@misc{oai:niigata-u.repo.nii.ac.jp:00004690, author = {Asatsuma, Satoru}, month = {Sep}, note = {The aim of this reserch is clarify details of funtional expression of α-amylase in rice. Studies of suspension-cultured cell system provided evidences that phosphate is a potent stimulator for α-amylase secretion and starch degradation. Studies of transgenic rice plants with suppressed expression or overexpression of α-amylase showed that α-amylase is involved in degradation of plastidial starch in living cells. These results indecate that there exist multi-subcellular localization of α-amylase encoded by single gene in rice. Furthermore, α-amylase I-1 fused with green fluorescence protein was targeted into the plastids in bombarded onion epidermal cells, indicating that rice α-amylase I-1 molecule contains a plastid targeting signal common to both rice and onion cells. The cotents are follows. Novel Regulatory Factor for α-Amylase Secretion and Stach Degradation in Rice Cell Culture Effects of phosphate on the Ca2+ uptake and the sucrose-controlled secretion of α-amylase molecules in cultured cells were investigated, Phosphate markedly stimulated Ca2+ uptake into rice cells, particulary at the outer cell layer of the cell cluster. Phosphate at 10 mM was found to increse the synthesis and extracelluar liberaton of α-amylase II-4 molecules in the sucrose-supplemented cells. The distribution pattern of enzyme in rice cell clusters induced by phosphate was similar to that of Ca2+ uptake, Phosphate did not increase the level of mRNA of α-amylase II-4, indicating that phosphate stimulates the translation and posttranslational secretory processes of α-amylase II-4 molecules in the presence of sucrose. Furthermore, phosphate enhanced both the Ca2+ uptake and α-amylase II-4 synthesis in the microsomes. These results strongly suggested that the ratio of phosphate to sugar is important for regulating the Ca2+ uptake, and that phosphate and sugar precisely coordinate the Ca2+-mediated synthesis and extracelluar liberation of α-amylase II-4 molecules in rice cells. In addition, phosphate reduced the starch content in rice cells, same as the sugar-starved cells. The results may indicate that α-amylase plays a role for degrading starch in living cells. Involment of α-Amylase in Starch Degradation in Rice Plastids To determine role of α-amylase isoform I-1 in the degradation of starch in rice leaf chloroplasts, I renerated a series of transgenic rice plants with suppressed expression of α-amylase I-1. In the lines with suppressed expression or overexpression of α-amylase I-1 at both mRNA and protein levels, seed germinaton and seedling growth were markedly delayed in comparison with those in the wild-type plants., Houever, the growth retardation was overcome by supplementation of sugars. Interestingly, a significant increse of starch accumulation in the young leaf tissues was observed under a sugar-supplemented condition. In contrast, the starch content of leaves was reduced in the plants overexpressinf α-amylase i-1. In immunocytochemicalanalysis with specific anti-α-amylase I-1 antiserum, immuno-gold particles deposited in the choroplasts and estracellular space in young leaf cells. I further examined the expression and targeting of α-amylase I-1 fused with the green fluorescent protein in re-differentiated green cells, and showed that the fluorescence of expressed fusion protein co-localized with the chlorophyll autofluorescence in the transgenic cells. In addition, mature protein species of α-amylase I-1 bearing an oligosaccharide side chain were detected in the isolated chloroplasts. Based on these results, I concluded that α-amylase I-1 targets the chloroplasts through the ER-Golgi system and plays a significant role in the starch degradation in rice leaves. Overexpression of α-Amylase affects Starch accumulation in Ripening Seeds of Rice To determine physiological function of α-amylase isoform II-4, I generated a series transgenic rice plants with overexpression of α-amylase II-4. In the line with overexpression of α-amylase II-4 at both the mRNA and protein levels, the ability of seed germination and seedling growth was unaltered. Unlike α-amylase I-1, starch accumulation in leaf tissues overexpressing α-amylase II-4 was scarcely changed. However, the seeds produced in the transgenic rice plants exhibited an abnormal accumulation of starch, as well as that in rice overexpressing α-amylase I-1. Based on these in endosperm of ripening seed and controls accumulation of reserve stach in rice., 新大院博(農)甲第63号, 新大院博(農)甲第63号}, title = {Studies on Functional Expression of α-Amylase in Rice}, year = {2005} }