Minisymposium 6: Epigenetics

Abs # 16004: Rmr6 functions in paramutation and developmental epigenetics

Presenter:	Parkinson, Susan E., sep@uclink.berkeley.edu
Authors	Parkinson, Susan E. (A)   Hollick, Jay B. (A)
Affiliations:	(A): University of California, Berkeley

The rmr6 (required to maintain repression6) locus encodes one of several trans-acting factors required to maintain paramutation-induced silencing at the purple plant1 (pl1) locus. The pl1 gene encodes an R2R3 Myb-type transcriptional activator that regulates the expression of genes required for anthocyanin biosynthesis. The Pl-Rhoades (Pl-Rh) allele can adopt various epigenetic states of expression that are directly reflected by visible levels of pigmentation. Strong expression states (referred to as Pl) undergo transcriptional repression when coupled in a heterozygote with weak expression states (referred to as Pl’), an example of paramutation. Pl-Rh alleles that segregate from a Pl/Pl' heterozygote remain repressed and direct silencing of naive Pl states in subsequent progeny. Molecular and genetic analyses indicate that RMR6 is necessary for the maintenance of Pl’ repression. Pl’/Pl’ plants with an rmr6 mutant genotype have high levels of anthocyanin pigmentation. Run-on transcription experiments demonstrate that these plants have increased pl1 transcription rates and pl1 transcript levels, relative to non-mutant siblings. Also, approximately 36% of Pl’ alleles transmitted from rmr6 mutant genotypes lose the capacity to repress Pl states. In addition to its role in maintaining epigenetic repression of Pl’, RMR6 ensures proper plant development. Phenotypic analyses implicate RMR6 involvement in plant growth and apical internode elongation, establishment of abaxial/adaxial polarity, floral organ identity, and lateral meristem repression. RMR6 also ensures proper tissue-specific expression patterns of specific regulatory factors. Double mutant analysis suggests that RMR6 restricts sk1 (silkless1) expression from the apical inflorescence. Similarly, RMR6 appears to restrict pl1 expression from aleurone tissues, a site where the paralogous colorless1 (c1) factor is expressed. The action of RMR6 in pl1 paramutation and its important role in developmental progression supports the hypothesis that the epigenetic regulation of paramutation is related to normal mechanisms of developmental gene regulation.