Maternal RNA-directed DNA methylation is required for seed development inBrassica rapa

Some organisms deploy small RNAs from accessory cells to maintain genome integrity in the zygote, a mechanism that has been proposed but not demonstrated in plants. Here we show that maternal mutations in the Pol IV-dependent small RNA pathway cause abortion of developing seeds in Brassica rapa . Surprisingly, small RNA production is required in maternal somatic tissues, but not in maternal gametes or the developing zygote. We propose that parental influence over zygotic genomes is a common strategy in eukaryotes and that outbreeding species such as B. rapa are key to understanding the role of small RNAs during reproduction.