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Photosynthetically active leaves of sugarbeet (Beta vulgaris L.) were incubated in 75 mM sucrose or glucose solution up to 24 h. The steady-state level of transcripts significantly decreased for the small subunit of ribulose-1,5bisphosphate carboxylase/oxygenase (Rubisco) (rbcS) and the cytosolic fructose-l,6-bisphosphatase and moderately for the large subunit of Rubisco (rbcL). The transcript level of sucrose phosphate synthase did not change. When leaf glucose, sucrose and starch levels were measured, internal sugar concentration was dramatically increased. Enzyme activities of fructose-l,6~-bisphosphatase and Rubisco remained unchanged in the presence of sugar. The accumulation of sugar caused the decrease of the sucrose phosphate synthase activity under the selective assay condition. 24 h after leaves had been transferred from sugar-containing solution to water, the sugar-induced repression on these genes was reversible and selective activity of sucrose phosphate synthase was recovered fully. The osmotic effect to the observed repression could be excluded because incubation with osmoticum did not lead to major changes in enzyme activities and transcript levels. Photosynthetic rates in sugar-treated leaves were declined at larger magnitudes than control, but it was mostly due to decrease of stomatal conductance rather than altered gene expression. Chlorophyll content remained relatively stable. These data provided evidence that end-products serve as molecular signals for repression of some genes encoding key enzymes of carbon metabolism and sucrose phosphate synthase appears to be differently regulated from other genes of carbon metabolism in photosynthetically active leaves.

End-Product Repression of Genes Involving Carbon Metabolism in Photosynthetically Active Leaves of Sugarbeet