| Summary: | The glyoxylate cycle is known to take part in the net conversion of storage lipids to sugar germinating oilseeds. Two enzymes are exclusive for this cycle, malate synthase (MS) and isocitrate lyase (ICL) and their synthesis is coordinately regulated. These enzymes are active during postgerminative growth of seeds but are repressed in mature plants. However, they appear again when plants senesce. Genes for both enzymes are regulated by carbohydrate status. The aim of this study was to examine carbohydrate regulation. Expression of <I>Ms </I>and <I>Icl</I> genes in cucumber roots was low but increased upon excision and dark-incubation during a six day period in the absence of exogenous sugar. However, when sucrose was added to the incubation medium their expression was repressed. Hairy roots obtained using <I>Agrobacterium rhizogenes</I> strain A4 showed the same pattern of expression. Transgenic hairy roots containing <I>Ms </I>and <I>Icl</I> promoters fused to the GUS reporter gene, had a low level of GUS activity. This GUS activity increased dramatically when roots were excised and incubated in the absence of sugar, indicating regulation at the transcriptional level. Histochemical staining showed that GUS activity is concentrated in root tips and lateral root primordia where demand for carbohydrate is presumably greatest. Defoliation and shading experiments were carried out to examine the expression of <I>Ms </I>and <I>Icl</I> in roots of whole plants under natural conditions. In both cases, MS and ICL mRNA increased and roots showed a decline in sugar content. Thus, indication of <I>Ms </I>and <I>Icl</I> expression takes place in roots when supply of carbohydrate from the shoot is impaired. Results are consistent with the hypothesis that gene expression in the roots is controlled by carbohydrate supply from the shoot. Identification of regulatory elements in the <I>Icl</I> promoter required for the sugar response were made possible using transgenic cucumber hairy roots. Deletions of the <I>Icl</I> gene were assayed, which located a 200 bp region necessary for the sugar response more than 1 kbp upstream of the transcriptional start.
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