A rare deletion of BZ1 in the ivy-leaf morning glory eliminates anthocyanin-based pigmentation.

Anthocyanin biosynthesis is a highly branched network with upstream mutations leading to the loss of many other branches beyond just anthocyanins like flavonols and isoflavonoids. 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 the first 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 BZ1. BZ1 is the gene most strongly and consistently differentially expressed between unpigmented and pigmented flowers in F2 siblings. A deletion of half of the BZ1 gene which includes the start codon is the genetic basis underlying a loss of BZ1 expression and, consequently, anthocyanins in unpigmented plants. The BZ1 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 BZ1. Furthermore, we confirm cosegregation of the BZ1 deletion with the unpigmented phenotype in F3 progeny from a cross where all unpigmented plants are homozygous for the BZ1 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.