TISSUE-SPECIFIC PATTERNING SPECIFIED BY A MULTICOPY PERICARP COLOR1 ALLELE IN MAIZE

Open Access
- Author:
- Robbins, Michael Lawrence
- Graduate Program:
- Plant Biology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 27, 2007
- Committee Members:
- Surinder Chopra, Committee Chair/Co-Chair
Kathleen Marie Brown, Committee Member
David Robert Huff, Committee Member
Paula Mc Steen, Committee Member - Keywords:
- transposon
maize
enhancer
flavonoid
mosaic
DNA methylation - Abstract:
- The extensive allelic series for the maize <I>pericarp color1</I> (<I>p1</I>) gene is a valuable resource for investigating how sequence and/or structure differences influence tissue-specific gene expression. Previous results showed that the single-copy <I>P1-rr</I> (red pericarp and red cob glumes) allele has considerably less DNA methylation as compared with the six-copy, tandemly-repeated <I>P1-wr</I> (white pericarp and red cob glumes) allele. Herein, the mechanism by which the tandem repeats of <I>P1-wr</I> influence the epigenetic suppression in pericarp tissue was investigated. The disruption of a single <I>P1-wr</I> copy by a <I>Mu1</I> transposon insertion into the 5’UTR induced a gain of pericarp pigmentation in the <I>P1-wr-mum6</I> allele. This gain of pericarp pigmentation was associated with DNA hypomethylation at a distal floral-organ-specific enhancer element. Interestingly, this hypomethylated state led to the expression from wild type copy(s) that were not interrupted by the transposon in the <I>P1-wr</I> mum6 allele. The correlation of copy number and silencing was further strengthened by the characterization of <I>P1-mosaic</I> (<I>P1-mm</I>) which has a <I>P1-wr</I> type gene structure but contains a reduced copy number. <I>P1-mm</I> has both a variegated pericarp pigmentation and a highly perturbed DNA methylation pattern. Interestingly, <I>P1-mm</I> was hypomethylated at the aforementioned distal enhancer and hypermethylated downstream of the transcription start site. These opposing DNA methylation changes are envisaged to explain the unpredictable, mosaic pigmentation pattern of <I>P1-mm</I>. Collectively, the <I>P1-wr-mum6</I> and <I>P1-mm</I> studies showed that the tandem-repeat structure of <I>P1-wr</I> is important for the epigenetic regulation that determines its tissue-specific expression. To further understand why the presence of multiple gene copies is conducive to the suppression of <I>p1</I> in pericarp tissue we studied the dominant <I>Unstable factor for orange1</I> (<I>Ufo1</I>) mutation. The presence of <I>Ufo1</I> induces ectopic pericarp pigmentation in <I>P1-wr</I> plants by a mechanism which correlates with the partial loss of DNA methylation at <I>P1-wr</I>. In addition to endogenous <I>P1-wr</I>, data herein suggests that <I>Ufo1</I> can also reactivate suppressed <I>p1</I> transgenes. Interestingly, although <I>Ufo1</I> has been previously characterized as unstable, its interaction with one <I>p1</I> transgene was highly penetrant and required the continued presence of <I>Ufo1</I>. To identify genes that are important for the range of <I>P1-wr Ufo1</I> expression in pericarp, we crossed a fully-penetrant <I>P1-wr Ufo1</I> stock with RNAi lines that target candidate chromatin-related genes. We found that the Retinoblastoma-associated Protein1 (RBAP1) may function to suppress the <I>Ufo1</I> mutation leading to its variable expression patterns. Collectively, this research broadens the understanding of how tandem repeats of genes influence the epigenetic control of tissue specific expression.