Flavanols in nuclei of tree species: facts and possible functions

This review presents a new conceptual model for the involvement of low molecular flavanols in chromatin remodelling and genome organization. The experiments are based on the property of flavanols to associate with nuclei as revealed by blue staining after treatment with the p-dimethylaminocinnamaldehyde reagent. From a critical standpoint, this puzzling finding is nearly incompatible with current views about nuclear organization. Therefore, it was necessary to collect a whole host of data to confirm this new aspect and to gain some insight into possible regulatory roles of histone-flavanol assemblies. A lot of research has been devoted to this topic over the last 13 years. In particular, conifer nuclei were found to contain flavanols, whereas the nuclei of most angiospermous tree species investigated until now reacted negatively. Camellia sinensis (tea bush), being a broad-leaved dicotyledonous species indeed has nuclei with prominent flavanol staining. A subnuclear patterning of flavanols can be observed which is regulated by genetic and epigenetic mechanisms. Broadly speaking, flavanols of nuclei range from evenly diffuse to mosaic-like mottling and from pale to dark blue. The diffuse type is apparently characteristic of a more silenced nuclear state, whilst mild to prominent mottling implicates a transcriptionally more activated state. Dark blue flavanol blobs within the mottled mosaic state indicate a heterochromatin pattern whilst pale blue stippling tends to euchromatin. Environmental stress conditions such as drought combined with heat induce key signals for down-regulation of nuclear flavanols. In this article, various aspects of nuclear flavanol localization are summarized and discussed.