When cucumber roots are excised and incubated without a carbon source, isocitrate lyase (ICL) and malate synthase (MS) mRNAs increase significantly in amount. However, if sucrose is added to the excised roots, the mRNAs do not accumulate. Hairy roots obtained by transformation with Agrobacterium rhizogenes show the same response. Transgenic hairy roots containing the Icl and Ms gene promoters fused to the GUS reporter gene, have very low GUS activity which increases dramatically when roots are incubated in the absence of sugar, indicating regulation at the transcriptional level. Staining of sugar-deprived roots shows that GUS activity is concentrated mainly in root tips and lateral root primordia, where demand for carbohydrate is greatest. In order to determine if Icl and Ms genes are expressed in roots of whole plants under conditions which may occur in nature, cucumber plants were subjected to reduced light intensity or defoliation. In both cases increases were observed in ICL and MS mRNAs. These treatments also reduced root sugar content, consistent with the hypothesis that sugar supply could control expression of Icl and Ms genes in roots of whole plants.