Genomic insights into citrus domestication and its important agronomic traits

Citrus originated in Southeast Asia, and it has become one of the most important fruit crops worldwide. Citrus has a long and obscure domestication history due to its clonal propagation, long life cycle, wide sexual compatibility, and complex genetic background. As the genomic information of both wild and cultivated citrus becomes available, their domestication history and underlying traits or genes are becoming clear. This review outlines the genomic features of wild and cultivated species. We propose that the reduction of citric acid is a critical trait for citrus domestication. The genetic model representing the change during domestication may be associated with a regulatory complex known as WD-repeat-MYB-bHLH-WRKY (WMBW), which is involved in acidification and anthocyanin accumulation. The reduction in or loss of anthocyanins may be due to a hitchhiking effect of fruit acidity selection, in which mutation occurs in the common regulator of these two pathways in some domesticated types. Moreover, we have summarized the domestication traits and candidate genes for breeding purposes. This review represents a comprehensive summary of the genes controlling key traits of interest, such as acidity, metabolism, and disease resistance. It also sheds light on recent advances in early flowering from transgenic studies and provides a new perspective for fast breeding of citrus. Our review lays a foundation for future research on fruit acidity, flavor, and disease resistance in citrus.