Pl-Bh, an anthocyanin regulatory gene of maize that leads to variegated pigmentation

Abstract

Anthocyanins are purple pigments that can be produced in virtually all parts of the maize plant. The spatial distribution of anthocyanin synthesis is dictated by the organ-specific expression of a few regulatory genes that control the transcription of the structural genes. The regulatory genes are grouped into families based on functional identity and DNA sequence similarity. The C1/Pl gene family consists of C1, which controls pigmentation of the kernel, and Pl, which controls pigmentation of the vegetative and floral organs. We have determined the relationship of another gene, Blotched (Bh), to the C1 gene family. Bh was originally described as a gene that conditions blotches of pigmentation in kernels homozygous for recessive c1, suggesting that Bh could functionally replace C1 in the kernel. Our genetic and molecular analyses indicate that Bh is an allele of Pl, that we designate Pl-Bh. Pl-Bh differs from wild-type Pl alleles in two respects. In contrast to the uniform pigmentation observed in plants carrying Pl, the pattern of pigmentation in plants carrying Pl-Bh is variegated. Pl-Bh leads to variegated pigmentation in virtually all tissues of the plant, including the kernel, an organ not pigmented by other Pl alleles. To address the molecular basis for the unusual pattern of expression of Pl-Bh, we cloned and sequenced the gene. The nucleotide sequence of Pl-Bh showed only a single base-pair difference from that of Pl. However, genomic DNA sequences associated with Pl-Bh were found to be hypermethylated relative to the same sequences around the wild-type Pl allele. The methylation was inversely correlated with Pl mRNA levels in variegated plant tissues. Thus, we conclude that DNA methylation may play a role in regulating Pl-Bh expression.