Morphological Plasticity of the Mitotic Apparatus in Plants and Its Developmental Consequences.

Abstract

More than 100 years after the discovery of mitosis, the role of the mitotic apparatus (MA) in cell and organismal development is still the subject of lively debate (e.g., Kaplan, 1992). The developmental impact of cell division looms perhaps even larger in plants than in other organisms because plant cells are not free to migrate during morphogenesis. Therefore, plants place great stock in modulating division planes to produce new directions for growth (Sinnott, 1960). In addition, precisely placed walls serve to set off unequal compartments that then differentiate in divergent pathways. Thus, although their principal function, partitioning two equal sets of chromosomes to separate cytoplasmic compartments, remains the same, mitosis and cytokinesis assume an additional dimension in plants. Whereas animals also depend on precise division planes (Strome, 1993), the process occupies an especially prominent role in plant development (Gunning, 1982). My main purpose in this review is not to address division plane determination per se but to explore a characteristic of plant mitosis that has received much less attention. The subject of my discussion is the morphological plasticity of the MA, that is, its ability to be tilted, squeezed, or otherwise deformed under the confining influence of the cell wall and still work. Given new information about a number of proteins, we may be on the verge of understanding the molecular basis of mitotic architecture. Furthermore, with a fuller appreciation of mitotic plasticity, division plane determination can be seen from another perspective-that of a mechanism to compensate for MA asymmetry or deformation. During my discussion, I'll also make a brief foray into the evolution of microtubule organization patterns in plants.