Sugars as Central Integrators of Plant Immunity and Stress Responses

Sugars are fundamental metabolites that sustain plant growth and development, but accumulating evidence demonstrates that they also function as regulatory hubs in plant defence. In this review, we synthesise current knowledge on how carbohydrate dynamics coordinate metabolism, signalling, and structural responses to shape plant resistance against both pathogens and herbivores. We highlight how spatial and temporal regulation of sugar transport, mediated by SWEET, SUT/SUC, and STP transporters, controls carbon availability at plant-heterotroph interfaces, while sucrose-cleaving enzymes such as invertases and sucrose synthase modulate local sugar composition to support defence metabolism or restrict nutrient access to attackers. Beyond metabolic functions, sugars act as signalling molecules that regulate immune responses through complex sensing and energy-signalling networks. Central to this framework are trehalose-6-phosphate and the antagonistic kinases SnRK1 and TOR, which integrate carbon status with growth-defence decisions. These pathways are closely interconnected with phytohormone signalling, forming a bidirectional regulatory network that enables plants to tailor defence responses according to attacker lifestyle, including biotrophic and necrotrophic pathogens as well as sap-sucking and chewing herbivores. In parallel, carbohydrate-based cell wall remodelling reinforces physical barriers and generates damage-associated molecular patterns that activate immune signalling. We further discuss how carbohydrates contribute to immune priming, enhancing resistance while minimising fitness costs. Collectively, this review highlights sugars as central regulators that coordinate carbon allocation with immune activation across multiple biological scales, providing promising opportunities for developing sustainable strategies to enhance crop resistance against pathogens and herbivores.