EXPRESSION OF GLYOXYLATE CYCLE GENES IN CUCUMBER ROOTS RESPONDS TO SUGAR SUPPLY AND CAN BE ACTIVATED BY SHADING OR DEFOLIATION OF THE SHOOT

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