Chapter

The Plant Cytoskeleton

Volume 2 of the series Advances in Plant Biology pp 207-225

Date:

Microtubule Organization in the Phragmoplast

  • Bo LiuAffiliated withDepartment of Plant Biology, University of California Email author 
  • , Takashi Hotta
  • , Chin-Min Kimmy Ho
  • , Yuh-Ru Julie Lee

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Abstract

The phragmoplast harnesses the actions of microtubules and actin microfilaments to deliver Golgi-derived vesicles for the assembly of the cell plate which divides the cytoplasm of the mother cell. This review emphasizes on how microtubules are organized in the phragmoplast to allow cytokinesis to take place in a spatially and temporally regulated fashion. The phragmoplast microtubule array consists of two mirrored sets of anti-parallel microtubules with their plus ends facing the division site. More and more proteins have been found to be associated with the phragmoplast, especially in the categories of microtubule-associated proteins or MAPs and microtubule-based motor kinesins. They exert different regulatory roles in making the phragmoplast microtubule array. The evolutionarily conserved γ-tubulin complex and its interacting proteins are responsible for microtubule nucleation in order to generate new microtubules. The plus ends of anti-parallel microtubules at the cell division site are cross-linked by one or more proteins in the MAP65 family. At the division site, the Kinesin-12 motors keep the microtubule plus ends in position by sliding newly polymerized microtubule segments apart. Proteins in the conserved end-binding protein 1 (EB1) and MAP215 families promote microtubule polymerization and stabilization and maintain the integrity of the phragmoplast microtubule array. The functions of these factors are orchestrated to establish this highly dynamic microtubule array which undergoes continuous reorganization. We propose that there are two classes of microtubules in the phragmoplast, interdigitating microtubules (IMTs) and non-IMTs. Bundles of IMTs are surrounded by non-IMTs to form an array of a mini-phragmoplast. During cytokinesis, microtubules in old mini-phragmoplasts are disassembled in the central region upon the completion of vesicle delivery. In the meantime, new mini-phragmoplast microtubule arrays are born toward the periphery of the phragmoplast until the cell plate is completely assembled. Questions like how microtubule depolymerization at the minus end is regulated remain to be answered.