Sugar co-ordinates plant defence signalling

Recognition of microbial molecules triggers energy-intensive defence systems. Although successful defence should therefore depend on energy availability, whether and, if so, how cellular metabolic information is molecularly input into defence remains unclear. We show that sugar, especially glucose-6-phosphate (G6P), plays a key role in regulating the types and amplitudes of defence outputs in Arabidopsis thaliana. Under sugar-sufficient conditions, protein and phosphorylation levels of calcium-dependent protein kinase 5 (CPK5) are elevated by induced expression and G6P-mediated suppression of protein phosphatases, priming defence responses. Furthermore, recognition of bacterial flagellin activates sugar transporters, leading to increased cellular G6P, which elicits CPK5-independent signalling promoting synthesis of the phytohormone salicylic acid (SA) involved in anti-bacterial defence. In contrast, while perception of fungal chitin does not promote sugar influx or SA accumulation, chitin-induced synthesis of the anti-fungal compound camalexin requires basal sugar influx activity. These results suggest that, by monitoring G6P levels, plants determine defence priming levels and execute appropriate outputs against bacterial and fungal pathogens. Together, our findings provide a comprehensive view of the roles of sugar in plant defence.