| Abstract: | In plants as well as in animals, hundreds to thousands of 45S rRNA gene copies
localize in Nucleolus Organizer Regions (NORs), and the activation or repression of
specific sets of rDNA depends on epigenetic mechanisms. Previously, we reported that
the Arabidopsis thaliana nucleolin protein NUC1, an abundant and
evolutionarily conserved nucleolar protein in eukaryotic organisms, is required for
maintaining DNA methylation levels and for controlling the expression of specific
rDNA variants in Arabidopsis. Interestingly, in contrast with animal
or yeast cells, plants contain a second nucleolin gene. Here, we report that
Arabidopsis NUC1 and NUC2 nucleolin genes are
both required for plant growth and survival and that NUC2 disruption
represses flowering. However, these genes seem to be functionally antagonistic. In
contrast with NUC1, disruption of NUC2 induces CG
hypermethylation of rDNA and NOR association with the nucleolus. Moreover, NUC2 loss
of function triggers major changes in rDNA spatial organization, expression, and
transgenerational stability. Our analyses indicate that silencing of specific rRNA
genes is mostly determined by the active or repressed state of the NORs and that
nucleolin proteins play a key role in the developmental control of this process. |
