FLOURY ENDOSPERM6 encodes a CBM48 domain‐containing protein involved in compound granule formation and starch synthesis in rice endosperm

Summary Starch is the most widespread form of energy storage in the plant kingdom. Although many enzymes and related factors have been identified for starch biosynthesis, unknown players remain to be identified, given that it is a complicated and sophisticated process. The endosperm of rice ( O ryza sativa ) has been used for the study of starch synthesis. Here, we report the cloning and characterization of the FLOURY ENDOSPERM 6 ( FLO 6 ) gene in rice. In the flo6 mutant, the starch content is decreased and the normal physicochemical features of starch are changed. Significantly, flo6 mutant endosperm cells show obvious defects in compound granule formation. Map‐based cloning showed that FLO 6 encodes a protein of unknown function. It harbors an N –terminal transit peptide that ensures its correct localization and functions in the plastid, and a C –terminal carbohydrate‐binding module 48 ( CBM 48) domain that binds to starch. Furthermore, FLO 6 can interact with isoamylase1 ( ISA 1) both in vitro and in vivo , whereas ISA 1 does not bind to starch directly. We thus propose that FLO 6 may act as a starch‐binding protein involved in starch synthesis and compound granule formation through a direct interaction with ISA 1 in developing rice seeds. Our data provide a novel insight into the role of proteins with the CBM 48 domain in plant species.