Two Arabidopsis Phragmoplast-Associated Kinesins Play a Critical Role in Cytokinesis during Male Gametogenesis

In plant cells, cytokinesis is brought about by the phragmoplast. The phragmoplast has a dynamic microtubule array of two mirrored sets of microtubules, which are aligned perpendicularly to the division plane with their plus ends located at the division site. It is not well understood how the phragmoplast microtubule array is organized. In Arabidopsis thaliana, two homologous microtubule motor kinesins, PAKRP1/Kinesin-12A and PAKRP1L/Kinesin-12B, localize exclusively at the juxtaposing plus ends of the antiparallel microtubules in the middle region of the phragmoplast. When either kinesin was knocked out by T-DNA insertions, mutant plants did not show a noticeable defect. However, in the absence of both kinesins, postmeiotic development of the male gametophyte was severely inhibited. In dividing microspores of the double mutant, microtubules often became disorganized following chromatid segregation and failed to form an antiparallel microtubule array between reforming nuclei. Consequently, the first postmeiotic cytokinesis was abolished without the formation of a cell plate, which led to failures in the birth of the generative cell and, subsequently, the sperm. Thus, our results indicate that Kinesin-12A and Kinesin-12B jointly play a critical role in the organization of phragmoplast microtubules during cytokinesis in the microspore that is essential for cell plate formation. Furthermore, we conclude that Kinesin-12 members serve as dynamic linkers of the plus ends of antiparallel microtubules in the phragmoplast.