Metabolic regulation of glyoxylate-cycle enzyme synthesis in detached cucumber cotyledons and protoplasts

When cotyledons are detached from cucumber (Cucumis sativus L. cv. Masterpiece) plants 14 d after seed imbibition, accumulation of the glyoxylate-cycle enzymes malate synthase (MS) and isocitrate lyase (ICL) occurs rapidly in the dark but not in the light. This is primarily due to an increase in transcript abundance. It has been proposed that glyoxylate-cycle enzymes are synthesised in response to lipid degradation. However, MS and ICL synthesis in detached cucumber cotyledons begins before a decline in lipid can be detected. Furthermore, the activation of MS and ICL gene expression does not correlate with loss of chlorophyll, carotenoid, protein or RNA. These results are discussed in terms of the activation of genes encoding glyoxylate-cycle enzymes and their possible function in detached and senescing organs. Previous work (Graham et al., 1992, Plant Cell 3, 349-357) has suggested that sucrose or other carbohydrates can repress the synthesis of MS and ICL. Consistent with this proposal, incubation of detached cotyledons in the light leads to a fivefold increase in sucrose content. However, when MS and ICL synthesis occurs in the dark, no change in the amount of sucrose is detected. Incubation of cotyledons in the dark in the presence of 25 mM sucrose leads to a complete inhibition of the accumulation of MS and ICL. Similarly, cotyledon protoplasts incubated in darkness accumulate MS and ICL, but not if 25 mM sucrose is included in the medium. These results collectively support the view that MS and ICL synthesis responds to the metabolic activities of plant cells.