The microtubule-associated kinase-like protein RUNKEL functions in somatic and syncytial cytokinesis

The microtubule (MT)-associated putative kinase RUNKEL (RUK) is an important component of the phragmoplast machinery involved in cell plate formation in Arabidopsis somatic cytokinesis. Since loss-of-function ruk mutants display seedling lethality, it was previously not known whether RUK functions in mature sporophytes or during gametophyte development. In this study we utilized RUK proteins that lack the N-terminal kinase domain to further examine biological processes related to RUK function. Truncated RUK proteins when expressed in wild-type Arabidopsis plants cause cellularization defects not only in seedlings and adult tissues but also during male meiocyte development, resulting in abnormal pollen and reduced fertility. Ultrastructural analysis of male tetrads revealed irregular and incomplete or absent intersporal cell walls, caused by disorganized radial MT arrays. Moreover, in ruk mutants endosperm cellularization defects were also caused by disorganized radial MT arrays. Intriguingly, in seedlings expressing truncated RUK proteins, the kinesin HINKEL, which is required for the activation of a mitogen-activated protein kinase signaling pathway regulating phragmoplast expansion, was mislocalized. Together, these observations support a common role for RUK in both phragmoplast-based cytokinesis in somatic cells and syncytial cytokinesis in reproductive cells.