International Journal of Environment and Climate Change

Drought tolerance depends on coordinated adjustments in root water relations, metabolism, hormonal signalling, redox homeostasis and shoot physiology. Endophytic microorganisms can influence these processes, yet the links between microbial recruitment, internal root establishment and systemic drought adaptation remain incompletely resolved. This review synthesises evidence concerning the proposed sequence from drought-induced changes in root physiology and exudation to microbial attraction, attachment, root entry, endophytic colonisation, host signalling and whole-plant responses. Root exudates may affect microbial chemotaxis, motility and biofilm formation, whereas successful entry can depend on developmental root zones, microbial motility and modification of cell-wall barriers. Internal establishment also requires interactions with host immune regulation and signalling pathways associated with calcium, reactive oxygen species and abscisic acid. Evidence from experimentally studied endophytes, particularly Piriformospora indica, links local root colonisation with changes in distal-root responses, drought-associated gene expression, antioxidant protection, osmotic adjustment and photosynthetic performance. However, microbial association at the root surface does not establish internal colonisation, and improved physiological performance does not identify a particular long-distance signal. The available evidence supports a framework in which endophytes modify endogenous plant signalling and physiological processes, with potential systemic consequences under water deficit. Direct causal links among recruitment, internal establishment and root-to-shoot signalling remain insufficiently resolved. Further research integrating spatially verified colonisation with temporal signalling measurements and realistic drought treatments is needed to test this proposed continuum.