Starch is glucose polymer linked by α-1,4 and α-1,6 glucosidic bonds. The balance of activity between starch synthases (SSs) and branching enzymes (BEs) is important for formation of amylopectin structure. SSI and SSIIIa isozymes account for 60 and 30%, respectively, of soluble SS activity in developing rice endosperm. Complete loss of both SSI and SSIIIa activities induces sterility. By contrast, a double mutant (
ss1L/ss3a) generated by crossing the leaky
ss1 mutant, whose SS activity is approximately ca. 16% of the wild type, and the
ss3a null mutant is fertile. Although there is only a significant residual SS activity in the developing endosperm of
ss1L/ss3a, the yield and growth of the double-mutant line is sufficient. We analyzed the amylopectin chain-length distribution, thermal and pasting properties, morphologies of starch granules and amyloplasts, and crystallinity of endosperm starch in
ss1L/ss3a and the parental mutant lines. The chain-length distribution pattern of
ss1L/ss3a was similar to that of
ss3a and there was an additive effect of the reduction of SSI activity on the chain-length of amylopectin in the
ss3a background. These changes in the amylopectin chain-length distribution were considered to increase the gelatinization temperature of the starch. The gelatinized starch viscosity of the double-mutant and
ss3a lines was very low. Most
ss1L/ss3a starch granules were round-shaped, and the starch granular size was smaller than those of the parental mutant lines and the wild type. The starch crystallinity of
ss1L/ss3a was slightly reduced compared with that of the wild type.
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