Abstract

Notched belly grain (NBG) is a type of deformed grain shape that has been associated with inferior appearance and tastes in rice. NBG is coordinated by both environments and genetics. In this study, we report on the first map-based cloning of an NBG gene on chromosome 4, denotedNBG4, which is a novel allele ofDwarf 11encoding a cytochrome P450 (CYP724B1) involved in brassinosteroid (BR) biosynthesis. A 10-bp deletion in the 7th exon knocked down the level of theNBG4transcript and shifted the reading frame of the resulting protein. In addition to the dwarf and clustered panicle as previously reported in the allelic mutants,nbg4grains also displayed retarded germination and NBG due to the physical constraint of deformed hulls caused by abnormal hull elongation.NBG4is constitutively expressed with the highest level of expression in immature inflorescences. In all, 2294 genes were differentially expressed innbg4and wild-type (WT), and evidence is presented thatNBG4regulatesOsPPS-2,OsPRA2,OsYUCCA1,sped1-D, andDwarfthat play critical roles in determining plant architecture, panicle development, and seed germination. This study demonstrated thatNBG4is a key node in the brassinosteroid-mediated regulation of rice grain shape.Keywords:rice;Oryza sativaL.;notched belly grain;germination;Dwarf 11;brassinosteroids;cytochrome P450