotopic tracing. Future research should integrate microbial colonisation with plant responses and validate findings across diverse soils, genotypes, climates, and management systems. Microbial inoculants should therefore be considered context-dependent tools within integrated crop management, not universal substitutes for good agronomic practices.