Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2002
Publication Date: 3/1/2003
Citation: CHOPRA, S., COCCIOLONE, S.M., BUSHMAN, B.S., SANGAR, V., MCMULLEN, M.D., PETERSON, T. THE MAIZE UNSTABLE FACTOR FOR ORANGE1 IS A DOMINANT EPIGENETIC MODIFIER OF A TISSUE SPECIFICALLY SILENT ALLELE OF PERICARP COLOR1. GENETICS. 2003. v. 163. p. 1135-1146. Interpretive Summary: Certain genetic phenomena do not depend solely on the DNA sequence of the genes but also on what is know as their "state." Taken together, these non-sequence-based differences in phenotype are known as "epigenetic" phenomena. It has become clear that epigenetic regulation of gene expression is a common and widespread process affecting traits, particularly affecting gene expression in transgenic plants. An understanding of the molecular basis of epigenetically-modified phenotypes is essential to predictive trait modification by transgenic approaches. We have studied a special case were the presence of one gene, unstable factor for orange1 (ufo1), directing ectopic expression of a specific allele of another gene, pericarp color1 (p1) in tissues were that allele of p1 is usually silent. The alleles of p1 that are affected by ufo1 all have a duplicated, i.e., multicopy, gene structure. This structure also occurs in many transgenes that undergo epigenetic gene silencing. ufo1 modifies the p1 gene by reducing its methylation and this hypomethylation is correlated to increase mRNA for p1 and for structural genes of the flavonoid pathway regulated by p1. These results are important to researchers in understanding the relationships of methylation to gene expression and of epigenetic modification to trait expression.
Technical Abstract: We have characterized Ufo1 (unstable factor for orange1), a dominant, allele-specific modifier of expression of the maize pericarp color1 (p1) gene. The p1 gene encodes a Myb-homologous transcriptional activator of genes required for biosynthesis of red phlobaphene pigments. The P1-wr allele specifies colorless kernel pericarp and red cobs; whereas, Ufo1 modifies P1-wr expression to confer pigmentation in kernel pericarp, as well as vegetative tissues which normally do not accumulate significant amounts of phlobaphene pigments. In the presence of Ufo1, P1-wr transcript levels and transcription rate are increased in kernel pericarp. The P1-wr allele contains approximately six p1 gene copies present in a hypermethylated, multicopy tandem array. In P1-wr Ufo1 plants, methylation of P1-wr DNA sequences is reduced, whereas the methylation state of other repetitive genomic sequences was not detectably affected. The phenotypes produced by the interaction of P1-wr and Ufo1 are unstable, exhibiting somatic mosaicism and variable penetrance. Moreover, the changes in P1-wr expression and methylation are not heritable: meiotic segregants which lack Ufo1 revert to the normal P1-wr expression and methylation patterns. These results demonstrate the existence of a class of modifiers of gene expression whose effects are associated with transient changes in DNA methylation of specific loci.