During the first phase of Arabidopsis female gamete formation (megasporogenesis), a somatic ovule cell differentiates into a megaspore mother cell and divides to generate four haploid megaspores. In the next phase (megagametogenesis), one of these megaspores undergoes syncytial mitosis and differentiates to form the female gametophyte. It’s known that a somatic small RNA (sRNA) pathway restricts reproductive potential to this functional megaspore but what controls the megasporogenesis to megagametogenesis transition? Here (), Matthew Tucker and co-workers examine gene expression patterns in ovule tissues and show that an sRNA pathway is also involved in this phase of female gamete formation. The researchers report that ARGONAUTE5 (AGO5), a putative sRNA pathway effector, is expressed around reproductive cells during megasporogenesis and show that a unique semi-dominant ago5-4 insertion allele disrupts the initiation of megagametogenesis. Expression of a viral RNAi suppressor protein in the somatic cells flanking the megaspores produces a similar phenotype. Thus, the researchers conclude, at least two somatic sRNA pathways contribute to female gametophyte development in Arabidopsis.
