The rise and fall of the phragmoplast microtubule array

https://doi.org/10.1016/j.pbi.2013.10.008Get rights and content

Highlights

  • The phragmoplast is assembled by mini-phragmoplast modules.

  • The microtubule overlapping zone in the phragmoplast is established by the coordinated activities of the microtubule bundling factor MAP65-3 and the motor Kinesin-12.

  • Microtubule nucleation on stabilized anti-parallel microtubules depends on augmin and γ-tubulin.

  • MAP65 cross-links microtubules nucleated from opposite sides of the phragmoplast to establish a stable anti-parallel microtubule bundle.

  • Disassembly of phragmoplast microtubules is triggered by phosphorylation of MAP65.

The cytokinetic apparatus, the phragmoplast, contains a bipolar microtubule (MT) framework that has the MT plus ends concentrated at or near the division site. This anti-parallel MT array provides tracks for the transport of Golgi-derived vesicles toward the plus ends so that materials enclosed are subsequently deposited at the division site. Here we will discuss a proposed model of the centrifugal expansion of the phragmoplast that takes place concomitantly with the assembly of the cell plate, the ultimate product of vesicle fusion. The expansion is a result of continuous MT assembly at the phragmoplast periphery while the MTs toward the center of the phragmoplast are disassembled. These events are the result of MT-dependent MT polymerization, bundling of anti-parallel MTs coming from opposite sides of the division plane that occurs selectively at the phragmoplast periphery, positioning of the plus ends of cross-linked MTs at or near the division site by establishing a minimal MT-overlapping zone, and debundling of anti-parallel MTs that is triggered by phosphorylation of MT-associated proteins. The debundled MTs are disassembled at last by factors including the MT severing enzyme katanin.

Introduction

The innovation of the phragmoplast marks a significant advancement in the evolution from green algae to land plants [1]. The phragmoplast contains a core of two mirrored sets of anti-parallel microtubules (MTs) whose plus ends are concentrated at or near the midzone of this cytokinetic apparatus (Figure 1) [2]. The ultimate mission of this bipolar MT array is to allow Golgi-derived vesicles to be transported unidirectionally toward MT plus ends so that the materials enclosed in these vesicles are deposited for the assembly of the cell plate. Phragmoplast MTs, like those MTs in spindles during mitosis, undergo continuous remodeling throughout cytokinesis [3, 4]. Upon the completion of anaphase, MTs are polymerized and coalesced in the spindle midzone, and a bipolar array is established as the Kinesin-5 motor acts on anti-parallel MTs to slide them against each other (Figure 1a) [5, 6••, 7]. Concomitant with the addition of more MTs to the array, the MTs are shortened at their minus ends, which are facing the reforming daughter nuclei (Figure 1b). One of the most spectacular phenomena in cytokinesis is the centrifugal expansion of the phragmoplast MT array from the cell center to the edge. During the expansion, new MT filaments are added to the periphery of the phragmoplast while older ones in the inner part of the phragmoplast array are disassembled (Figure 1c). This is particularly challenging because those opposing activities occur simultaneously within a few microns of each other. The assembly of new MTs uses tubulin subunits released from the depolymerization of older MTs [3].

Before MT-associated proteins (MAPs) and MT-based motors were identified, microscopic observations had revealed many structural details of the phragmoplast in elegant systems like the endosperm of the African blood lily Haemanthus and tobacco BY-2 cells [8, 9]. To date, those magnificent images still inspire us to dig into mechanisms underlying plant cytokinesis. In the past two decades or so, a mechanistic understanding of cytokinesis has been greatly advanced in the model system Arabidopsis thaliana by utilizing its powerful genetics together with live-cell imaging. Particularly, mutational analyses have led to the identification of proteins important for cytokinesis, many of which are conserved among eukaryotes [10, 11]. Inactivation of a number of factors that regulate fundamental aspects of MT dynamics certainly leads to the eventual failure in cytokinesis [12]. However, there are factors that act specifically on phragmoplast MTs and regulate the rise and fall of the phragmoplast MT array [13].

While a number of insightful reviews have summarized the role of genes and proteins that regulate phragmoplast organization and cytokinesis [13, 14, 15, 16], here we will focus on several critical MT reorganization events during phragmoplast expansion. We envisage that the phragmoplast MT array is assembled by anti-parallel MT modules of mini-phragmoplasts which contain a core of interdigitating MT bundles surrounded by non-interdigitating MTs [6••]. The centrifugal expansion of the array is brought about by the amplification of the mini-phragmoplast modules toward the periphery and the disassembly of old ones at the inner part of the phragmoplast, where the cell plate is formed. Figure 2 illustrates hypothesized sequential events that take place after interdigitating MTs are formed from coalesced MTs in the central spindle. At early stages during mini-phragmoplast development, Kinesin-12 is recruited to the plus ends of anti-parallel MTs in order to define the minimal MT-overlapping zone. Then the γ-tubulin complex is recruited to interdigitating MTs via the augmin complex to activate branched MT nucleation/polymerization preferentially toward the division plane. The newly polymerized anti-parallel MTs are captured by MAP65 toward their plus ends, followed by MT-induced multimerization of MAP65 to generate interdigitating MT bundles. While these events are taking place, additional non-interdigitating MTs are polymerized alongside the interdigitating MTs. When a mini-phragmoplast completes its mission, the MAP65 proteins associated with its MTs are phosphorylated by a MAP kinase pathway [14]. Consequently, MAP65 dimers are no longer in their multimeric form and dissociate from MTs. The resulting debundled MTs in the mini-phragmoplast are then disassembled via severing by enzymes like katanin and depolymerization by unknown MT depolymerases.

Section snippets

Anti-parallel MTs in the phragmoplast

The phragmoplast midzone is the destination of Golgi-derived vesicles and the site of cell plate assembly. The midzone is characterized by the meeting of the two halves of the phragmoplast MTs. Earlier observations by electron microscopy in the Haemanthus endosperm showed that these MTs are often interdigitated, associating with electron dense material [8]. Such a MT interdigitation phenomenon also has been shown in the moss Physcomitrella patens [17]. Although MT interdigitation was readily

Amplification of MTs at the phragmoplast periphery

The centrifugal expansion of the phragmoplast MT array requires MTs to be continuously amplified at the periphery. However, MT nucleation and polymerization take place along the entire phragmoplast, likely on existing MTs [36••]. Consistent with this, the MT-nucleating factor γ-tubulin decorates phragmoplast MTs with biases toward their minusends [37]. New MTs are preferentially polymerized toward the phragmoplast midzone and share the polarities of extant ones [36••]. This feature of shared

Phragmoplast MT turnover

New MTs are polymerized at the phragmoplast periphery and as soon as cell plate assembly is underway, older ones toward the center are disassembled. The addition of new MTs is at the expense of depolymerization of old ones because taxol-mediated inhibition of MT depolymerization blocks phragmoplast expansion [44]. When MTs are bundled, they become resistant to depolymerization challenges. The mitogen-activated protein kinase (MAPK) cascade, including the NPK, NQK, and NRK (PQR) kinases, is

Conclusions and perspectives

The modular model discussed here serves as a gateway for future investigations of the assembly and development of the phragmoplast MT array. A number of critical questions immediately arise in regard to specific activities of critical factors and the coordination of different events in the phragmoplast. Among them, how is Kinesin-12 recruited to the phragmoplast midzone? Why does MAP65-3 only bundle MTs toward their plus ends? How do MAP65-3 and Kinesin-12 coordinate to establish the narrow

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

The cytokinesis work in our laboratory was supported by the National Science Foundation of USA under Grant MCB-1243959. We want to thank Mr. Jia Jin Chen for his graphic illustrations in the figures. We are particularly grateful to Dr. Sherryl Bisgrove for her critical comments on the manuscript.

References (54)

  • A. Bannigan et al.

    A conserved role for kinesin-5 in plant mitosis

    J Cell Sci

    (2007)
  • C.M. Ho et al.

    Interaction of antiparallel microtubules in the phragmoplast is mediated by the microtubule-associated protein MAP65-3 in Arabidopsis

    Plant Cell

    (2011)
  • T. Asada et al.

    TKRP125, a kinesin-related protein involved in the centrosome-independent organization of the cytokinetic apparatus in tobacco BY-2 cells

    J Cell Sci

    (1997)
  • P.K. Hepler et al.

    Microtubules and early stages of cell-plate formation in the endosperm of Haemanthus katherinae Baker

    J Cell Biol

    (1968)
  • T. Kakimoto et al.

    Cytoskeletal ultrastructure of phragmoplast–nuclei complexes isolated from cultured tobacco cells

    Protoplasma Suppl

    (1988)
  • R. Söllner et al.

    Cytokinesis-defective mutants of Arabidopsis

    Plant Physiol

    (2002)
  • G. Jürgens

    Cytokinesis in higher plants

    Annu Rev Plant Biol

    (2005)
  • K. Steinborn et al.

    The Arabidopsis PILZ group genes encode tubulin-folding cofactor orthologs required for cell division but not cell growth

    Genes Dev

    (2002)
  • C.M. McMichael et al.

    Cytoskeletal and membrane dynamics during higher plant cytokinesis

    New Phytol

    (2013)
  • M. Sasabe et al.

    Regulation of organization and function of microtubules by the mitogen-activated protein kinase cascade during plant cytokinesis

    Cytoskeleton (Hoboken)

    (2012)
  • M.S. Otegui et al.

    Midbodies and phragmoplasts: analogous structures involved in cytokinesis

    Trends Cell Biol

    (2005)
  • Y. Hiwatashi et al.

    Kinesins are indispensable for interdigitation of phragmoplast microtubules in the moss Physcomitrella patens

    Plant Cell

    (2008)
  • M. Otegui et al.

    Three-dimensional analysis of syncytial-type cell plates during endosperm cellularization visualized by high resolution electron tomography

    Plant Cell

    (2001)
  • J.R. Austin et al.

    Quantitative analysis of changes in spatial distribution and plus-end geometry of microtubules involved in plant-cell cytokinesis

    J Cell Sci

    (2005)
  • Y.R.J. Lee et al.

    Identification of a phragmoplast-associated kinesin-related protein in higher plants

    Curr Biol

    (2000)
  • S. Müller et al.

    The plant microtubule-associated protein AtMAP65-3/PLE is essential for cytokinetic phragmoplast function

    Curr Biol

    (2004)
  • C.M. Ho et al.

    Arabidopsis microtubule-associated protein MAP65-3 cross-links antiparallel microtubules toward their plus ends in the phragmoplast via its distinct C-terminal microtubule binding domain

    Plant Cell

    (2012)
  • Cited by (47)

    • Plant Cell Biology: Shifting CORDs to Fine-Tune Phragmoplast Microtubule Turnover

      2019, Current Biology
      Citation Excerpt :

      Three potential targets of CORD4 that could promote katanin activity indirectly include MAP65, augmin and CLASP (Figure 1C). Reversible cross-linking of antiparallel microtubules by MAP65 in the midzone is a critical part of phragmoplast progression [11]. Burkhardt and Dixit recently demonstrated that MAP65-mediated microtubule bundling inhibits the binding of KTN1 to microtubules in interphase arrays [12].

    • Plant cell division — defining and finding the sweet spot for cell plate insertion

      2019, Current Opinion in Cell Biology
      Citation Excerpt :

      In tobacco, Arabidopsis and P. patens microtubule turnover in the phragmoplast center is regulated by a conserved mitogen-activated protein kinase (MAPK) cascade, NACK-PQR that negatively regulates the bundling activity of microtubule cross-linkers of the PRC1/AseI/MAP65 family, in parallel with Aurora kinase and cyclin-dependent kinase [43–46]. In plant cytokinesis several MAP65 isoforms contribute, to stabilize antiparallel microtubule overlaps in the phragmoplast midzone and their loss results in wider midzones and fragmentary cell plates [24,44,47,48]. A conserved WD40 protein Budding Uninhibited by Benzimidazole 3 (BUB3), formerly implicated in spindle checkpoint assembly, acquired a novel function in plant cytokinesis [49•].

    • Phragmoplast expansion: the four-stroke engine that powers plant cytokinesis

      2018, Current Opinion in Plant Biology
      Citation Excerpt :

      This opinion piece summarizes the available data on this topic and addresses the gaps in knowledge with speculations on the events in the midzone that comprise the ‘engine’ for phragmoplast expansion. The stages that precede the expansion phase, including phragmoplast establishment, are reviewed elsewhere [7,11,13]. The majority of microtubules polymerize inward from the phragmoplast distal zones towards the midzone.

    • Myosin XI localizes at the mitotic spindle and along the cell plate during plant cell division in Physcomitrella patens

      2018, Biochemical and Biophysical Research Communications
      Citation Excerpt :

      To divide, plant cells form the phragmoplast, a scaffolding structure that assembles the new cell wall in the middle of the cell, dividing from inside out [1,2]. This plant specific structure comprises microtubules, microfilaments, motor proteins, and several regulators [2–6]. Microtubules are organized in two antiparallel sets, which overlap at the equator of the phragmoplast [7,8].

    View all citing articles on Scopus
    View full text