Epigenetic control, development and natural genetic variation in plants

Plant life strategies differ radically from those of most animals. Plants are not motile, and can only face stress by developing appropriate physiological responses. In addition, many developmental decisions take place during post-embryonic life in plants, whereas vertebrate and invertebrate development is nearly complete by the time of birth. For instance, while the germ line is typically set aside early during embryogenesis in animals, plants produce gametes from stem cell populations that were previously used for the vegetative growth of shoots. Nevertheless, plants and animals have similar nuclear organization, chromatin constitution and gene content, which raises the question as to whether or not fundamental differences in the use of genetic information underlie their distinct life strategies. More specifically, we would like to know if chromatin and the epigenetically defined, heritable cell fates that it can confer play comparable roles in plants and animals. Here we review our current knowledge on chromatin-mediated epigenetic processes in plants. Based on available evidence, we argue that epigenetic regulation of gene expression plays a relatively minor role in plants compared to mammals. Conversely, plants appear to be more prone than other multicellular organisms to the induction of chromatin-based, epigenetically modified gene activity states that can be transmitted over many generations. These so-called "epimutations" may therefore represent a significant proportion of the natural genetic variation seen in plants. In humans, epimutations are frequently observed in cancers, and given their metastable nature, they could also play an important role in familial disorders that do not demonstrate clear Mendelian inheritance.