A MAP kinase is activated late in plant mitosis and becomes localized to the plane of cell division

In eukaryotes, mitogen-activated protein kinases (MAPKs) are part of signal ing modules that transmit diverse stimuli, such as mitogens, developmental cues, or various stresses. Here, we report a novel alfalfa MAPK, Medicago M AP kinase 3 (MMK3). Using an MMK3-specific antibody, we detected the MMK3 p rotein and its associated activity only in dividing cells. The MMK3 protein could be found during all stages of the cell cycle, but its protein kinase activity was transient in mitosis and correlated with the timing of phragm oplast formation. Depolymerization of microtubules by short treatments with the drug amiprophosmethyl during anaphase and telophase abolished MMK3 act ivity, indicating that intact microtubules are required for MMK3 activation . During anaphase, MMK3 was found to be concentrated in between the segrega ting chromosomes; later, it localized at the midplane of cell division in t he phragmoplast. As the phragmoplast microtubules were redistributed from t he center to the periphery during telophase, MMK3 still localized to the wh ole plane of division; thus, phragmoplast microtubules are not required to keep MMK3 at this location. Together, these data strongly support a role fo r MMK3 in the regulation of plant cytokinesis.