Arabidopsis NRP1 and NRP2 encode histone chaperones and are required for maintaining postembryonic root growth

NUCLEOSOME ASSEMBLY PROTEIN1 (NAP1) is conserved from yeast to human and was proposed to act as a histone chaperone. While budding yeast contains a single NAP1 gene, multicellular organisms, including plants and animals, contain several NAP1 and NAP1-RELATED PROTEIN (NRP) genes. However, the biological role of these genes has been largely unexamined. Here, we show that, in Arabidopsis thaliana, simultaneous knockout of the two NRP genes, NRP1 and NRP2, impaired postembryonic root growth. In the nrp1-1 nrp2-1 double mutant, arrest of cell cycle progression at G2/M and disordered cellular organization occurred in root tips. The mutant seedlings exhibit perturbed expression of approximately 100 genes, including some genes involved in root proliferation and patterning. The mutant plants are highly sensitive to genotoxic stress and show increased levels of DNA damage and the release of transcriptional gene silencing. NRP1 and NRP2 are localized in the nucleus and can form homomeric and heteromeric protein complexes. Both proteins specifically bind histones H2A and H2B and associate with chromatin in vivo. We propose that NRP1 and NRP2 act as H2A/H2B chaperones in the maintenance of dynamic chromatin in epigenetic inheritance.