Study On The Function Of Histone Methyltransferase SDG25 And Histone Chaperone NAP1 In Arabidopsis Thaliana

The SET domain proteins are recognized as generally having histone methyltransferase activity. The Arabidopsis SDG25is clustered together with Trithorax group (TrxG) SET domain proteins in physlogenetic analysis. We showed that the recombinant SDG25protein could methylate in vitro histone H3using oligonucleosomes as a substrate. Using T-DNA insertion lines, we showed that loss-of-function of SDG25caused an early-flowering phenotype, which is associated with a reduced level of expression of the flowering repressor FLC. By western-blot and ChIP analysis, we demonstrated that H3K36di-methylation was weakly reduced at FLC chromatin. Together our results establish a function for the previously uncharacterized gene SDG25in the FLC activation and flowering time regulation.Histone chaperones play a crucial role in mediating nucleosome assembly/disassembly in diverse chromatin metabolism and epigenetic regulation. Nucleosome Assembly Protein1(NAP1) represents the primary chaperone of H2A and H2B. The Arabidopsis genome contains6genes encoding NAP1homologues belonging to two phylogenic groups:NAP1-subgroup and NRP-subgroup. The NAP1-subgroup comprises4members. We showed that AtNAP1;4is specifically expressed in pollen and in the elongation zone of the root. Plant growth is unaffected after knocking-down the other three ubiquitously expressed AtNAP1-subgroup genes. Nevertheless, the two allelic triple mutants m123-1and m123-2exhibit perturbed genome transcription and show hypersensitivity to DNA damage caused by UV-C irradiation. Ch1P assays show that AtNAP1;3binds chromatin with higher enrichment at genes involved in the nucleotide excision repair (NER) pathway and in vitro DNA repair assays show that the DNA repair efficiency of the triple mutants m123-1and m123-2is lower than that of wild-type plants. Furthermore, we found that the Atnap1;3-2allele expresses a truncated protein, AtNAPl;3-T, which lacks34amino acids at the C-terminus. This mutant allele exhibits ABA-hyposensitive and a decreased tolerance to salt stress phenotype. The expression of some ABA-responsive genes was found to be perturbed in the mutant and in the AtNAP1;3T-overexpression transgenic plants. Together our work indicates that AtNAP1proteins can function as modulators of chromatin structure in response to different environmental stresses.The ml23-I and m123-2mutants are hypersensitive to bleomycin treatment that causes DSBs in plant genome. Homologous recombination (HR) is important for DSB repair in all eukaryotes. Using GUS-based recombination substrates, I demonstrated that somatic HR is impaired in ml23-1and m123-2as well as nrpl-1nrp2-1mutant plants under standard or divers stress growth conditions, and that nrp1-1nrp2-1dominates fas2-4specifically in HR in a way that the nrpl-Inrp2-1lfas2-4plants exhibit a low HR similar to nrp1-lnrp2-1but a morphological phenotype largely similar to fas2-4. Though precise function of AtNAP1/NRP proteins in HR still needs deepened understanding, it is very tempting to speculate that AtNAP/NRP proteins as primarily identified histone H2A/H2B chaperone in Arabidopsis, contribute to initial steps such as extensive end resection after DSB, strand invasion and heteroduplex extension during HR.