Auxin metabolism and signaling: Integrating independent mechanisms and crosstalk in plant abiotic stress responses

Plants encounter various abiotic stresses that substantially impact their growth, development, and productivity. As immobile organisms, plants rely on sophisticated stress sensing, signaling, and regulation mechanisms to adapt and survive under challenging conditions. Auxin, a major plant growth regulator, plays a central role in modulating these responses by influencing various molecular, biochemical, and physiological processes. It is synthesized through multiple biosynthetic pathways and tightly regulated via metabolism, transport, and signal transduction. Auxin directly governs stress responses through distinct, stress-specific mechanisms that operate independently of other hormones, as supported by recent studies on auxin-regulated gene expression and signaling modules under individual stress conditions. In parallel, auxin acts as a central integrator in hormonal crosstalk networks, interacting with abscisic acid (ABA), brassinosteroids (BRs), cytokinin (CK), ethylene (ET), jasmonic acid (JA), melatonin (Mel), and salicylic acid (SA) to fine-tune adaptive responses. This review delves into the dual aspects of auxin's role by first analyzing its stress-specific, independent mechanisms and then exploring its integrative functions through hormonal crosstalk during abiotic stress conditions. By shedding light on these distinct regulatory frameworks, this review emphasizes auxin's multifaceted role in enhancing plant resilience and explores potential agricultural strategies for improving stress tolerance.