Maintenance of H3K9me2 heterochromatin in the pollen vegetative nucleus requires ARID1 nuclear body formation.

Mature pollen contains a vegetative nucleus with decondensed chromatin and two sperm cells engulfed within the vegetative cytoplasm. Chromatin decondensation in plants usually accompanies a decrease in heterochromatin markers such as H3K9me2, H3K27me1, and H2A.W. However, H3K9 methyltransferases are detected primarily in the vegetative nucleus. This finding raises two questions: whether heterochromatin persists in the vegetative nucleus, and by what mechanism it is regulated. Here, we show that the vegetative nucleus harbors a basal level of H3K9me2, and that ARID1, an integrator coordinating pollen mitosis and heterochromatin reprogramming during sperm cell formation, promotes the maintenance of this basal level via its C-terminal tail-mediated nuclear body formation. The heterochromatin nature of ARID1 nuclear bodies is further confirmed by their colocalization with H2A.W and H3K27me1. Domain truncation analysis reveals that nuclear body formation of ARID1 depends on its C-terminal forty-nine amino acids. Deletion of this short C-terminal region abolished nuclear body formation and inhibited the interaction with H3K9 methyltransferase SUVH6. Disrupting the nuclear body localization of ARID1 significantly impairs its recruitment to H3K9me2 target loci. Therefore, the ARID1 nuclear body is necessary for maintaining H3K9me2 in the vegetative nucleus, which ultimately influences the regulation of nuclear size. Our findings demonstrate that the vegetative nucleus, despite its decondensed state, maintains a basal level of H3K9me2 that is regulated by nuclear body-localized ARID1.