The Cloning And Characterization Of The Genes For Starch Synthesis In Rice Endosperms And Studies On The Molecular Physiological Effects Of High Temperature On Rice Grain Quality Forming

The quality of rice grain is characterized by the component, structure and accumulation of starch. Six genes for starch synthesis were cloned and characterized in rice. And the effects of high temperature on the quality of rice grain and the genes involved in starch synthesis were investigated. The results were summarized as follows:1 A cDNA clone encoding glucose 6-phosphate/phosphate translocator (GPT) was isolated from a cDNA library of immature seeds of rice and named as OsGPT. The putative amino acids sequence of OsGPT shared 91% identity with maize GPT. The OsGPT gene was mapped on chromosome 8 linked to a quantitative trait locus for 1000-grain weight. OsGPT was mainly expressed in heterotrophic tissues. These results suggest that glucose 6- phosphate imported via GPT should be used for starch biosynthesis in amyloplast of rice endosperms.2 Two isoforms of fructokinases were detected in rice endosperm by Native-PAGE, and their activities were inhibited by 50 mmol/L ATP, but not inhibited by 50 mmol/L fructose. Two cDNA clones, OsFKJ and OsFKJI, encoding fructokinases were isolated from rice endosperm by polymerase-chain-reaction. The deduced proteins possess an ATP-binding motif and a putative specific substrate recognition site identified in bacterial fructokinases. The identity of OsFKJ and OsFKJI as fructokinase was confirmed by expression of the enzyme activity in E. coli. OsFKI and OsFKJI expressed predominantly both of immature seeds and roots. The mRNA corresponding to OsFKlI accumulated to higher levels in young developing rice seeds and its transcript was remarkably higher than that of OsFKJ. The results indicate that both OsFKI and OsFKII are sink specific. Both genes exist as a single copy in the rice genome and were mapped on chromosome 1 and chromosome 8, respectively. OsFKI is linked to a quantitative trait locus for 1000-grain weight on chromosome 1.3 Nine isoforms of starch synthase were detected in rice endosperm by Native-PAGE, and they could be divided into three classes according to the electrophoretic mobility. Three genes, SSSII-1, SSSII-2 and SSSII-3 were identifiedand cloned in rice, and the identity of the three genes as starch synthase was confirmed by expression of enzyme activity in E. coli. Each of the genes existed as a single copy in rice genome and was mapped on chromosome 10, chromosome 2 and chromosome 6, respectively. SSSII-1 is linked to a QTL for 1000-grain weight, while SSSII-J is linked to the alk gene. Lower transcript of SSSII-2 was constitutively expressed. SSSH-1 was expressed mainly in immature seeds and leaves. SSSII-3 was only expressed in endosperm of rice. The results indicate that SSSII-J and SSSII-3 are involved in the forming of starch yield and grain quality in rice.4 Fifty-six genes for starch synthesis were mapped on rice JRGP-RFLP-MAP (2000) by bioinformatics method, and the loci of thirty-one genes among them were found linked with QTLs for 1000-grain weight, gelatinization temperature and breakdown viscosity. The results suggest that these genes be involved in the yield and quality of rice grains.5 Rice grain quality is prone to environmental temperature. To investigate physiological molecular mechanisms of the effect of high temperature on rice grain, a non-waxy indica rice (zhe733) was grown under two temperature conditions, (29/35 癈) and (22/28 癈), during ripening stage in two phytotrons. The activities and gene expression of the key enzymes for the biosynthesis of amylose and amylopectin were examined. The activity and the expression level of soluble starch synthase I (SSSI) in endosperm was slightly higher at 29/35 癈 than that at 22/28 癈. In contrast, the activities and expression levels of rice branching enzymel (RBE1) and granule binding starch synthase (GBSS) of endosperm were lower at 29/35 癈 than those at 22/28 癈. The results suggest that the decreased activity of RBE1 result in the reduction of the branching frequency of the longer branches of amylopectin,...