Auxin signaling networks and hormonal cross-talk in plant responses to biotic stress.

Plants are continuously challenged by biotic stressors, including pathogens, pests, and parasitic organisms, which threaten their growth, development, and productivity. As sessile organisms, plants depend on intricate signaling networks to perceive these threats and activate appropriate defense responses. Auxin, a central regulator of plant growth and development, has emerged as a key modulator of plant-pathogen and plant-insect interactions. Beyond its classical role in developmental plasticity, auxin influences plant defense by shaping transcriptional regulation, modulating immune signaling pathways, and coordinating metabolic adjustments. Recent studies have revealed extensive cross-talk between auxin and other phytohormones, such as jasmonic acid (JA), salicylic acid (SA), ethylene (ET), and cytokinin (CK), which fine-tunes the balance between growth and immunity. Despite these advances, the molecular mechanisms underlying auxin-mediated responses to biotic stress remain incompletely understood, particularly with respect to hormonal cross-communication and context-dependent signaling. This review synthesizes recent progress in elucidating the molecular basis of auxin signaling under biotic stress, with an emphasis on hormone interactions, regulatory networks, and downstream defense responses. Advancing this mechanistic understanding will create new opportunities to engineer crop resilience and foster sustainable agricultural practices.