Molecular identification and comparison of the starch synthase bound to starch granules between endosperm and leaf blades in rice plants.

Normal (nonglutinous) rice plants (Oryza sativa and O. glaberrima) contain more than 18% amylose in endosperm starch, while waxy (glutinous) plants lack it in this starch. In contrast, leaf starch contained more than 3.6% amylose even in waxy plants. SDS-PAGE analysis of proteins bound to endosperm starch granules in the normal plants revealed a single band with a Mr of 60 kd, whereas waxy plants did not exhibit a similar band. The activity of starch synthase (NDP-glucose-starch glucosyltransferase) was completely inhibited by antibody against the 60-kd protein. Thus, we conclude that the 60-kd protein is the waxy protein encoded by the Wx allele, which also plays a role in the synthesis of nonglutinous starch in endosperm tissue. In leaf blades, the proteins bound to starch granules separated into five bands with Mr's of 53.6 to 64.9 kd on SDS-PAGE. Analysis of these proteins by immunoblotting using antiserum against Wx protein and inhibition of starch synthase activity by the synthase antibody revealed that none of these proteins was homologous to Wx protein. We suggest that the synthesis of amylose in leaf blades is brought about by a protein encoded by a gene(s) different from the Wx gene expressed in the endosperm.