b and pl Paramutation in Maize: Heritable Transcription States Programmed during Development

Abstract

As discussed in the accompanying chapters, most epigenetic regulatory mechanisms result in mitotically stable, but meiotically unstable, changes. The regulatory mechanism(s) associated with paramutation in flowering plants is exceptional because the changes initiated during somatic development are meiotically heritable; in a sense, blurring the distinction of epigenetic versus genetic alterations. Specifically, paramutation is an interaction between two alleles that leads to a directed alteration in transcription that is both mitotically and meiotically heritable. That such meiotically heritable changes appear to be confined to plants (Jorgensen 1993) suggests either that plants have evolved distinct regulatory mechanisms or that the basic differences in reproductive biology allow such somatic changes to be sexually transmitted (Patterson and Chandler 1995a). An understanding of paramutation should reveal how homologous chromosomes communicate in somatic cells and how transcription states that are programmed early in development can be stably transmitted through numerous cell divisions and into the next generation.

Paramutation was first described at the r locus in maize (Brink 1958) and has been studied at two other maize genes, b (Coe 1966) and pl (Hollick et al. 1995). All three loci encode transcription factors that regulate the purple anthocyanin pigment biosynthetic pathway. The r and b genes encode functionally equivalent basic-helix-loop-helix (bHLH) proteins, with different alleles at each gene expressed under distinct tissue-specific and developmental controls (Goff et al. 1990; Ludwig and Wessler 1990; Ludwig et al. 1990; Radicella et al. 1992). All b alleles examined have only a single coding region, whereas most r alleles contain...