ERECT PANICLE2 Encodes a Novel Protein That Regulates Panicle Erectness in Indica Rice

Rice (Oryza sativa L.) inflorescence (panicle) architecture is an important agronomic trait for rice breeding. A number of high-yielding japonica rice strains, characterized by an erect panicle (EP) of their architecture, have been released as commercial varieties in China. But no EP-type indica varieties are released so far. Here, we identified two allelic erect-panicle mutants in indica rice, erect panicle2-1 (ep2-1) and erect panicle2-2 (ep2-2), exhibiting the characteristic erect panicle phenotype. Both mutants were derived from spontaneous mutation. We cloned the EP2 gene by way of a map-based cloning strategy, and a transgenic complementation test rescued the phenotype of ep2-1. Anatomical investigations revealed that the ep2 mutants have more vascular bundles and a thicker stem than that of wild-type plants, explaining the panicle erectness phenotype in ep2 mutants. It was shown that EP2 was specifically expressed in the vascular bundles of internodes by GUS staining and RT-PCR. EP2 encodes a novel plant-specific protein, which localizes to the endoplasmic reticulum with unknown biochemical function. In addition, EP2 also regulates other panicle characteristics, such as panicle length and grain size, but grain number per panicle shows little change, indicating that the mutation of the ep2 gene could be applied in EP-type indica rice breeding.