Biosynthesis starch synthase

Mutations of SSIIa have been observed in wheat and rice.208 In wheat, each of the three wheat genomes were mutated to entirely eliminate expression of the SSIIa gene product, Sgp-1 protein in one line 209 the result was reduced starch amounts and an altered starch structure. In rice, two classes of starch have been found. In Indica rices, the starch is of the long chain variety, while in Japonica, it is of the short chain variety.208 Genetic analysis showed that the mutation in Japonica rice led to a loss of starch synthase II.209 Thus, in higher plants, it seems that loss of SSII activity in dicots and SSIIa activity in monocots have the same results with respect to reduced starch content, due to a lowered amount of amylopectin and altered amylopectin chain size distribution. Thus, these genes may have the same function in starch biosynthesis. 

What roles do the starch synthase isoforms play in the formation of the crystalline starch granule and amylopectin structures How is amylose formed Why are starch granules from different species different in size and in the number per cell New methodology and much effort have resulted in major advances in the understanding of starch biosynthesis, but many questions remain unanswered. Here we discuss some of these open questions and possible answers. 

Maddelein, M.-L., Bellanger, F., Deirue, B., Libessart, N., D hulst, C., Van den Koomhuyse, N., Fontaine, T Wieruszeski, J.-M., Decq. A., and Ball, S. 1994a. Genetic dissection of starch metabolism in the monocellular alga Chlamydomonas reinhardtii Determination of granule-bound and soluble starch synthases functions in amylopectin biosynthesis. J. Biol. Chem. 269, 25150-25157. 

Chlamydomonas reinhardtii monocellular algal mutants defective in amylose biosynthesis and granule-bound starch synthase activity accumulate a structurally modified amylopectin. J. Bac-teriol. 1992 174 3612-3620. 

Zhang, X Szydlowski, N Delvalle, D D Hulst, C James, MG Myers, AM. Overlapping functions of the starch synthases SSII and SSIII in amylopectin biosynthesis in Arabidopsis. BMC Plant Biol, 2008, 8(96). 

Some of the cell wall mutants, such as the mur mutants in Arabidopsis, affect dovmstream enzymes, which supply substrates for the glycosyl transferases involved in cell wall synthesis (Williamson et al. 2002). These types of mutations usually lead to a significant overall reduction in the rate of cellulose synthesis. Sucrose synthase (SuSy) (FC2.4.1.13) catalyzes the reversible conversion of sucrose and UDP to UDP-glucose and fructose thereby channeling sucrose into numerous pathways, including cell wall and starch biosynthesis. 

Chourey PS., Taliercio E.W., Carlson S.J., and Ruan Y.L. 1998. Genetic evidence that the two isozymes of sucrose synthase present in developing maize endosperm are critical, one for cell wall integrity and the other for starch biosynthesis. Mol Gen Genet 259 88-96. 

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