The new cell walls that bisect plant cells during cytokinesis develop from the cell plate. The precise plane of cell division, determined during late interphase, usually corresponds to the shortest path that will halve the volume of the parent cell. The first sign of this spatial orientation is rearrangement of the cytoskeleton. Microtubules in the cytoplasm become concentrated into a ring called the preprophase band. The band disappears before metaphase but predicts the future plane of cell division.
Although the plane of cell division does not determine the shape of plant organs, the symmetry of cell division—the distribution of cytoplasm between daughter cells—is important in determining cell fate. Not all plant cells divide into two equal halves during mitosis. Although chromosomes are allocated to daughter cells equally during mitosis, the cytoplasm may sometimes divide asymmetrically. Asymmetrical cell division, in which one daughter cell receives more cytoplasm than the other during mitosis, usually signals a key event in development.
For example, the formation of guard cells typically involves both an asymmetrical cell division and a change in the plane of cell division. An epidermal cell divides asymmetrically, forming a large cell that remains an unspecialized epidermal cell and a small cell that becomes the guard cell “mother cell.” Guard cells form when this small mother cell divides in a plane perpendicular to the first cell division. Thus, asymmetrical cell division generates cells with different fates—that is, cells that mature into different types.
Asymmetrical cell divisions also play a role in the establishment of polarity, the condition of having structural or chemical differences at opposite ends of an organism. Plants typically have an axis, with a root end and a shoot end. Such polarity is most obvious in morphological differences, but it is also apparent in physiological properties, including the movement of the hormone auxin in a single direction and the emergence of adventitious roots and shoots from “cuttings.” Adventitious roots form within the root end of a stem cutting, and adventitious shoots arise from the shoot end of a root cutting.
The first division of a plant zygote is normally asymmetrical, initiating polarization of the plant body into shoot and root. This polarity is difficult to reverse experimentally, indicating that the proper establishment of axial polarity is a critical step in a plant’s morphogenesis.