Epigenetics and its implications for plant biology 2. The 'epigenetic epiphany': epigenetics, evolution and beyond

Abstract

Scope: In the second part of a two-part review, the ubiquity and universality of epigenetic systems is emphasized, and attention is drawn to the key roles they play, ranging from transducing environmental signals to altering gene expression, genomic architecture and defence.

Key issues: The importance of transience versus heritability in epigenetic marks is examined, as are the potential for stable epigenetic marks to contribute to plant evolution, and the mechanisms generating novel epigenetic variation, such as stress and interspecific hybridization.

Future prospects: It is suggested that the ramifications of epigenetics in plant biology are immense, yet unappreciated. In contrast to the ease with which the DNA sequence can be studied, studying the complex patterns inherent in epigenetics poses many problems. Greater knowledge of patterns of epigenetic variation may be informative in taxonomy and systematics, as well as population biology and conservation.

Fig. 1.

Formation of synthetic Arabidopsis suecica allotetraploids (T = A. thaliana genome, A = A. arenosa genome).

Fig. 2.

How hybrid formation may lead to the generation of ectopic promoters and aberrant transcripts.

Fig. 3.

Hypothetical model showing outcome of divergence in miRNA expression patterns in two plant species and their hybrid. (A) Two species, 1 and 2, share the same target gene (gene 1.1 and 1.2) that promotes cell divisions and outgrowth of a tissue (expression domain indicated by lines). However, the two species differ in the expression pattern of microRNA genes (microRNA1 and microRNA2, respectively), that both target this gene (expression domain marked by lines). (B) After miRNA-induced degradation, the phenotypic outcome on the tissue is seen. The hybrid differs markedly from both parents as the actions of both miRNA1 and miRNA2 have limited expression to only a small group of cells in the tissue.

Fig. 4.

Epigenetic control of floral symmetry in Linaria vulgaris (common toadflax). Right: wild-type L. vulgaris where the Lcycloidea locus controlling dorso-ventral patterning is expressed normally. Note the zygomorphic flower with the ventral petal lobe displaying a spur. Left: the peloric epimutant shows hypermethylation of the Lcyc sequence and loss of expression of Lcyc. The flowers show greater radial symmetry as the ventral petal lobe is repeated five times. The plant shown here is homozygous for the Lcyc epiallele; heterozygotes show normal floral development.