ADP–glucose pyrophosphorylase (AGPase) catalyzes the first limiting step in starch biosynthesis in plants. However, the direct transcriptional activator of the AGPase genes has not yet been determined. We have isolated a WRKY transcription factor cDNA,
AtWRKY20, from
Arabidopsis thaliana and purified the corresponding protein. Transient expression of
AtWRKY20 by particle bombardment enhanced expression of the promoter of
ApL3, encoding a sugar-inducible AGPase large subunit gene of
A. thaliana, in leaves of
A. thaliana. AtWRKY20 bound to the
ApL3 promoter
in vitro. The expression of
AtWRKY20 was strongly induced by sucrose or, to a lesser extent, by mannitol, and the expression pattern of the
ApL3 gene mimicked that of the
AtWRKY20 gene. Transient expression experiments demonstrated that AtWRKY20 also activated the promoter of Koganesengan
ibAGP1 encoding an AGPase small subunit gene of sweet potato var. Koganesengan. A 5’–end deletion analysis revealed a negative regulatory region from –1371 to –641 and a positive regulatory region from –640 to –180 in the Koganesengan
ibAGP1 promoter. AtWRKY20 interacted directly with the region between positions –623 and –490 in the Koganesengan
ibAGP1 promoter. These results suggest that AtWRKY20 functions directly as a transcriptional activator of the
ApL3 promoter and regulates the expression of
ApL3 induced by sucrose or osmoticum in
A. thaliana. Moreover, AtWRKY20 can enhance the expression of the Koganesengan
ibAGP1 promoter directly in sweet potato.
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