The pl gene encodes a regulatory protein that controls the transcription of a number of structural genes of the anthocyanin biosynthetic pathway i n maize. pl alleles have been classified phenotypically into two categories: dominant (Pr) alleles lead to intense, light-independent pigmentation in vegetative and floral organs of the plant; recessive "sun-red" alleles (pr) 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.

Pioneer Hi-Bred International, Johnston, lowa 50130-0038The pl gene encodes a regulatory protein that controls the transcription of a number of structural genes of the anthocyanin biosynthetic pathway i n maize. pl alleles have been classified phenotypically into two categories: dominant (Pr) alleles lead to intense, light-independent pigmentation in vegetative and floral organs of the plant; recessive "sun-red" alleles (pr) 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.

No abstract

orange pericarp (orp) is a seedling lethal mutant of maize caused by mutations in the duplicate unlinked recessive loci orp1 and orp2. Mutant seedlings accumulate two tryptophan precursors, anthranilate and indole, suggesting a block in tryptophan biosynthesis. Results from feeding studies and enzyme assays indicate that the orp mutant is defective in tryptophan synthase beta activity. Thus, orp is one of only a few amino acid auxotrophic mutants to be characterized in plants. Two genes encoding tryptophan synthase beta were isolated from maize and sequenced. Both genes encode polypeptides with high homology to tryptophan synthase beta enzymes from other organisms. The cloned genes were mapped by restriction fragment length polymorphism analysis to approximately the same chromosomal locations as the genetically mapped factors orp1 and orp2. RNA analysis indicates that both genes are expressed in all tissues examined from normal plants. Together, the biochemical, genetic, and molecular data verify the identity of orp1 and orp2 as duplicate structural genes for the beta subunit of tryptophan synthase.

orange pericarp (ofp) is a seedling lethal mutant of maize caused by mutations in the duplicate unlinked recessive loci O r p l and orp2. Mutant seedlings accumulate two tryptophan precursors, anthranilate and indole, suggesting a block in tryptophan biosynthesis. Results from feeding studies and enzyme assays indicate that the ofp mutant is defective in tryptophan synthase p activity. Thus, orp is one of only a few amino acid auxotrophic mutants to be characterized in plants. Two genes encoding tryptophan synthase p were isolated from maize and sequenced. Both genes encode polypeptides with high homology to tryptophan synthase p enzymes from other organisms. The cloned genes were mapped by restriction fragment length polymorphism analysis to approximately the same chromosomal locations as the genetically mapped factors orpl and orp2. RNA analysis indicates that both genes are expressed in all tissues examined from normal plants. Together, the biochemical, genetic, and molecular data verify the identity of orpl and orp2 as duplicate structural genes for the p subunit of tryptophan synthase.