AtMPK4 is required for male-specific meiotic cytokinesis in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades have been implicated in regulating various aspects of plant development, including somatic cytokinesis. The evolution of expanded plant MAPK gene families has enabled the diversification of potential MAPK cascades, but functionally overlapping components are also well documented. Here we report that Arabidopsis MPK4, an MAPK that was previously described as a regulator of disease resistance, can interact with and be phosphorylated by the cytokinesis-related MAP kinase kinase, AtMKK6. In mpk4 mutant plants, anthers can develop normal microspore mother cells (MMCs) and peripheral supporting tissues, but the MMCs fail to form a normal intersporal callose wall after male meiosis, and thus cannot complete meiotic cytokinesis. Nevertheless, the multinucleate mpk4 microspores subsequently proceed through mitotic cytokinesis, resulting in enlarged mature pollen grains that possess increased sets of the tricellular structure. This pollen development phenotype is reminiscent of those observed in both atnack2/tes/stud and anq1/mkk6 mutants, and protein-protein interaction analysis defines a putative signalling module linking AtNACK2/TES/STUD, AtANP3, AtMKK6 and AtMPK4 together as a cascade that facilitates male-specific meiotic cytokinesis in Arabidopsis.