MADS31 supports female germline development by repressing the post-fertilization program in cereal ovules

The female germline of flowering plants develops within a niche of somatic ovule cells, also referred to as the nucellus. How niche cells maintain their own somatic developmental program, yet support the development of adjoining germline cells, remains largely unknown. Here we report that MADS31, a conserved MADS-box transcription factor from the B-sister subclass, is a potent regulator of niche cell identity in barley. MADS31 is preferentially expressed in nucellar cells directly adjoining the germline, and loss-of-function mads31 mutants exhibit deformed and disorganized nucellar cells, leading to impaired germline development and partial female sterility. Molecular assays indicate that MADS31 encodes a potent transcriptional repressor, repressing genes in the ovule that are normally active in the seed. One prominent target of MADS31 is NRPD4b, a seed-expressed component of RNA polymerase IV/V that is involved in gene silencing via RNA directed DNA methylation. NRPD4b is directly repressed by MADS31 in vivo and is de-repressed in mads31 ovules, while overexpression of NRPD4b recapitulates the mads31 ovule phenotype. This coincides with specific changes in histone methylation and is consistent with NRPD4b being directly repressed by MADS31 to maintain ovule niche functionality. Our findings reveal a new mechanism by which somatic ovule tissues maintain their own identity before transitioning to the post-fertilization program.