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Background  Starch is the most important carbohydrate in plant storage tissues. Multiple isozymes in at least four enzyme classes are involved in starch biosynthesis. Some of these isozymes are thought to interact and form complexes for efficient starch biosynthesis. Of these enzyme classes, starch synthases (SSs) and branching enzymes (BEs) play particularly central roles.    Results  We generated double mutant lines ( ss1/be1  and  ss1    L    /be2b ) between SSI (the largest component of total soluble SS activity) and BEI or BEIIb (major BEs in developing rice endosperm) to explore the relationships among these isozymes. The seed weight of  ss1/be1  was comparable to that of wild type, although most  ss1/be2b  seeds were sterile and no double recessive plants were obtained. The seed weight of the double recessive mutant line  ss1    L    /be2b , derived from the leaky  ss1  mutant ( ss1    L   ) and  be2b , was higher than that of the single  be2b  mutant. Analyses of the chain-length distribution of amylopectin in  ss1/be1  endosperm revealed additive effects of SSI and BEI on amylopectin structure. Chain-length analysis indicated that the BEIIb deficiency significantly reduced the ratio of short chains in amylopectin of  ss1    L    /be2b . The amylose content of endosperm starch of  ss1/be1  and  ss1    L    /be2b  was almost the same as that of wild type, whereas the endosperm starch of  be2b  contained more amylose than did that of wild type. SSI, BEI, and BEIIb deficiency also affected the extent of binding of other isozymes to starch granules.    Conclusions  Analysis of the chain-length distribution in amylopectin of the double mutant lines showed that SSI and BEI or BEIIb primarily function independently, and branching by BEIIb is followed by SSI chain elongation. The increased amylose content in  be2b  was because of reduced amylopectin biosynthesis; however, the lower SSI activity in this background may have enhanced amylopectin biosynthesis as a result of a correction of imbalance between the branching and elongation found in the single mutant. The fact that a deficiency of SSI, BEI, or BEIIb affected the affinity of other starch biosynthetic isozymes for the starch granule implies that there is a close interaction among SSI, BEI and BEIIb during amylopectin biosynthesis in rice endosperm.

Relationships between starch synthase I and branching enzyme isozymes determined using double mutant rice lines