Control of Rice Embryo Development, Shoot Apical Meristem Maintenance, and Grain Yield by a Novel Cytochrome P450

ABSTRACT Angiosperm seeds usually consist of two major parts: the embryo and the endosperm. However, the molecular mechanism(s) underlying embryo and endosperm development remains largely unknown, particularly in rice, the model cereal. Here, we report the identification and functional characterization of the rice GIANT EMBRYO ( GE ) gene. Mutation of GE resulted in a large embryo in the seed, which was caused by excessive expansion of scutellum cells. Post-embryonic growth of ge seedling was severely inhibited due to defective shoot apical meristem (SAM) maintenance. Map-based cloning revealed that GE encodes a CYP78A subfamily P450 monooxygenase that is localized to the endoplasmic reticulum. GE is expressed predominantly in the scutellar epithelium, the interface region between embryo and endosperm. Overexpression of GE promoted cell proliferation and enhanced rice plant growth and grain yield, but reduced embryo size, suggesting that GE is critical for coordinating rice embryo and endosperm development. Moreover, transgenic Arabidopsis plants overexpressing AtCYP78A10 , a GE homolog, also produced bigger seeds, implying a conserved role for the CYP78A subfamily of P450s in regulating seed development. Taken together, our results indicate that GE plays critical roles in regulating embryo development and SAM maintenance. SUMMARY The GIANT EMBRYO ( GE ) gene encodes a CYP78A subfamily P450 monooxygenase, which controls rice embryo development and coordinates embryo and endosperm growth. GE also plays a pivotal role in shoot apical meristem (SAM) maintenance and grain yield improvement.