Kazuyuki Saitou

Activation of ADP-Glucose Pyrophosphorylase Gene Promoters by a WRKY Transcription Factor, AtWRKY20, in Arabidopsis thaliana L. and Sweet Potato (Ipomoea batatas Lam.)

Abstract 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.

Regulation of Expression of D3-type Cyclins and ADP-Glucose Pyrophosphorylase Genes by Sugar, Cytokinin and ABA in Sweet Potato (Ipomoea batatas Lam.)

Abstract The productivity of sweet potato is governed by both the rate of cell division and sink activity of its tuberous root. The aim of this study was to reveal the mechanisms that regulate cell division activity and sink activity during tuberous root formation. As an indicator of the cell division activity, we used the transcript level of two D3-type cyclins, which regulate cell cycle progression through the formation of the regulatory subunit of the cyclin-dependent kinase complex. As an indicator of photosynthetic product sink activity, we used the gene expression of ADP-glucose pyrophosphorylase (AGPase), one of the key enzymes of starch synthesis. During tuberous root formation, the expression of D3 cyclin genes increased to the maximal levels and then decreased. In contrast, the expression of the AGPase gene increased continuously. Sucrose enhanced the expression of D3 cyclin and AGPase genes, but a high concentration of sucrose repressed the expression of a D3 cyclin gene. In the presence of sucrose, cytokinin increased the expression of D3 cyclins, but abscisic acid (ABA) did not. However, cytokinin and ABA repressed the induction of AGPase gene expression by sucrose. These results suggested that sugars, cytokinin and ABA regulate the cell division activity and the sink activity in sweet potato.

Effects of Nitrogen on the Expression of Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Small Subunit Multigene Family Members in Rice (Oryza sativa L.)

Abstract Five ribulose-1,5-bisphosphate carboxylase/oxygenase small subunit genes identified in the rice (Oryza sativa) genome are designated as OsRBCS1, 2, 3, 4 and 5. OsRBCS1 transcripts were not detectable. The mRNA levels of OsRBCS2, OsRBCS3, OsRBCS4 and OsRBCS5 in leaf blades were increased by NH4NO3 supply to nitrogen-starved plants, but the extent of increment varied with the genes. The mRNA levels of OsRBCS2, OsRBCS3 and OsRBCS4 in leaf blades were increased by NH4NO3 or glutamine supply to detached shoots. On the other hand, NH4NO3 and glutamine had little effect on the expression of OsRBCS5 gene. The mRNA level of OsRBCS4 was increased only by cytokinin supply to the detached shoots. The expression of the promoters of OsRBCS2 and OsRBCS3 was enhanced by NH4NO3 supply.