Morphogenesis requires the coordination of cell growth, division, and cell differentiation. Female gametogenesis in flowering plants, where a single haploid spore undergoes continuous growth and nuclear division without cytokinesis to form an eight-nucleate coenocytic embryo sac before cellularization, provides a good system to study the genetic control of such processes in multicellular organisms. Here we reported the characterization of an Arabidopsis female gametophyte mutant, slow walker2 (swa2), in which the progression of the mitotic cycles and the synchrony of female gametophyte development were impaired, causing an arrest of female gametophytes at two-, four- or eight-nucleate stages. Delayed pollination test showed that a portion of the mutant ovules were able to develop into functional embryo sacs and could be fertilized. SWA2 encodes a nucleolar protein homologous to yeast NOC1/MAK21 that, together with NOC2, is involved in pre-ribosome export from the nucleus to cytoplasm. Similarly, SWA2 can physically interact with a putative Arabidopsis NOC2 homologue. SWA2 is expressed ubiquitously throughout the plant, at high levels in actively dividing tissues and gametophytes. Together, we conclude that SWA2 most likely plays a role in ribosome biogenesis that is essential for the coordinated mitotic progression of the female gametophyte.

Morphogenesis requires the coordination of cell growth, division, and cell differentiation. Female gametogenesis in flowering plants, where a single haploid spore undergoes continuous growth and nuclear division without cytokinesis to form an eight-nucleate coenocytic embryo sac before cellularization, provides a good system to study the genetic control of such processes in multicellular organisms. Here we reported the characterization of an Arabidopsis female gametophyte mutant, slow walker2 (swa2), in which the progression of the mitotic cycles and the synchrony of female gametophyte development were impaired, causing an arrest of female gametophytes at two-, four- or eight-nucleate stages. Delayed pollination test showed that a portion of the mutant ovules were able to develop into functional embryo sacs and could be fertilized. SWA2 encodes a nucleolar protein homologous to yeast NOC1/MAK21 that, together with NOC2, is involved in pre-ribosome export from the nucleus to cytoplasm. Similarly, SWA2 can physically interact with a putative Arabidopsis NOC2 homologue. SWA2 is expressed ubiquitously throughout the plant, at high levels in actively dividing tissues and gametophytes. Together, we conclude that SWA2 most likely plays a role in ribosome biogenesis that is essential for the coordinated mitotic progression of the female gametophyte.