OsToc159 is essential for amyloplast development and starch granule initiation in rice endosperm

In cereal endosperm, starch is biosynthesized within specialized plastids termed amyloplasts through the coordinated activity of a suite of enzymes. The development of amyloplasts requires the import of nuclear-encoded preproteins from the cytosol via the translocon. However, little is known about how the translocon influences amyloplast development and starch synthesis in cereal endosperm. Here, we functionally characterize the rice floury endosperm mutant b303, which is defective in amyloplast development and starch granule initiation. Map-based cloning of B303 identifies a mutation in OsTOC159, which encodes a GTPase receptor subunit of the translocon at the outer envelope membrane of chloroplasts (TOC) complex. OsToc159 is highly expressed in developing endosperm and localizes to the amyloplast membrane. Combined with biochemical evidence, protoplast-based import assays suggest that OsToc159 potentially modulates the plastid import of a subset of starch biosynthetic proteins. Moreover, OsToc159 likely interacts with another GTPase receptor, OsToc34, to synergistically facilitate the posttranslational translocation of isoamylase 1 (ISA1). Collectively, our data suggest that OsToc159 may function as a receptor involved in the import of a subset of plastid proteins required for amyloplast development and starch biosynthesis.