Control of Cell Axis

Cell movement constitutes a basic mechanism in animal development, for instance during gastrulation or during the development of neural systems. Plant cells with their rigid cell walls cannot move and therefore had to evolve alternative mechanisms to organize their Bauplan. In plants, morphogenesis is controlled by the initiation of a cell axis during cell division and by the expression of this axis during subsequent cell expansion. Axiality of both division and expansion is intimately linked with specific microtubular arrays such as the radial array of endoplasmic microtubules, the preprophase band, the phragmoplast, and the cortical cytoskeleton. This chapter will review the role of microtubules in the control of cell axis, and attempt a synthesis of classical research with recent developments in the field. During the last few years, our understanding of two central enigmas of plant microtubule organization has been advanced substantially. It had been observed for a long time that the spatial configuration of the phragmoplast was guided by events that take place prior to mitosis. However, the premitotic microtubular arrays disappear at the time when the spindle appears. It was therefore unclear how they could define the formation of a phragmoplast. The deposition of an endosomic belt adjacent to the phragmoplast, in combination with highly dynamic exploratory microtubules nucleated at the spindle poles, provides a conceptual framework for understanding these key events of cell axiality. The microtubule–microfibril concept, which is central to understanding the axiality of cell expansion, has been enriched by molecular candidates and elaborate feedback controls between the cell wall and cytoskeleton. Special attention is paid to the impact of signalling to cortical microtubules, and to the mechanisms of microtubule reorientation. By means of live-cell imaging it has become possible to follow the behaviour of individual microtubules and thus to assess the roles of treadmilling and mutual sliding in the organization of microtubular arrays. Direction-dependent microtubule lifetimes, spatial patterns of post-translational modifications, and new mutants with deviating orientation of microtubules shed light on a complexity that is still far from being understood, but reveals a network of highly dynamic, nonlinear interactions that are endowed with pattern-generating properties. The chapter concludes with potential approaches to manipulation of the cell axis either through cell division or through cell expansion. 1 Cell Axis and Plant Development During the growth of any organism, volume increases with the third power of the radius. Surface extension, however, increases only with the second power and thus progressively lags behind. In order to balance these two processes, the surface has to be enlarged substantially, either by internal or external exten-