Phytohormonal Regulation of Plant Immunity Under Biotic Stress: Mechanisms, Crosstalk, and Sustainable Management Strategies

Biotic stress caused by pathogens and herbivores remains a major constraint on crop productivity. Plant immunity is regulated by an interconnected hormonal network in which salicylic acid, jasmonic acid, and ethylene interact with abscisic acid, auxin, cytokinins, gibberellins, brassinosteroids, strigolactones, and peptide hormones. This review synthesizes evidence on hormonal control of receptor-mediated immunity, reactive oxygen species, secondary metabolism, pathogen manipulation, and growth–defense trade-offs. Three conclusions emerge. First, hormonal effects are conditional on pathogen lifestyle, infection stage, tissue, dosage, genotype, and environment; therefore, individual hormones cannot be assigned universally protective or susceptibility-promoting roles. Second, pathogens exploit hormonal crosstalk, while constitutive defense activation can reduce growth, fertility, yield, or abiotic-stress tolerance. Third, omics, biosensors, spatial profiling, and genome editing can identify regulatory nodes, but their agricultural value depends on causal validation and field performance. We propose phytohormonal immune architecture as a framework for integrating these processes. Effective deployment should prioritize inducible, tissue-specific, and dose-controlled regulation through priming, beneficial microbes, breeding, promoter engineering, and targeted delivery. Hormone-based strategies should complement established crop-protection practices and be evaluated jointly for pathogen restriction, productivity, environmental stability, and physiological cost.