Maternal components of RdDM are required for seed development in Brassica rapa

RNA directed DNA methylation (RdDM) is an epigenetic process in which plant double-strand RNAs are processed into small RNAs (sRNA) that add repressive DNA methylation to homologous DNA sequences. RdDM primarily acts on repetitive DNA and transposable elements (TEs). Despite its important biological functions, loss of RdDM components in Arabidopsis does not show much phenotypic consequences. In contrast, mutations in RdDM components in genomes with higher transposons content like maize and tomato result in severe phenotypic consequences including reproductive defects. Grover et al. show that mutations in Pol IV-dependent sRNA pathway cause severe reproductive defects, specifically during seed production in Brassica rapa. High rates of abortion occur when the RdDM mutants are crossed as the female parent, and not when the mutants are used as pollen donors. No vegetative growth defects associated with loss of RdDM were observed in these mutants. However, mutant siliques were smaller and produced fewer viable seeds. 24nt siRNA, the abundant class of siRNAs in reproductive tissues, are strongly reduced in the B. rapa mutants. The authors show that seed production defect is due to loss of p4-siRNA activity in the maternal sporophytic tissue even though the defects manifest later during seed development. In conclusion, the authors suggest that the control of developing progeny by parental small RNA is a common biological mechanism and B. rapa, as a recently outbreeding species, may be invaluable in understanding the role of RdDM in plant reproduction. Plant J. 10.1111/tpj.13910