The rise and fall of the phragmoplast microtubule array
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.
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2019, Current BiologyCitation 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].
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2019, Current Opinion in Cell BiologyCitation 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•].
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Myosin XI localizes at the mitotic spindle and along the cell plate during plant cell division in Physcomitrella patens
2018, Biochemical and Biophysical Research CommunicationsCitation 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].
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