Histone reader involved in homologous recombination-mediated repair of double-strand breaks (DSBs) at stalled or collapsed replication forks (PubMed:35298257). Specifically recognizes and binds histone H3.1 lacking methylation at 'Lys-27' (H3K27me1) (PubMed:35298257). Involved in structural and functional stabilization of chromatin; represents a link between response to DNA damage and epigenetic gene silencing (PubMed:14966212, PubMed:15082530, PubMed:16728410, PubMed:21705754). Required for cell arrangement in root and shoot apical meristems (PubMed:14966212, PubMed:15082530, PubMed:16728410, PubMed:21705754). May be involved, when interacting with TSA1, in the organization of spindle microtubules (PubMed:15964904). Interacts (via TPR repeats) with TSA1 (via C-terminal domain). During anaphase, relocalizes in spots close to the ends of spindle microtubules ahead of separating chromatids. Preferentially expressed in the shoot apex than in the leaves and stems. Expressed in the shoot apex and root tips in 5-day-old seedlings. At the shoot apex it is expressed in young leaves and in broad areas between two cotyledons. Weakly or not expressed in vascular tissues except vascular tissue near the shoot apical meristem. In the root, it extends from the root apical meristem to the elongation zone. Under cell cycle-dependent control with a peak of expression at the S phase. Abnormal enlargement of the shoot apical meristem associated with a disrupted cellular and subcellular organization. Release of transcriptional gene silencing of heterochromatic genes, and ectopic gene expression and organogenesis. Under-expression of FLC and early flowering in short days. Belongs to the Tonsoku family.