Nitrate is a negative signal for fructan synthesis, and the fructosyltransferase-inducing trehalose inhibits nitrogen and carbon assimilation in excised barley leaves.

•  Fructan biosynthesis in barley (Hordeum vulgare) has been shown to be upregulated by sugar signalling and downregulated by nitrogen. The relationship between these two regulations is investigated. •  Excised third-leaves of barley were fed nitrate or glutamine under two light intensities. Other leaf blades were supplied in the dark for 24 h with nitrate and trehalose in the presence of validamycin A, a trehalase inhibitor. •  In the light, nitrate, but not glutamine, decreased fructan contents and sucrose:fructan 6-fructosyltransferase protein without affecting the levels of sucrose and other carbohydrates. In darkened leaves, trehalose increased and nitrate decreased the fructan contents and total sucrose:fructosyltransferase activity without altering the concentration of sucrose. The effect on fructan contents of trehalose disappeared, whereas that of nitrate remained in subsequent incubations in water under light. Trehalose decreased and nitrate increased the light- and CO2 -saturated rate of photosynthesis without significantly affecting the initial Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase) activity. Trehalose feeding decreased the activation of nitrate reductase and amino acid levels, and blocked the positive effect of nitrate on the maximal activity of this enzyme. •  The results indicate that nitrate, and not a downstream metabolite, is a negative signal for fructan synthesis, independent from the positive sugar signalling and overriding it. Trehalose signalling inhibits nitrogen and carbon assimilation, at the same time, inducing fructosyltransferase activity.