Microtubules in Plant Cells

Microtubules in plant cells are responsible for a series of structures and processes absent from animal cells. In plant cells, microtubules stack at the cell cortex as parallel loops. These loops act as the tracks on which cellulose synthase moves while it synthesizes cellulose microfibrils. As such, the cellulose microfibrils are produced parallel to the cortical microtubules. Disrupting either of these processes results in isotropic growth. Next, during cell division, cortical microtubules will accumulate into a band around the nucleus known as the preprophase band (PPB). The PPB dissolves before spindle assembly leaving behind division markers that label where the cell plate will join the cell wall. Lastly, microtubules play another important role in plant cell division during the formation of the phragmoplast. The phragmoplast is a complex of microtubules, actin, and vesicles that is generated after nuclear separation and serves as the cytokinetic mechanism of plant cell division. To generate the cell plate precursor to the cell wall that will eventually divide the new plant cells, microtubules are arranged parallel to one another pointing towards the region demarcated by the division markers. Along the microtubules are transported vesicles containing callose that will fuse to form the cell plate, growing it until it reaches the pre-existing cell wall. This mechanism generates a precursor cell wall made of callose that will eventually be replaced with cellulose. Additionally, the formation of the cell plate involves production of primary plasmodesmata due to the presence of the endoplasmic reticulum (ER). The ER blocks the cell plate in certain areas generating holes in the cell wall that will become the primary plasmodesmata between the newly divided cells.