Essential protein that regulates phragmoplast microtubule organization during cell plate expansion in cytokinesis during cell division, both somatic and syncytial (PubMed:11089872, PubMed:19268593, PubMed:23451828). Required for endosperm cellularisation (PubMed:12421698, PubMed:23451828). In pollen development, involved in cellularisation during microsporogenesis by regulating radial microtubules (MT) organization in microspore mother cells (PubMed:23451828). Seems to not have kinase activity (PubMed:19268593). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Binds to microtubules (MT). Colocalizes also with mitotic preprophase band. Expressed in proliferating tissues of seedlings, lateral roots, young rosette leaves, siliques, flowers, embryos and stems (including apical meristem). Cell-cycle-regulated. Seedling lethal (PubMed:19268593). Abnormal cell division with defective cytokinesis due to abnormal phragmoplast organization and arrested cell plate expansion, leading to enlarged cells and nuclei as well as incomplete cell walls (PubMed:11089872, PubMed:19268593). In metaphase and anaphase, enlarged cells with several mitotic spindles or a single greatly enlarged spindle (PubMed:19268593). Impaired endosperm development with altered cellularisation (PubMed:12421698, PubMed:23451828). Cellularization defects caused by disorganized radial microtubules (MT) arrays in seedlings and adult tissues, as well as during male meiocyte development. Irregular and incomplete or absent intersporal cell walls in male tetrads, resulting in abnormal pollen and reduced fertility. Sterile and aborted ovules (PubMed:23451828). Belongs to the protein kinase superfamily. Ser/Thr protein kinase family.