Deciphering actomyosin functions in plant cytokinesis: A half-century odyssey

Half a century ago, it was discovered that in somatic plant cells actin microfilaments play a critical role in division plane orientation but not in cell plate assembly during cytokinesis. Because plant cytokinesis is brought about by the microtubule-based phragmoplast, the actomyosin system must be required for the centrifugally expanding phragmoplast to recognize the cortical division zone established by the microtubular preprophase band (PPB). It has been intriguing to learn how the two cytoskeletal elements communicate with each other in the two temporally separated cytokinetic apparatuses of the PPB and the phragmoplast. Half a century later, it is now clear that the PPB recruits the actomyosin system to the cortical division zone where the Myosin XI motors form macromolecular assemblies with Kinesin-12 motors and other microtubule-associated proteins. These Cytoskeleton-Associated Motor assemblies at the PPB site (CAMPs) play critical roles in the maintenance of the division site established by the PPB. They receive microtubules emanating from the edge of the expanding phragmoplast so that the developing cell plate can be inserted into the spatially defined position. Therefore, the actomyosin system joins microtubules to orchestrate the recognition of the PPB-defined position by the phragmoplast in order to execute cytokinesis in a spatiotemporally regulated manner in somatic plant cells.