DNA methylation and differentiation in plants.

DNA methylation in plants appears to be a phenomenon more complex than in animals both for site specifity and the possible occurrence of two types of DNA methyltransferases. As in animals, DNA methylation appears to play a role in plant gene expression and differentiation (though recently acquired evidence indicated a less immediate relationship); other suggested roles for DNA methylation include genomic imprinting, paramutation and, to some extent, the gene silencing phenomenon observed in transgenic plants. However, while results obtained in mouse strictly link the presence of an active DNA methyltransferase with normal embryogenesis, in plants only circumstantial evidence points to the involvement of this enzyme in carrot somatic embryogenesis. What appears puzzling is the observation that hypomethylating mutants of Arabidopsis thaliana show normal development and morphology. The involvement of DNA methylation in plastid development is also debated. Of some interest appears the occurrence in plant cells of inhibitors of DNA methyltransferase activity although their biological significance and chemical nature remain to be elucidated.