Histone chaperone involved in maintining knox genes silencing throughout leaf development (PubMed:16243907). Involved in heterochromatic and euchromatic gene silencing, especially upon salt stress (PubMed:25600486). Involved in gene expression reprogramming during dedifferentiation probably by modifying histone H3.3 recruitment at the nucleolus (PubMed:25086063, PubMed:25600486). Contributes to maintenance of silencing of pericentromeric repeats and certain transposons (PubMed:25600486). Component of the HIRA complex made of UBN1, UBN2, ASF1A, CABIN1 and HIRA (PubMed:25086063, PubMed:25600486). Binds to histone H3.3, UBN1, UBN2 and ASF1A (PubMed:25086063). Interacts with AS1 (PubMed:16243907). Localized at rDNA loci in the nucleolus. A number of isoforms are produced. According to EST sequences. Expressed in the meristem and developing leaf primordia. Plants are embryo lethal when homozygous (PubMed:16243907). Retarded root growth. Abnormal gene expression reprogramming during dedifferentiation in root-derived protoplasts (PubMed:25086063). Reduced fertility leading to less seeds production. Reduced histone H3 levels and decreases nucleosome occupancy at both actively transcribed genes and heterochromatic regions (PubMed:25600486). Belongs to the WD repeat HIR1 family.