Morphological Characteristics and Gene Mapping of Purple Apiculus Formation in Rice

Apiculus color of grain is an important trait which is used as a morphological marker in rice (Oryza sativa. L). In the present study, the purple apiculus mutant named as Ospa was developed from an indica cultivar using ethyl methanesulfonate mutagenesis. The Ospa mutant showed increased grain size, thousand-grain weight, and anthocyanin accumulation compared with the wild-type (WT). Histological analysis revealed that the size and number of cells in parenchyma layers of spikelet hulls was significantly higher in Ospa mutant than the WT. By map-based cloning, OsPA was located within the 60.74 kb region on the long arm of chromosome 1 where a T to C substitution was detected in the third exon of LOC_Os01g39580. The encoded polypeptide, predicted as anthocyanin regulatory Lc protein, contained the basic-helix-loop-helix (bHLH) domain. The mutation has changed cysteine to arginine in the amino acid sequence and modified the predicted secondary structure in the conserved bHLH domain of the encoded peptide. As the RT-qPCR analysis revealed, OsPA was specifically expressed in the young panicles of Ospa mutant. The OsPA gene might have regulated the grain anthocyanin content by up-regulating the expression of the anthocyanin regulatory MYB gene (OsC), and the anthocyanin biosynthesis genes (OsF3′H, OsF3′5 ′H, OsDFR, and OsANS). Our study will provide new genetic material for a further study of the genetic mechanism of tissue-specific pigmentation, and the developed gene-based markers could be utilized in marker-assisted selection along with purple apiculus trait in rice.