Salicylic Acid-Centred Defence Signalling and Hormonal Crosstalk under Natural Farming Systems: Mechanisms of Disease and Insect Suppression

Abstract

Salicylic acid (SA) occupies a central and integrative role in plant immunity, mediating systemic acquired resistance against biotrophic and hemibiotrophic pathogens whilst engaging in extensive crosstalk with jasmonic acid, ethylene, and abscisic acid signalling pathways. Under natural farming systems — encompassing organic cultivation, agroecological management, and related low-input approaches — the chemical milieu and microbial architecture of the rhizosphere undergo substantial restructuring in ways that carry consequential effects on plant hormone physiology and defence capacity. This review critically examines the molecular basis of SA biosynthesis, receptor-mediated signal perception, and systemic immunity, with particular attention to the antagonism between SA and jasmonic acid signalling that fundamentally shapes how plants allocate defensive resources between biotrophic pathogens and insect herbivores. The roles of beneficial rhizosphere microorganisms enriched under natural farming — including plant growth-promoting rhizobacteria, arbuscular mycorrhizal fungi, and biocontrol agents such as Trichoderma and Bacillus species — in priming SA-mediated and induced systemic resistance pathways are evaluated. The significance of organic soil amendments, including composts and vermicomposts, in modulating endogenous SA concentrations and influencing defence gene expression is also examined. Particular attention is given to the trade-offs inherent in SA–jasmonic acid hormonal antagonism, which may affect resistance to chewing herbivores in agroecosystems. The review concludes that natural farming systems, through their enhancement of rhizosphere microbial diversity and activity, create conditions broadly conducive to plant defence priming, though further field-based molecular studies are needed to translate these insights into durable management strategies.