Fine mapping and candidate gene analysis of purple pericarp gene Pb in rice (Oryza sativa L.)

Purple rice is a type of rice with anthocyanins deposited in its grain pericarp. The rice Pb gene control-ling purple pericarp character is known to be on chromosome 4,and the purple color is dominant over white color. In this study,we fine mapped the Pb gene using two F2 segregating populations,i.e. Pei’ai 64S(white) × Yunanheixiannuo(purple) and Pei’ai 64S × Chuanheinuo(purple) . In the first-pass map-ping,the Pb gene was located in the region downstream the SSR marker RM3820. In the fine mapping,the candidate region was saturated with InDel and CAPS markers developed specifically for this study. Eventually,the Pb gene was mapped within the 25-kb region delimited by the upstream marker RID3 and the downstream marker RID4. The delimited region contained two annotated genes,Ra and bhlh16(TIGR Rice Genome,R.5) . The former is a homologue of the Myc transcription factor Lc controlling anthocyanin biosynthesis in maize,and the latter is a homologue of the TT8 gene,which is also an Myc transcription factor gene controlling the pericarp pigmentation in Arabidopsis thaliana. Sequence analysis showed that the exon 7 of the Ra gene of Yunanheixiannuo and Chuanheinuo had a 2-bp(GT) deletion compared with those of the white rice varieties Pei’ai 64S,9311 and Nipponbare. A CAPS marker,CAPSRa,was developed according to the GT deletion for analysis of the two F2 segregating populations and 106 rice lines. The results showed that all F2 plants with white pericarp,and all non-purple rice lines(63 white and 22 red) contained no GT deletion,but all 20 purple rice lines con-tained the GT deletion. These results suggested that the Ra gene may be the Pb gene and the purple pericarp characteristic of rice is caused by the GT deletion within exon 7 of the Ra gene.