Paternal easiRNAs regulate parental genome dosage in Arabidopsis

The regulation of parental genome dosage is of fundamental importance in animals and plants, exemplified by X chromosome inactivation and dosage compensation. The "triploid block" is a classical example of dosage regulation in plants that establishes a reproductive barrier between species differing in chromosome number. This barrier acts in the endosperm, an ephemeral tissue that nurtures the developing embryo and induces the abortion of hybrid seeds through a yet unknown mechanism. Interploidy hybridizations involving diploid (2x) maternal parents and tetraploid (4x) pollen donors cause failure in endosperm cellularization, leading to embryo arrest. Here we show that paternal epigenetically activated small interfering RNAs (easiRNAs) are responsible for the establishment of the triploid block-associated seed abortion in Arabidopsis thaliana. Paternal loss of the plant-specific RNA polymerase IV suppressed easiRNA formation and rescued triploid seeds by restoring small RNA-directed DNA methylation at transposable elements (TEs), correlating with reduced expression of paternally expressed imprinted genes (PEGs). We propose that excess of paternally derived easiRNAs in diploid pollen prevents establishment of DNA methylation, leading to triploid seed abortion. Our data further suggest that easiRNAs form a quantitative signal for chromosome number and their balanced dosage is required for post-fertilization genome stability and seed viability.