Ainsworth, C. C. [ed.]: Sex Determination in Plants
(BIOS Scientific Publishers Ltd, Oxford, ISBN 1 85996 042 1, Oxford, United Kingdom 1999)

Chapter 6 (pp. 101-120) - Boris Vyskot
The role of DNA methylation in plant reproductive development

Abstract:
During the last few years, data have accumulated which show that DNA methylation and core histone acetylation play an important role in the long-term control of gene expression and development. They are obviously responsible for (or at least accompany) mitotic (cell memory) and meiotic transmission (epigenetic inheritance and genomic imprinting) of gene expression. DNA demethylation and/or histone acetylation seem to be a necessary step toward potential gene activation. However, not all eukaryotic genes are regulated by methylation and these exceptions, which include Drosophila, nematodes, ciliate protozoans and yeasts, illustrate that DNA methylation is only one of several possible modifying controls used in epigenetic regulation. This suggests that methylation as a device for transcriptional regulation appeared relatively late in the evolution of animal and plant kingdoms. The molecular effects of methylation in transcriptional regulation have not yet been elucidated fully, but recent data implicate transcriptional repressors of methylated DNA accompanied by a specific chromatin assembly and indicate that CpG methylation alters the chromatin structure by preventing the histone octamer from interaction with an otherwise high affinity positioning sequence in the promoter region. DNA methylation obviously plays a pleiotropic role in eukaryotic cells and organisms. It represents a defence mechanism used to inactivate mobile genetic elements and other types of intrusive DNA (e.g., transgenes). It is involved in the condensed, heterochromatic structure of repetitive DNA sequences. It ensures monoallelic expression of some genes depending on their parent-of-origin (genomic imprinting). It could accompany permanently or transiently inactive chromosomes or their complete sets (e.g., one X chromosome inactivation in female mammals), and it participates in a long-term inactivation of developmentally controlled genes. Recent data demonstrate that DNA hypermethylation and/or core histone underacetylation are chromatin modifications which characterise a transcriptional inertness in both constitutive and facultative heterochromatin. They do not seem to be causative processes of transcriptional inactivation as demonstrated by experiments on the mammalian inactive X chromosome; both DNA hypermethylation and histone H4 underacetylation follow the silencing of X-linked genes. In flowering plants, both the most important mechanisms of epigenetic control, DNA methylation and histone acetylation, have been unambiguously demonstrated. The experimental data on Melandrium albumpresented here show that its sex expression is under DNA methylation control and indicate that in dioecious plants, due to the gender separation, epigenetic mechanisms could also play other roles (dosage compensation, genomic imprinting) as in mammals. The isolation of genes involved in M. album sex expression or genes located on the sex chromosomes will clearly help to shed light on these processes.

Contents:
6.1. Introduction
6.2. DNA methylation and development
6.2.1 Methylation patterns during mammalian ontogenesis
6.2.2 Role of DNA methylation in plant growth and development
6.3. Experimental studies on dioecious Melandrium album
6.3.1 Epigenetic sex reversal
6.3.2 Different roles of X and Y chromosomes in plant development:
when does the differentiation of the sexes begin?
6.3.3 Is there dosage compensation of X-linked genes?
6.4. Conclusions

Schematic comparison of the chromosome constitutions of female and male developing seeds of dioecious Melandrium album,populations of which regularly display a more/less strong female bias (60-70% of adult plants are females). Sets of autosomes are indicated as the A, the chromosomes coming from the male parent (pollen donor) are shown in italics.