ABSTRACT
Shure et al., 1983; Klösgen et al., 1986). Thus, it appears that GBSS is specifically involved in amylose biosynthesis and that no other SS can provide this function. Genetic evidence indicates that all five SSs have specific functions that cannot be provided by any other SS class. SSI preferentially elongates the shortest chains with a degree of polymerization (DP) of 4-10 glucosyl units, and SSII catalyzes formation of medium-length chains with DP of 12-24 glucosyl units. Only one form of SSIII genes, SSIIIb, is exclusively expressed in rice leaves (Ohdan et al., 2005). In a study performed by Zhang et al. (2008), analysis of SS II functions in Arabidopsis leaves was extended by characterization of mutations in the AtSS2 gene, which encodes SSII. The effects of eliminating SSII on amylopectin structure in Arabidopsis leaves closely resembled the phenotype caused by SSII deficiency in other plant species (Nakamura et al., 2005; Morell et al., 2003; Umemoto et al., 2002). In all these species, SSII mutations cause altered amylopectin structure such that the frequency of DP12 to DP28 chains is decreased while the abundance of linear glucan chains of DP6 to DP10 is significantly increased. Inactivation of both genes responsible for SSII and SSIII together causes synergistic defects more severe than those resulting from mutation of only one of the two enzymes with regard to starch content and amylopectin structure.Additional to starch synthases, starch branching enzymes (SBE/BE, EC 2.4.1.18) are involved in starch biosynthesis introducing -(1,6)-bonds by cleaving internal -(1,4) linkages and transferring glucan segments of six or more glucose residues to the same or neighboring chain of the amylopectin molecule (Borovsky et al., 1975). Also, BE exists in multiple isoforms. Two classes of BEs have been distinguished: I (B) and II (A) (Burton et al., 1995; Rahman et al., 2001). In maize, mutation of the gene encoding BEIIb produces a high-amylose starch phenotype (Yun and Matheson, 1993). Similar mutation of BEII in potato resulted in the increase of amylose content and modification of amylopectin structure due to the higher content of average chain length, i.e., DP 23-60 (Jobling et al., 1999). Taken together, results obtained with different BE mutants suggest that both classes of BE together participate in the formation of the proper structure of the starch granule.
