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