The pl gene encodes a regulatory protein that controls the transcripti
on of a number of structural genes of the anthocyanin biosynthetic pat
hway 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 wh
ich only tissues exposed to light become pigmented. Based on these obs
ervations, two alternate pathways leading to anthocyanin synthesis in
the plant have been proposed: one requiring light and the other bypass
ing the light requirement through the action of Pl. To evaluate this h
ypothesis, we have analyzed light-independent and light-dependent alle
les of pl. Sequence analysis revealed that the two types of alleles ha
ve very distinct promoters but have the capacity to encode very simila
r proteins. The protein encoded by one recessive allele was shown to b
e 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 express
ion is increased approximately sixfold by exposure to light. These res
ults 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 o
f a threshold effect in which light exposure boosts pl mRNA expression
past a crucial level necessary to generate sufficient PL protein mole
cules to activate transcription of the anthocyanin structural genes.