Shaping plant microtubule networks via overlap formation

Abstract

Microtubules are dynamic polymers that are vital for a vast range of cellular functions. For virtually all of these functions microtubules are organized into specifically tailored networks. Plant cells present us with a remarkably diverse range of microtubule networks, one of which supports the unique manner in which these cells divide. The physical separation of two daughter cells at the end of cell division in plants is achieved by formation of an interior, disc-shaped membrane compartment called the cell plate that ultimately gives rise to a dividing cell wall segment. Cell plate construction starts in the centre of the cell and continues radially outwards until the cell plate fuses with the existing plasma membrane. Cell plate assembly is fuelled by fusion of vesicles that are targeted to the division plane by the microtubules of the phragmoplast apparatus. Phragmoplast microtubules are organized in a bipolar configuration that is propagated during radial expansion of the cell plate. In contrast to most animal cells, plants do not use centrosomes as foci of microtubule growth initiation. Instead, plant microtubule networks are striking examples of self-organizing systems that emerge from physically constrained interactions of dispersed microtubules. How microtubule-based activities including growth, shrinkage, severing, sliding, nucleation and bundling interrelate to jointly generate a required ordered structure is a fascinating but underexplored question. Evidence mounts that adapter proteins sense the local geometry of microtubules to locally modulate the activity of proteins involved in microtubule growth regulation and severing. One particular geometry that is a prime candidate for serving as an integrative hub in a bipolar configuration such as the phragmoplast, is the antiparallel overlap formed between opposing microtubules. The finding that proteins fostering this interaction are also important in interphase plant cells hints that overlaps may have roles in constructing microtubule networks beyond bipolar arrays. Insight into control of overlap formation gained from plants has broader relevance since many other microtubule assemblies encountered in eukaryotes rely on these structures as well. pp.2909-2923. Hotta, T. et al., 2012. Characterization of the Arabidopsis augmin complex uncovers its critical function in the assembly of the acentrosomal spindle and phragmoplast microtubule arrays.