| The quality of rice grain is characterized by the component, structure and accumulation of starch. Starch synthase is the key enzyme involved in starch biosynthesis and composed of five classes of isoforms. The relative contribution of the classes to starch synthase activity was responsible to the amylose components, chain length profiles of amylopectin and structure of starch granule. The starch synthase gene family in rice was presented in this study. The characterization, exon/intron structure, evolution, expression analysis and function of rice starch synthase genes were investigated. The results were summarized as follows:1 The plant starch synthase gene family was divided into five distinct classes, named GBSS, SSI, SSII, SSIII and SSIV based on gene structures. Two GBSS, one SSI, three SSII, two SSIII and two SSIV comprised the rice starch synthase gene family. In addition, one truncated SSI was identified through searches of the recently published draft sequence of the rice genome. Members of the rice starch synthase gene family were diffused in different rice chromosome loci, which suggests that the duplication of these genes may occur at the beginning of the Gramineae evolution.2 The C-termini sequence of these proteins showed high sequence homology with the glycogen synthase of Escherichia coli, but little sequence similarity with the yeast and vertebrate glycogen synthases. These results indicate that the origin gene of plant starch synthase and that of the vertebrate were from distinct ancestor. The diverse N-termini of SSs were similar in each class of isoform, which indicates their distinct role in the synthesis of starch. Given the similarity between exon and intron structure from rice and other species and the existence of these five classes in Chlamydomonas genome, it is clear that the separation of GBSS, SSI, SSII, SSIII and SSIV predates the separation of monocots and dicots. Alignment of amino acid sequences of the plant starch synthases suggests that the development of the SS genes should be associated with several duplications. The first duplication produced two genes, one of which further duplicated twice and diverged to GBSS, SSI and SSII gene classes, and the other duplicated second time to yield SSIII and SSIV gene classes. After the separationof monocots and dicots, but before the separation of the Gramineae, duplication of each class of the SS gene occurred at least once, respectively, to form the SS gene family in cereals.3 Enzymatic activity of recombinant OsSSs demonstrated these SS genes encode functional starch synthase enzymes. The tissue specific expressions of the OsSS gene family were tested by RT-PCR, Western blots and isoenzymic analysis. OsSSII-3, OsSSIIIb, OsGBSSI and OsSSIVa were expressedspecifically in endosperm, whereas OsSSII-2, OsGBSSII and OsSSIVb were specifically in leaves. OsSSI, OsSSII-1 and OsSSIIIa were expressed in both leaves and grains, while OsSSIVa and OsSSIVb were expressed scarcely in rice. Gircadian expression pattern of GBSSII in rice leave is distinct. Simultaneously, expressions of OsGBSSII and OsSSII-2 were induced by N-starvation and sugars. N-starvation-induced OsGBSSII expression may depend on the sugar-regulating pathway and sugar regulated OsGBSSII expression is glycolysis -dependent.4 The results of SDS-PAGE, Western blots and N-termini amino acid sequence analysis identified that soluble isoforms of OsSSI, RBE3 and OsSSII-3 were also bound to starch granules in developing rice endosperms, besides OsGBSSI, whereas OsGBSSII and OsSSII-2 were bound to starch granules of rice leaves.The protein amount of OsSSII-3 in indica was increased relative to that of OsSSII-3 in japonica rice and even exceeded the amount of OsSSI in indica. This high ratio of OsSSII-3 amount to OsSSI in indica would explain why starch from indica rice has a higher gelatinisation temperature than starch from japonica rice and is more resistant to disintegration in alkali or urea.The evolution, partial expression pattern, contribution to starch synthesis a...
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