Characterization of a new histone deacetylase 6 (hda6-11) mutant allele in Arabidopsis thaliana

Histone deacetylases (HDACs or HDAs) are highly conserved among eukaryotes. Arabidopsis HDA6 is a Class I, RPD3-like HDAs that plays a role in transcriptional gene silencing, seed and embryo development, regulation of circadian rhythm, and stress tolerance. HDA6 also plays a role in selectively silencing ribosomal RNA (rRNA) genes. Arabidopsis thaliana has hundreds of 45S ribosomal RNA (rRNA) genes tandemly arrayed at chromosomal loci known as Nucleolus Organizer Regions (NORs) on chromosomes 2 (NOR2) and 4 (NOR4). As part of rRNA gene dosage control during development, rRNA genes on NOR2 undergo selective gene silencing while those on NOR4 remain active. To study HDA6 domains' functions, several mutant alleles comprising mutations from different domains of HDA6 have been created and characterized. In this study, we characterized a new hda6 mutant allele which we named hda6-11. We show that hda6-11 mutants carry a T-DNA insertion in the first exon of HDA6 gene, resulting in a null mutation. The hda6-11 mutants displayed slow growth and delayed flowering phenotypes similar to other hda6 mutants. Using Chi-square goodness-of-fit analysis of genotyping data from segregating progenies involving two null mutant alleles of HDA6 (hda6-7 and hda6-11), we show that hda6 null mutations affect plant survivability. Lastly, we show that the null mutant hda6-11 and hda6-8, which carries a base substitution mutating a conserved amino acid in the Essential SAS Family Acetyltransferase1-Reduced Potassium Dependency3 (Rpd3; E-R) motif, failed to silence NOR2 genes, confirming the role of HDA6 in large-scale selective gene silencing of rRNA genes during development in Arabidopsis.