Loss of Function of the Cytochrome P450 Gene CYP78B5 Causes Giant Embryos in Rice

Rice (Oryza sativa L.) is one of the most important food crops in the world. Most kernel oil and micro-nutrients are located in the embryo whereas starch is mainly in the endosperm. Although embryo size is important for rice grain yield and food processing, little is known about the genetic mechanisms controlling embryo size. We characterized a rice giant embryo mutant, giant embryo (ge), which was derived from wild-type Kinmaze by chemical mutagenesis. The large embryo size resulted from larger cell size. The gene controlling the giant embryo trait was cloned by a map-based approach, and shown to encode cytochrome P450 protein CYP78B5, which was previously designated as CYP78A13. Gain-of-function transgenic experiments on OsGE/CYP78B5 in the ge mutant restored the normal seed phenotype. Expression analysis showed that the OsGE/CYP78B5 gene product accumulated mainly in young developing seeds. Subcellular localization indicated that OsGE/CYP78B5 resides in the peroxisomes, and the N-terminal 51 amino acid residues function as a targeting signal. The IAA levels were one-third those of wild type in GE mutants at 6 days after fertilization, yet the embryonic cells were much larger. OsGE/CYP78B5 also regulates expression levels of auxin-responsive and cyclin-related genes. Isolation of GE provides an opportunity to manipulate the nutritive composition and distribution of the grain by marker-assisted selection and transgenic approaches.