Abstract
Expression of some α‐amylase genes in cereals is suppressed by sugars and activated by sugar starvation. A 100‐bp sugar response sequence (SRS) identified in the promoter of a rice α‐amylase gene, αAmy3, contains three essential motifs: the GC box, the G box, and the TATCCA element. To study the mechanism of sugar regulation of αAmy3 transcription, an ABA‐responsive bZIP protein, OsBZ8, which binds specifically to the G box in αAmy3 SRS was characterized and function analysed. In sucrose‐starved rice suspension cells and embryos, decline in OsBZ8 mRNA levels coincided with the induction of αAmy3 mRNA accumulation. In vivo gain‐ and loss‐of‐function studies by transient expression assays in rice embryos revealed that OsBZ8 suppresses SRS activity through the G box and overrides the activity of an activator, OsMYBS1, which binds to the TATCCA element. Gel mobility shift assays revealed that OsBZ8 binds specifically to the G box in vitro. These studies suggest that OsBZ8 is a suppressor responsible for sugar repression of αAmy3 expression, and OsMYBS1 is responsible for sugar starvation induced expression of αAmy3.
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