Kyoko Toyofuku

Promoter elements required for sugar-repression of the RAmy3D gene for α-amylase in rice

There is increasing evidence showing that cereal α‐amylase gene expression is controlled not only by the classical hormonal regulation, but also by feed‐back sugar repression. We demonstrated by in situ hybridization that the sugar repression of rice α‐amylase gene RAmy3D takes place in scutellar epithelium cells of callus‐forming rice embryos. We also used a transient expression system to study the cis‐acting elements involved in the sugar repression of the RAmy3D promoter activity. Site‐directed mutagenesis of the 50‐bp nucleotide sequence from −172 to −123 revealed that consensus sequences of G motif (TACGTA) and TATCCA T/C motif (GATA motif as its antisense sequence) are responsible for sugar repression. The promoter sequences required for sugar repression are reported and discussed.

Sugar transporters involved in flowering and grain development of rice

Developing seeds offer a fine experimental system for study on sugar transport mechanism(s) in sink tissue. The sugar transporters identified as being specific to the seed development may play a crucial role for flowering and grain development. The rice sucrose transporter OsSUTI and the monosaccharide transporters OsMST1-3 have been previously characterized. To investigate sugar transport processes during flowering and in developing grains of rice, we newly isolated two genomic clones OsSUT2 (Oryza sativa sucrose transporter 2) and OsMST5 (Oryza sativa monosaccharide transporter 5) and their corresponding cDNAs. OsSUT2 and OsMST5 are encoded by open reading frames of 1485 and 1557bp encoding 495 and 519 amino acids, respectively. The putative amino acid sequence of OsSUT2 showed 63.0 and 62.4 % identity to that of OsSUTI and barley transporter HvSUT1, respectively. Northern blot analysis revealed that OsSUT2 and OsMST5 mRNA accumulates in panicles before pollination. In situ hybridization analysis showed that OsSUT2 transcript is specific to the developing pollen. These data presented suggest that both OsSUT2 and OsMST5 play a role during the development at the early stage of the seed development. On the other hand OsSUTI was expressed at the early stage of the grain development, suggesting a different physiological role compared to OsSUT2.

Glucose modulates the abscisic acid-inducible Rab16A gene in cereal embryos

Glucose effects on the expression of the abscisic acid-inducible Rab16A gene were examined in rice and barley embryos. Glucose feeding to rice embryos negatively affects the endogenous abscisic acid content and represses the promoter activity of the Rab16A gene. Glucose repression of the Rab16A gene takes place both at a transcriptional and a post-transcriptional level. Modulation of the abscisic acid content in rice embryos triggered by glucose did not directly influence the expression of the rice α-amylase gene RAmy3D, which is known to be under glucose control. The possible interaction between the glucose and abscisic acid signaling pathway is discussed.

Characterization of Rice Functional Monosaccharide Transporter, OsMST5

cDNA of a monosaccharide transporter in rice, OsMST5 ( Oryza sativa monosaccharide transporter 5) was cloned and its sugar transport activity was characterized by heterologous expression analysis. The amino acid sequence and topology were similar to the sequences and topology of other plant monosaccharide transporters. Yeast cells co-expressed with OsMST5 cDNA transported some monosaccharide substrates. The transport rate increased when ethanol as an electron donor was added, so the transporter was an energy-dependent active one. Most of the OsMST5 was expressed in panicles before pollination, indicating that it is associated with pollen development in rice.

Expression of α-Amylase Isoforms and the RAmy1A Gene in Rice (Oryza sativa L.) during Seed Germination, and its Relationship with Coleoptile Length in Submerged Soil

Summary A significant difference in the seed germinability was observed between the two rice cultivars, ‘Nipponbare’ and ‘Suweon 287’, under anoxia (i.e., during germination in submerged soil at 18°C), although little difference was seen under aerobic (in air) or hypoxic (in water) conditions. The number of α-amylase isoforms synthesized in germinating seeds was inversely proportional to the O2 concentrations at the early germination stage. The formation of isoform B was promoted by oxygen supply, while isoform H was undetectable if the seeds were unable to germinate. The activity of isoform H was highly correlated with the coleoptile length in the submerged soil at 18°C, indicating that isoform H is a critical factor for seed germination under anoxia. The expression of the rice α-amylase RAmylA gene was repressed when the seeds germinated under hypoxia or anoxia. The interactions between oxygen stress, gibberel-lin, and carbon metabolites on the expression of α-amylase in rice are discussed.