leondumoulin.nl: Actin: A Dynamic Framework for Multiple Plant Cell Functions ( Developments in Plant and Soil Sciences): C.J. Staiger, F. Baluska, D. Volkmann, .
Table of contents
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- Mutations in Actin-Related Proteins 2 and 3 Affect Cell Shape Development in Arabidopsis
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- Cytoskeleton-Plasma Membrane-Cell Wall Continuum in Plants. Emerging Links Revisited
A Long-Term Perspective W. Actin Depolymerizing Factor; D. Sucrose Metabolism and the Actin Cytoskeleton: Susy as Actin-Binding Protein; H. From Flow to Track; F. Actin in Characean Rhizoids and Protonemata; M. Actin in Characean Internodal Cells; I. Rho Gtpases and the Actin Cytoskeleton; H. Actin in Pollen and Pollen Tubes; L.
Actin in Formation of Stomatal Complexes; A. Although available data are rather scarce for higher plants, and critical linker molecules between the cytoskeleton and ECM are still missing Pont-Lezica et al. The integrated cytoskeleton and its dynamic adhesion to the plasma membrane allow these complex interactions between mechanical forces and chemical signals Critchley, ; Geiger and Bershadsky, ; Sheetz, What is still unknown in plants is the molecular nature of linkers between elements of the cytoskeleton and components of the cell wall.
In this Update , we provide a brief survey of linker molecules in animal cells and highlight emerging plant-specific linkers. We hope that our Update will stimulate more activity in this exciting and very important area of research. Physical coupling between cytoskeleton and ECM is relatively well understood in animal cells.
Several molecules are well known to act as linkers between elements of the cytoskeleton and ECM components. The most famous and best-understood are the integrins, which communicate signals between fibronectin, vitronectin, laminin, and other RGD-containing ECM proteins and the actin cytoskeleton within the cytoplasm. Integrins allow bidirectional signaling in all multicellular eukaryotes with the exception of plants and fungi Burke, ; Hynes, , ; Geiger et al. These actin-binding proteins then recruit the next cohort of actin-binding proteins such as profilin, VASP, and Arps, which drive local actin polymerization Critchley, ; Craig and Chen, ; Sakai et al.
After receiving inputs at the ECM-plasma membrane interface, integrins convey this information downstream via several signal transducers including members of the Ras family of small GTPases and mitogen-activated protein kinases MAPKs; Juliano, ; Schwartz and Ginsberg, Additionally, integrins signal into the cell interior via focal adhesion kinase, pactivated kinase, phosphatidylinositol 3-kinase, integrin-linked kinase, Tyr kinase c-Src, adenylate cyclase, protein kinase A, LIM-kinase, and protein kinase C Geiger et al.
Cadherins are calcium-dependent transmembrane adhesion molecules that play a key role in the maintenance of tissue architecture Adams and Nelson, due to their homophilic cell-to-cell interactions and abundant interactions with the dynamic actin cytoskeleton Kovacs et al.
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Cadherins accumulate at specialized cell-to-cell adhesion domains known as tight junctions, adherens junctions, and desmosomes Adams and Nelson, Like cadherins, these three less studied groups of cell adhesion receptors also perform homophilic and heterophilic interactions to hold adjacent cells together. At the ECM side, they interact with hyaluronan, but also with collagen, laminin, and fibronectin. Within the cytoplasm, these transmembrane proteins interact with the actin cytoskeleton Culty et al.
Hyaluronan receptors are linked to the actin cytoskeleton via actin-binding proteins including those of the band 4. These adhesion proteins have multiple structural functions related to vesicle trafficking and signaling Kobayashi et al. Inherent association of the actin cytoskeleton with the plasma membrane is due to interactions among actin-binding proteins and phosphatidylinositol-bisphosphate PIP 2 , which localizes to the inner leaflet of the plasma membrane Nebl et al.
Thus, it is not surprising that adhesion of the actin cytoskeleton to the plasma membrane is dependent on PIP 2 Raucher et al. PIP 2 was localized to discrete domains at the plasma membrane of maize Zea mays root cells. Generally, the actin cytoskeleton has been optimized during eukaryotic evolution for acting as a structural scaffold for diverse signaling complexes Juliano, Recent data from plants support the concept whereby the dynamic actin cytoskeleton is closely linked to the signaling cascades initiated at the plasma membrane Meagher et al.
Despite the presence of proteins immunologically related to both integrins and cadherins Kaminskyj and Heath, ; Katembe et al. This situation might be surprising in the face of rather well-conserved nature of the actin cytoskeleton Meagher et al. One could argue that the different organization of adhesion sites in plants is due to the unique molecular nature of plant cell walls.
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- Mutations in Actin-Related Proteins 2 and 3 Affect Cell Shape Development in Arabidopsis.
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Generally, the animal ECM is proteinaceous, and protein fibrils are the prime mechanical devices. In contrast, carbohydrates are the major building blocks of plant cell walls. Although higher plant cells also seem to use RGD-containing proteins to connect their cell walls with the plasma membrane Schindler et al. One hypothesis to explain the absence of integrins, cadherins, and other animal-type linkers in plants is that plant cells, especially root cells, are often exposed to hyperosmotic stress, which necessitates rapid and reversible retractions of their protoplasts from the cell walls, the so-called plasmolytic cycle Oparka and Crawford, ; Lang-Pauluzzi and Gunning, ; Komis et al.
To maintain mechanical integrity, osmotically stressed plant cells must retract their protoplasts from their cell walls almost immediately. Integrin- and cadherin-based adhesion complexes are apparently too complex to disintegrate rapidly. Therefore, plant cells may have designed other molecules and used other principles for the very dynamic interactions between cytoskeleton and cell walls.
Bruce Kohorn discussed putative plant-specific linker molecules, focusing on the four most appealing candidates: Progress made during the last three years has resulted in additional candidates including formins, plant-specific myosins of the class VIII, phospholipase D, and callose synthases.
In contrast to the cytoplasmic and transmembrane domains, which are well-conserved, the extracellular domains of WAKs are the most variable among the five Arabidopsis WAK isoforms WAK and contain motifs typical for animal proteins such as epidermal growth factor repeats, tenascin-like, collagen-like, and neurexin-like sequences He et al.
Mutations in Actin-Related Proteins 2 and 3 Affect Cell Shape Development in Arabidopsis
So far, the functional significance of these motifs remains unknown, although epidermal growth factor repeats suggest calcium-mediated dimerization of WAKs. Interestingly, the phosphorylated version of WAK1 was found to be firmly bound to plasma membrane-associated cell wall pectins Kohorn, , ; Anderson et al. Plasma membrane-associated pectins have adhesive properties Mollet et al. Intriguingly, depriving cells of boron, which cross-links RGII pectins in cell walls, results in inhibition of endocytosis of cell wall pectins Yu et al.
In addition, boron deprivation exerts immediate impacts on the cytoskeleton Yu et al. Similarly, aluminum binds cell wall pectins Horst et al. Obviously, both aluminum and boron bind cell wall pectins and affect events at the cell wall-cytoskeleton interface Horst et al. All this suggests that complex interactions between pectins, boron, and the cytoskeleton are important for the assembly of the cell wall-cytoskeleton continuum as well as for its maintenance via signal-mediated processes.
An intriguing possibility is that WAKs act as receptors for endocytosis of adhesive cell wall pectins. Interestingly, WAKs released from cell walls after pectinase treatments are still in a complex with cell wall pectins, or their fragments, because antibodies against pectins detect released WAKs on western blots Anderson et al.
Plant pectins resemble hyaluronan in many aspects: Both are abundant components of the ECM having structural as well as signaling functions; they both perform endocytic internalization; and their smaller fragments, internalized presumably via endocytosis, have important signaling roles at the plasma membrane and within the cytoplasm Van Cutsem and Messiaen, ; Thain et al. There are some additional data showing that glycine-rich proteins GRPs could be bound to pectins.
Studies using transgenic plants revealed that WAKs are essential for plant cell elongation Lally et al. In addition to the WAKs, recent database searches identified a large family of WAK-like kinases that might also be relevant for interactions between cell walls and the cytoskeleton Verica and He, Another emerging candidate for signaling-mediated interactions between the cell wall and cytoskeleton of plant cells are the AGPs, which are predicted to have both adhesive and signaling properties Schultz et al. Interestingly, AGPs bind to cell wall pectins Nothnagel, , and they might interact also with WAKs because they seem to localize to the same domains at the plasma membrane of BY-2 protoplasts Gens et al.
Viral MPs have been used as experimental probes to isolate plant factors involved in viral infection and plant transport processes and have identified candidate proteins associated with the nucleus, plasmodesmata, and the cytoskeleton Oparka, Since , when it was shown that viral MPs interact with actin microfilaments and microtubules Heinlein et al. Nevertheless, current data show that transport of MPs and spread of virus can occur independently of the plant cytoskeleton.
The following discussion highlights some of the recent evidence for and against the involvement of the plant cytoskeleton in the development of viral diseases in plants. Discovery of the direct interaction of TMV MP with microtubules and actin microfilaments led to the idea that the plant cytoskeleton might function as a scaffold for targeting viruses to plasmodesmata.
Single amino acid mutations in this motif can confer temperature sensitivity to the association of MP with microtubules and, at nonpermissive temperatures, reduce intercellular transport of viral RNA, giving evidence of a correlation between the MP's ability to interact with microtubules and cell-to-cell movement of virus Boyko et al. Association between TMV MPs and microtubules is resistant to high salt treatment, a feature that could be indicative of integration of the MP into the microtubule lattice. If this were the case, intracellular transport of MP and bound viral RNA might occur through microtubule treadmilling and polymerization-depolymerization dynamics Boyko et al.
In support of this idea, there are data indicating that MPs can influence microtubule dynamics. TMV MP introduced into mammalian cells associates with microtubule nucleating sites causing microtubule detachment from centrioles, a process that increases microtubule dynamics Boyko et al. In addition, a microtubule binding protein encoded by potato virus X PVX , in conjunction with PVX virions, has been shown to induce microtubule polymerization Serazev et al.
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However, despite the evidence of MP binding to microtubules and its apparent mediation of microtubule-based movement of virus, the results of a number of other studies indicate that viral spread and transport of MP can occur independently of plant microtubules. By contrast, intracellular and intercellular movement of TMV replication complexes has been shown to be affected by latrunculin B, an inhibitor of actin polymerization Kawakami et al. This pattern is thought to arise through the breakdown of MP at the center of the infection.
Building on a previous report of the ubiquitination and subsequent degradation of MP by the 26S proteasome Reichel and Beachy, , the authors interpret their data as indicating that plant microtubules, rather than being essential for cell-to-cell movement, may be part of a mechanism to degrade TMV MP. MPs of some viruses, such as grapevine fanleaf virus GFLV , form tubules that facilitate the movement of virus-like particles through plasmodesmata in infected host plants Fig. However, oryzalin-induced depolymerization of microtubules led to loss of polarity in the distribution of GFP-MP tubules that were found over the whole cell surface rather than preferentially accumulating at young cross walls.
Cytochalasin D or latrunculin B, inhibitors of actin polymerization, did not have any effect on tubule formation or their distribution, although simultaneous treatment with oryzalin and latrunculin B resulted in cytosolic localization of tubules. These results indicate that the GFP-MP tubules are transported via a microtubule-dependent pathway, although microfilaments function in the transport process if microtubules are depolymerized. During pollination with compatible pollen, growth of the pollen tube is extremely rapid and is dependent upon intact microtubule and actin microfilament cytoskeletons Hepler et al.
Cytoskeleton-Plasma Membrane-Cell Wall Continuum in Plants. Emerging Links Revisited
Plants employ a number of mechanisms to stop self-pollen from fertilizing the ovule, including the degradation of pollen RNA by stigma RNAases Kao and Tsukamoto, In Papaver, growth of self-pollen is rapidly and specifically arrested by stigmatic S self-incompatibility proteins via increased levels of cytoplasmic calcium in the pollen tube Thomas et al.
Treatment with S-protein causes rapid reorganization and depolymerization of endoplasmic actin microfilament bundles in incompatible pollen of Papaver rhoeas , indicating that actin microfilaments are one of the components targeted during arrest of pollen tube growth in this form of self-incompatibility Fig. Artificial elevation of cytosolic calcium levels increases the G-actin binding activity of pollen profilin, and causes the depolymerization of actin microfilaments Snowman et al.
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These data suggest that PrABP80 could function as the main regulator of actin depolymerization in this self-incompatibility response. In addition to the rapid arrest of pollen tube growth, S-protein treatment can also trigger programmed cell death of self-pollen Thomas and Franklin-Tong, , giving support for earlier suggestions of overlapping mechanisms in self-incompatibility and hypersensitivity-mediated plant defense against pathogens Dickinson, Depolymerization of actin microfilaments in a pollen tube triggered by a self-incompatibility response.
A, Normally growing pollen tube. B, Incompatible pollen tube 5 min after treatment with S-protein. C, Incompatible pollen tube 60 min after treatment with S-protein. Actin filaments were visualized by Alexaphalloidin staining. Reproduced with permission from Snowman et al. In most plant-microbe interactions, the development of structured cytoskeletal arrays at the interaction site is associated with outcomes that are beneficial to the plant.
Examples include the radial arrays of actin microfilaments and microtubules that accompany cytoplasmic aggregation during defense responses against invading pathogens, the dense network of actin microfilaments and microtubules that forms around arbuscules and peletons of mycorrhizal fungi, and the lattice of actin microfilaments or radial array of microtubules found among symbiosomes in root nodules. On the other hand, degradation of the plant cytoskeleton is often associated with outcomes of plant interactions with other organisms that are detrimental to the plant.
Both microtubule and actin microfilament arrays are lost during invasion by the Lotus mutant that is unable to develop a functional mycorrhizal symbiosis, and degradation of actin microfilaments results in arrest of self-pollen. Both cytoskeletal arrays are also disrupted or completely depolymerized during feeding cell formation by root-infecting nematodes de Almeida Engler et al. These observations underscore the important role played by the plant cytoskeleton in mediating the plant cell's response to biotic factors.
It is clear that remodeling of the plant cytoskeleton is instrumental in achieving structural responses to other organisms, for example, in forming an apoplastic barrier to arrest pathogen ingress. Changes in cytoskeletal organization may also facilitate signaling of the presence of symbionts or pathogens on the plant surface.
There is growing evidence that actin and microtubule arrays in plant cells participate in signaling cascades initiated at the plasma membrane, enabling adaption to environmental factors Abdrakhamanova et al.
It is likely that research over the next few years will further elucidate the role of the plant cytoskeleton in signal transduction and the plant response to pathogens and symbionts. Jones for critical reading of the manuscript, and Drs. Frankin-Tong for allowing us to use their previously published micrographs.
National Center for Biotechnology Information , U. Journal List Plant Physiol v. Daigo Takemoto and Adrienne R. This article has been cited by other articles in PMC. Open in a separate window. Changes in the Plant Cytoskeleton in Root Hairs following Inoculation Rhizobial attachment, or application of host-specific nodulation Nod factors Esseling and Emons, ; Riely et al.
Changes in the Plant Cytoskeleton in Cortical Cells in Response to Inoculation Attachment of bacteria or application of Nod factors at the root surface also induces cellular rearrangements and cell divisions in the root cortex and the formation of a nodule primordium. Involvement of the Plant Cytoskeleton in Viral Tubule Distribution MPs of some viruses, such as grapevine fanleaf virus GFLV , form tubules that facilitate the movement of virus-like particles through plasmodesmata in infected host plants Fig.
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