Studies on Starch Accumulation and Sucrose Transport in Rice Embryos during Seed Germination and Seedling Establishment

Part1: ndosperm starch would be hydrolyzed to glucose and maltose and further being sent to the newly synthesized shoot and root through embryo during rice (Oryza sativa L.) seed germination and seedling establishment. In embryos, the soluble sugar from endosperm could be converted to starch and transiently accumulated in scutellums for few days. Following, the starch resynthesis was began to be presented in the cells surrounding the vascular bundles of embryos. Since the hyperaccumulation of starch in embryo tissues was observed in shoot-removing seedlings and low levels of starch content were found in low-photosynthetic efficiency seedling embryos in dark conditions, it was suggested that the amount of starch biosynthesis in embryo tissues were depended on the sink tissue demand. Accroding to the expressions of granule-bound starch synthase (GBSS)Ⅰ, Ⅱ, and starch branching enzyme (SBE) Ⅰ, Ⅱ, Ⅳ responsive to dark treatment, it indicated that the starch level in embryo tissues of growing seedlings in dark was mainly controlled by starch synthetic efficiency. Moreover, SBEIII expression in dark was recovered when glucose was supplied into medium. It was suggested that sugar may function as one of the factors to regulate SBEⅢ gene expression in embryos of young seedlings.art 2:ucrose transporters are transmembrane proteins in charge of transporting sucrose across cell membrane. There are five sucrose transporter isogenes in rice (Oryza sativa L.) and named OsSUT1, 2, 3, 4, 5. During embryo germination, effect of glucose on OsSUT1 expression in embryos presented a bi-phase process. OsSUT1 expression was repressed in embryos when germinated in glucose-containing medium after 1-d treatment; however, it was enhanced after 5-d treatment. It was proposed that the differential effects of glucose on OsSUT1 expression were mainly affected by glucose treatment period. Besides, OsSUT2 expression was enhanced in embryos when germinated in glucose-containing medium after 1 and 5-d treatment. According to the effects of sugar analog, 3-O-Methylglucose, and hexokinase inhibitor, N-acetyl glucosamine, on OsSUT1 expressions, it was suggested that the signal transduction for regulating glucose-responsive OsSUT1 expressions in embryos were mediated by hexokinase-dependent pathway. On the other hand, glucose affected OsSUT2 expression was through a hexokinase-independent.