A rare deletion of IhUF3GT1 in the ivy-leaf morning glory eliminates anthocyanin-based pigmentation.

Abstract

Anthocyanin biosynthesis is a highly branched network with upstream mutations leading to the loss of many other branches beyond just anthocyanins like flavonols, isoflavonoids, and proanthocyanidins. Consequently, disentangling the specific effects of anthocyanins on plant fitness from those of other branches is difficult unless a mutant impacting the terminal step of anthocyanin biosynthesis can be found. We discovered an Ipomoea hederacea plant (ivy-leaf morning glory) lacking anthocyanins due to a genetic deletion in the terminal step of the anthocyanin biosynthetic pathway. Anthocyanin loss follows a recessive, Mendelian inheritance pattern. Anthocyanin loss is perfectly correlated with a lack of expression of the anthocyanin biosynthesis gene IhUF3GT1 (anthocyanidin-3-O-glucosyltransferase). IhUF3GT1 is the gene most strongly and consistently differentially expressed between unpigmented and pigmented flowers in F2 siblings. A deletion of half of the IhUF3GT1 gene which includes the start codon is the genetic basis underlying a loss of IhUF3GT1 expression and, consequently, anthocyanins in unpigmented plants. The IhUF3GT1 deletion is rare and unique to the unpigmented line. Genome coverage analyses demonstrate that no other pigmented I. hederacea line (of 123 screened) has a loss-of-function mutation in IhUF3GT1. Furthermore, we confirm cosegregation of the IhUF3GT1 deletion with the unpigmented phenotype in F3 progeny from a cross where all unpigmented plants are homozygous for the IhUF3GT1 deletion. Taken together, we describe the genetic basis of a novel unpigmented mutant in I. hederacea, creating a potentially important model for studying the fitness effects of anthocyanin loss.