Endophytes-mediated mitigation of sodium stress in plants via ABA-independent signaling pathways

Plants have evolved sophisticated mechanisms to sense and respond to sodium (Na⁺) stress, coordinating molecular, physiological, and structural defenses. However, under severe or prolonged salinity, these intrinsic systems are often overwhelmed, and plants increasingly rely on external biological partners to sustain growth and productivity. Endophytes, often described as the "second genome" of plants, are increasingly recognized as pivotal allies in enhancing Na⁺ tolerance in plants. This review critically evaluates whether endophytes confer Na⁺ stress resilience primarily through abscisic acid (ABA)-independent pathways or by modulating endogenous ABA signaling, which is a central regulatory hub during abiotic stresses. While certain endophytes are known to produce ABA or influence its metabolism in specific plant species, growing evidence suggests that their protective effects largely arise via ABA-independent mechanisms. These include secretion of growth-promoting phytohormones (e.g., indole-3-acetic acid, gibberellins, cytokinins), ACC deaminase activity, production of osmolytes, ion homeostasis regulation, membrane stabilization, photosynthetic preservation, antioxidant activation, root architecture modification, and priming of systemic tolerance. However, the degree and nature of these responses can vary substantially depending on the identity of both the plant host and endophyte species, as well as environmental context. Critical gaps remain in understanding the mechanistic links between microbial activity and ABA signaling. A conceptual model is proposed wherein endophytes initiate early protective responses via ABA-independent pathways, potentially delaying or reducing reliance on energetically costly ABA-dependent mechanisms and minimizing the trade-off between stress tolerance and growth. Clarifying these diverse microbial mechanisms offers a promising strategy toward targeted endophyte-based interventions for improving crop performance in Na⁺-affected soils.