Abstract
Expression of alpha-amylase genes in both rice suspension cells and germinating embryos is repressed by sugars and the mechanism involves transcriptional regulation. The promoter of a rice alpha-amylase gene alphaAmy3 was analyzed by both loss- and gain-of-function studies and the major sugar response sequence (SRS) was located between 186 and 82 base pairs upstream of the transcription start site. The SRS conferred sugar responsiveness to a minimal promoter in an orientation-independent manner. It also converted a sugar-insensitive rice actin gene promoter into a sugar-sensitive promoter in a dose-dependent manner. Linker-scan mutation studies identified three essential motifs: the GC box, the G box, and the TATCCA element, within the SRS. Sequences containing either the GC box plus G box or the TATCCA element each mediated sugar response, however, they acted synergistically to give a high level glucose starvation-induced expression. Nuclear proteins from rice suspension cells binding to the TATCCA element in a sequence-specific and sugar-dependent manner were identified. The TATCCA element is also an important component of the gibberellin response complex of the alpha-amylase genes in germinating cereal grains, suggesting that the regulation of alpha-amylase gene expression by sugar and hormone signals may share common regulatory machinery.
Highlights
Expression of ␣-amylase genes in both rice suspension cells and germinating embryos is repressed by sugars and the mechanism involves transcriptional regulation
The sugar-dependent repression of ␣-amylase gene expression provides an ideal model for studies on the molecular mechanisms that mediate glucose repression in plants. ␣-Amylases are endo-amylolytic enzymes which catalyze the hydrolysis of ␣1,4-linked glucose polymers that play an important role in the degradation of starch and glycogen in higher plants, animals, and many microorganisms. ␣-Amylases in plants are recognized as essential enzymes whose major function is hydrolysis of starch stored in the endosperm during germination of cereal grains
Studies with rice suspension cells have shown that ␣-amylase expression, carbohydrate metabolism, and vacuolar autophagy are coordinately regulated by sucrose levels in the medium [20]
Summary
Expression of ␣-amylase genes in both rice suspension cells and germinating embryos is repressed by sugars and the mechanism involves transcriptional regulation. The sugar-dependent repression of ␣-amylase gene expression provides an ideal model for studies on the molecular mechanisms that mediate glucose repression in plants. Studies with rice suspension cells have shown that ␣-amylase expression, carbohydrate metabolism, and vacuolar autophagy are coordinately regulated by sucrose levels in the medium [20]. Both the transcription rate and mRNA stability of ␣-amylase genes in cells increase in response to sucrose depletion in the culture medium [12]. Use of transgenic rice carrying an ␣-amylase gene promoter--glucuronidase (GUS) gene proved that the regulation
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