Independent Origin of Phenol Non-responsive Phenotype Caused By Phr1 Variation During Domestication of Asian and African Rice

Phenol color reaction has been used to distinguish between two subspecies of Asian rice (Oryza sativa), indica and japonica. The trait is controlled by one single Phr1 gene, which encodes a PPO enzyme that catalyzes the oxidation of phenolic compounds into brown or black pigments upon contact to phenol solution. In O. sativa, ssp. indica responds to phenol chemical assay by altering the rice hull color to black, whereas ssp. japonica remains unaffected due to mutations that render the gene non-functional. Although the different characteristics between subspecies in Asian rice is well known, there is no information about the variation of this trait in African rice, Oryza glaberrima, which was originated and domesticated independently of Asian rice. In this study, we found both phenol negative and positive lines in O. glaberrima and its wild ancestor O. barthii and detected the responsible non-functional mutation (1-bp deletion) in the exon 1 of the Phr1 gene. Geographical distribution of its haplotype suggested that this mutation originated in O. barthii in Mali and was later inherited by O. glaberrima. The predominance of the non-functional Phr1 alleles in O. glaberrima lines and the occurrence of the identical haplotypes in negative group of both O. barthii and O. glaberrima suggest that the negative phenol reaction was favored during domestication and breeding selection. The presence of a selection event is also supported by low nucleotide diversity of Phr1 locus. However, genetic diversity of Phr1 persists in African rice germplasm, as the functional alleles are still present in O. glaberrima. We also compared the nucleotide diversity of Phr1 in African rice with that in Asian rice and found that their origins of the phenol responsive phenotype are independent. These findings expand the current understanding of African rice domestication and offer the valuable molecular marker for improved rice breeding.