Changes in Chromatin Structure during Plant Development
Plants differ from animals in at least two important features: in the formation of seeds, which represent units of dispersal capable to survive unfavourable surroundings in a quiescent stage, and in an incomplete starting body plan, which is formed during the plant life from meristematic cells. A strong desiccation and metabolic inactivation occur during seed maturation. On the contrary, seed germination is coupled with water uptake and metabolic activation. In regards of these facts, processes of seed maturation and germination have to be connected with changes in chromatin structure which are able to ensure inactivation and later activation of the plant genome. We have immunohistochemically studied DNA methylation dynamics during the plant germination and subsequent plant growth. Our results show the highest DNA methylation signals on the sections of dry seeds restricted to the tips of cotyledons and apical and root meristems, while the other parts of the embryo are characterised by a lower 5-methylcytosine signal. During the seed germination, a very rapid non-replicative decrease of the DNA methylation signal in the endosperm nuclei was observed. Further changes in DNA methylation patterns also occurred without DNA replication: first, a strong decrease of the DNA methylation signal was found in the cells of the radicula and hypocotyl, a strong demethylation of the cotyledons was observed in the early postgerminative phase of plantlet development. The high DNA methylation signal remained apparent in cells of the central zone of the shoot apical meristem till the time of flowering. This high level of DNA methylation is in correlation with a low cell division activity. However, when the floral meristem is formed, the central zone of meristem is demethylated and the cells start to divide in a high intensity and form a flower. There are two mechanisms providing a protection of the cells in the central zone against possible mutations during the vegetative phase of development: (i) the high DNA methylation level resulting in a compact structure of nuclei and (ii) a low cell division rate. These data indicate that the cells of the central zone are stored during the early stage of plant development in a stock, similarly as the germline in many animal species. This temporary quiescence reduces mutation frequencies which would have been transmitted to sexual progeny. - This research was supported by the Grant Agency of the Czech Republic (521/99/0696).
J. Zluvova and B. Vyskot
Laboratory of Plant Developmental Genetics,