Role of the regulatory gene pl in the photocontrol of maize anthocyanin pigmentation.

The pl gene encodes a regulatory protein that controls the transcription of a number of structural genes of the anthocyanin biosynthetic pathway in maize. pl alleles have been classified phenotypically into two categories: dominant (Pl) alleles lead to intense, light-independent pigmentation in vegetative and floral organs of the plant; recessive "sun-red" alleles (pl) lead to light-dependent red pigmentation in which only tissues exposed to light become pigmented. Based on these observations, two alternate pathways leading to anthocyanin synthesis in the plant have been proposed: one requiring light and the other bypassing the light requirement through the action of Pl. To evaluate this hypothesis, we have analyzed light-independent and light-dependent alleles of pl. Sequence analysis revealed that the two types of alleles have very distinct promoters but have the capacity to encode very similar proteins. The protein encoded by one recessive allele was shown to be functional in transient assays. Measurements of husk mRNA levels by quantitative polymerase chain reaction showed that sun-red pl alleles are expressed at much lower levels than a Pl allele, but their expression is increased approximately sixfold by exposure to light. These results lead to the conclusion that the sun-red pl alleles are not null; instead, they synthesize functional mRNA and protein. We propose that the light-dependent pigmentation observed in pl plants is the result of a threshold effect in which light exposure boosts pl mRNA expression past a crucial level necessary to generate sufficient PL protein molecules to activate transcription of the anthocyanin structural genes.