Background and Aims: The main soluble sugars are important components of plant defence against pathogens, but the underlying mechanisms are unclear. Upon infection by Botrytis cinerea, the activation of several sugar transporters, from both plant and fungus, illustrates the struggle for carbon resources. In sink tissues, the metabolic use of the sugars mobilized in the synthesis of defence compounds or antifungal barriers is not fully understood.Methods: In this study, the nitrogen-dependent variation of tomato stem susceptibility to B. cinerea was used to examine – before and throughout the course of infection – the transcriptional activity of enzymes involved in sugar metabolism. Under different nitrate nutrition regimes, the expressions of genes that encode the enzymes of sugar metabolism (invertases, sucrose synthases, hexokinases, fructokinases and phosphofructokinases), as well as sugar contents, were measured before inoculation and in asymptomatic tissues surrounding the lesions after inoculation.Key Results: At high nitrogen availability, decreased susceptibility was associated with the over-expression of several genes two days after inoculation: sucrose synthase Sl-SUS1 and Sl-SUS3, cell wall invertase Sl-LIN5 to Sl-LIN9 and some fructokinase and phosphofructokinase genes. By contrast, increased susceptibility corresponded to the early repression of several genes that encode cell wall invertase and sucrose synthase. The evolution of sugar contents was coherent with the gene expressions.Conclusions: The activation of specific genes which encode sucrose synthase is required for enhanced defence. Since the over-expression of fructokinase is also associated with reduced susceptibility, it can be hypothesized that supplemental sucrose cleavage by sucrose synthases is dedicated to the production of cell wall components from UDP-glucose, or to the additional implication of fructose for the synthesis of antimicrobial compounds, or both.Read less <