Physiological and compensatory roles of three starch-branching enzymes in different rice organs

Starch-branching enzymes (BEs) generate amylopectin branches in starch, and three isoforms are found in the rice genome. BEI and BEIIa are expressed ubiquitously, on the other hand BEIIb is exclusively expressed in the endosperm. The loss of BEIIb generates opaque seeds with drastically reduced short amylopectin chains, whereas the loss of BEI slightly reduces long amylopectin chains. However, the loss of BEIIa has no obvious effect on amylopectin structure in the endosperm. Little is known about these isozymes' specific and compensatory roles in other tissues. In this study, all three combinations of double mutants were generated, and their starch properties were analyzed to reveal the function of the solely remaining isozyme, deduce the compensatory function of the missing isozymes, and clarify their roles in different organs. be2a be2b displayed severe sterility and hindered starch synthesis in the endosperm; be1 be2a displayed only minor alterations in endosperm starch; but be1 be2b displayed a drastic increase in amylose content in the endosperm. These results indicate that BEIIa has some compensatory function for BEIIb, and BEI alone is insufficient. Additionally, BEIIa or BEIIb is necessary for endosperm formation. Unlike the endosperm, short amylopectin chains were drastically reduced in the leaf sheath of be1 be2a and be2a be2b, and amylose content was drastically increased in the leaf sheath of be1 be2a and be2a be2b but not be1 be2b. These results indicate that BEIIa has a major role in the leaf sheath, and the generation of short amylopectin chains by BEII can be partially compensated by BEI.