The Roles of Microtubules in Tropisms Plant Science

Home , Chemotropism, Thermotropism

Plant Science 175 (2008) 747–755

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Plant Science

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Review The roles of microtubules in tropisms

Sherryl R. Bisgrove *

Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, B.C., Canada V5A 1 s6

ARTICLE INFO ABSTRACT

Article history: Plant tropisms, or growth towards or away from a stimulus, usually involve the bending of shoots or roots Received 11 March 2008 which reorient growth in a new direction. Plant responses to tropic cues, especially gravity and light, have Received in revised form 19 August 2008 been active areas of investigation for many years. Despite all of this attention we still do not understand Accepted 19 August 2008 how these responses are regulated. In this review possible roles for microtubules in tropisms are Available online 7 September 2008 discussed. Tropisms occur in a series of steps; directional cues are perceived and converted into biochemical signals that induce bending in roots and shoots. One model suggests that microtubules Keywords: function late in the response pathway, during organ bending. Microtubules have been linked to organ Microtubule Cytoskeleton bending by virtue of their role in regulating the direction of cell elongation. In elongating cells Cell expansion microtubules appear to function as guides for the deposition of cellulose microfibrils into the cell wall and Cell wall the placement of the microfibrils in the wall is thought to constrain the direction of cell elongation. Tropism According to the model bending occurs when different microtubule/microfibril alignments across the organ cause cells on the outer flank to elongate more than cells on the inner flank. In support of this idea is the observation that tropic signals can induce the appropriate changes in microtubule orientations across a bending organ. However, attempts to validate the hypothesis have produced conflicting results and the idea that microtubule alignment regulates cell expansion during organ bending is controversial. Microtubules have also been linked to organizational events associated with the plasma membrane. Although speculative, one possibility is that microtubules influence tropisms by positioning regulatory proteins and/or complexes in the plasma membrane. Two possible mechanisms by which microtubules could contribute to organizational events associated with the plasma membrane are outlined. In addition to cell expansion, microtubules are postulated to have roles in the perception of touch and gravity signals. Although microtubules are associated with touch sensing in animals, we know very little about the relevant receptors in plants. One way to assess how microtubules function during tropisms is to identify and study proteins that function in concert with microtubules. In particular, the analysis of microtubule- associated proteins whose mutant forms confer defects in tropic responses promises to provide additional insights into the roles of microtubules in tropisms. ß 2008 Elsevier Ireland Ltd. All rights reserved.

Contents

1. Introduction ...... 748 1.1. Signal perception ...... 748 1.2. Auxin redistribution and differential growth ...... 748 1.3. Possible roles for microtubules ...... 749 2. Cortical microtubule organization and cell elongation ...... 749 2.1. Cellulose deposition ...... 750 2.2. Microtubule reorientations and tropisms ...... 750 3. Beyond cellulose deposition: new roles for microtubules? ...... 751 4. Summary and future directions ...... 753 Acknowledgements...... 753 References...... 753

* Tel.: +1 778 782 5269; fax: +1 778 782 3496. E-mail address: [email protected].

1. Introduction mechanosensors [7]. However, the physiological relevance of these channels in intact plants is unknown. Plants are sessile and cannot move when conditions become The receptors that mediate gravity perception have not been unfavorable. They can, however, redirect their growth to identified, although two models that describe how gravity is position stems, roots, leaves, and flowers towards the best sensed have been put forward in the literature. The starch-statolith possible locations. Shoots grow towards light, maximizing hypothesis postulates that gravity-sensitive cells detect the falling photosynthetic activity. Roots point down along the gravity of intracellular masses called statoliths (see refs. [18,23] for vector and can be attracted to areas of higher moisture or reviews). The gravitational pressure model proposes that the nutrient content in the soil. When conditions change, plants settling of the protoplast within the cell wall is sensed by receptors respond by redirecting their growth in accordance with the new at the plasma membrane–extracellular matrix junction [18,24]. signals. Tropisms, these changes in the direction of growth in These receptors would be capable of detecting differences in response to stimuli, involve the bending of stems or roots that tension and compression between the plasma membrane and the reorient growth in the most favorable direction. Tropisms extracellular matrix at the top and bottom of the cell. Higher plant include responses to a number of cues including gravity cells that can sense gravity commonly contain starch-filled (gravitropism), light (phototropism), and touch (thigmotrop- amyloplasts that sediment and it is thought that this is the ism), as well as gradients of moisture (hydrotropism), chemicals primary mechanism by which gravity is perceived, although it is (chemotropism), and temperature (thermotropism). Although possible that protoplast settling is also detected [18,23]. How these tropisms have all been documented many have not been amyloplast sedimentation is converted into a biochemical signal is investigated in any further detail [1].Gravitropismand not understood, although it has been proposed that mechan- phototropism have received the most attention, although some osensitive ion channels are involved [11,14,16,25,26]. studies addressing hydrotropism and thigmotropism have been With the exception of hydrotropism, plant responses to most published in recent years [2–10]. other directional cues have been described mainly from a Tropisms can be either positive or negative depending on phenomenological perspective [1]. Recently genetic approaches whether growth occurs towards or away from the stimulus. The have been used in efforts to understand the mechanisms that response occurs through a series of steps. Directional cues are regulate hydrotropism [3–6,27,28]. Screens for mutants that are sensed and converted into biochemical signals. Under normal defective in hydrotropic responses have been done [29] and the conditions, plants receive many cues at once often from different characterization of the corresponding genes promises to yield directions. To cope with all of this information incoming signals insight into the molecular mechanisms that regulate hydrotropism must be integrated and transmitted to the responding cells where [1]. the changes in growth occur (see ref. [11] for a review). Gravity is a constant stimulus and changes in growth occur only when 1.2. Auxin redistribution and differential growth opposing cues out compete or over-ride gravity [11]. Because of the large amount of incoming information and the requirement for Signals controlling tropisms often result in a concentration an integrated growth response, the signaling pathways that gradient of auxin across a responding organ and this auxin gradient underlie tropisms are complex and they are not well understood. is responsible for redirecting growth [17,19,30]. Consider, for However, this is an active area of investigation and it is discussed in example, a seedling that has been placed on its side. Auxin several recent reviews [1,6,11–19]. accumulates to a higher level on the lower flanks of the hypocotyl and root. According to the Cholodony–Went hypothesis [31],an 1.1. Signal perception auxin gradient triggers a differential growth response in which cells on one side of the organ elongate more than cells on the other The receptors responsible for initiating phototropism, the side. Because the cells are held together and cannot move apart phototropins, were the first to be described at the molecular level from one another, differential growth results in the formation of a (see ref. [12] for a review). They are autophosphorylating protein kinases that are activated by blue light [12,17,20]. In addition to phototropins cryptochromes, another class of blue light receptors, and phytochromes, red/far-red reversible photoreceptors, also modulate phototropism [17,19,21]. Once activated, the phototropins transfer the light signal to proteins in downstream signaling pathways, but as of yet only a few of these intermediates have been identified [12,17,22]. Little is known about mechanoperception in plants, although studies in bacteria and animals are providing information about how mechanical stimuli are sensed in these organisms. The relevant receptors appear to be mechanosensitive ion channels Fig. 1. Bend formation in the hypocotyl and root of a seedling responding to light [16]. These channels are transmembrane complexes that open in and gravity. In the seedling on the left, the shoot (shaded in green) grows up response to mechanical forces exerted on the plasma membrane. towards light; it is negatively gravitropic and positively phototropic. The root, on In some cases mechanical forces are thought to induce a the other hand, has the opposite response; it grows down and away from the light. conformational change in the channel that opens it while in other Cells in both the hypocotyl and the root expand mainly in the longitudinal direction (indicated by double-headed arrows). The seedling in the middle has been placed on instances channels are opened indirectly via interactions with its side and it now perceives a change in the direction of the light and gravity signaling molecules or cytoskeletal proteins (see ref. [16] for vectors. Cells on the lower flank of the hypocotyl elongate more than the cells on the review). The first molecular characterization of mechanosensitive upper flank (designated by large and small double-headed arrows respectively). ion channel proteins in plant membranes has recently been This differential growth response produces a bend that reorients the shoot into an upright position (shown in the seedling on the right). A similar differential growth published [7]. These MscS (mechanosensitive channel of small response occurs in the root, but in this case cells on the upper flank elongate more conductance)-like proteins form pressure-sensitive channels in the than the cells on the lower flank and the root bends down, towards the gravity plasma membrane of protoplasts suggesting that they could be vector. S.R. Bisgrove / Plant Science 175 (2008) 747–755 749 bend (Fig. 1). The Cholodony–Went hypothesis and auxin transport discussed below). When microtubules are disrupted by pharma- have received a considerable amount of attention in the literature cological or herbicide treatments or by mutation, cells stop and the reader is referred to several other articles for more detailed elongating and swell by expanding radially (see refs. [47–53] for discussions [25,30,32–37]. The mechanism by which auxin induces examples). In addition, there are several mutations that alter differential cell elongation during organ bending is not understood, cortical microtubule organization and plants with these mutations but it is thought that auxin alters the rate at which cells elongate by have defects in the control of directional cell elongation. For inducing changes in cell wall extensibility [38,39]. Ethylene also example, microtubules become very short and cells expand influences organ bending by a mechanism that is not well radially when the temperature-sensitive mor1-1 mutants are understood but appears to involve the modulation of auxin grown at the restrictive temperature [51]. The lefty and spiral transport [36,40]. mutants comprise another class of microtubule mutants that have a twisted pattern of growth [54–57]. These mutant plants can 1.3. Possible roles for microtubules organize their microtubules into parallel arrays, but instead of a transverse alignment, the arrays form shallow helices with either a How might microtubules contribute to these growth right- or a left-handed orientation. Theoretically, twisted growth responses? Microtubules are thought to be involved in the occurs because cells are expanding perpendicularly to micro- differential growth response that leads to organ bending (see tubules that have a helical rather than a transverse orientation ref. [41] for review). Suggestions have also been made that [58,59]. The radially swollen 6 (rsw6) mutants represent a third microtubules function earlier in the gravity and touch pathways, class of temperature-sensitive microtubule mutants that have possibly during signal perception or transduction [16,26,41]. These defects in cell expansion. At the restrictive temperature rsw6 ideas and the evidence that supports them are discussed in this mutants have microtubules that are well organized into parallel review. In addition, recent analyses that hint at new possibilities arrays within each cell but the orientations of microtubules among for how microtubules might function during tropic responses are neighboring cells are highly variable [60]. Apparently rsw6 presented. mutants are defective in a mechanism that controls global microtubule array changes across cells. 2. Cortical microtubule organization and cell elongation The examples described above all illustrate the importance of cortical microtubules in cell elongation. Although the mechanisms Microtubules are long, tubule-shaped polymers of a- and b- by which microtubules affect cell elongation are not completely tubulin. Microtubules are actually highly dynamic, although they understood, it is thought they exert their influence by modifying appear stable in still photographs. They are almost always growing the cell wall and the wall then acts as a constraint for expansion or shrinking, providing them the flexibility to rearrange into [61,62,52]. Expansion is driven by water uptake into the vacuole. different arrays within the cell. Microtubules function in concert Water entering the vacuole causes an increase in volume that with a large group of microtubule-associated proteins or MAPs. exerts turgor pressure on the cell wall. When turgor exceeds the MAPs bind to microtubules and while bound they modify resistance imposed by the wall the cell expands (see refs. [62–66] microtubule dynamics or mediate microtubule interactions with for reviews). other proteins or structures in the cell. The microtubule arrays that Cellulose microfibrils and the linkages that hold them together appear in higher plants include preprophase bands, spindles, and are thought to be the major load-bearing elements in the wall. phragmoplasts in dividing cells, as well as the cortical arrays seen Because long and intact microfibrils are resistant to stretching in elongating cells. Plant MAPs are discussed in detail in several along their lengths, the wall yields to turgor by moving adjacent recent reviews [42–46]. microfibrils apart from one another (Fig. 2). Cellulose arrays in the Microtubules are found in the periphery (or cortex) of wall, therefore, constrain expansion to a direction that is elongating cells just below the plasma membrane where they perpendicular to the microfibril orientation. The microfibrils are are arranged in parallel hoops that encircle the cell (Fig. 2). embedded in a matrix of hemicelluloses, pectins and proteins, and Elongation occurs in a direction that is perpendicular to the cortical the extensibility of the wall also depends on the level of cross- microtubules and the arrangement of the cortical microtubules is linking between these molecules and the cellulose microfibrils. important for controlling the direction in which cells elongate (as When some of the connections that hold adjacent microfibrils

Fig. 2. Cells expand in a direction that is perpendicular to cortical microtubules and cellulose microfibrils in the cell wall. (A) A confocal section taken through expanding root cells reveals cortical microtubules (green) in the upper cell that are aligned transverse to the long axis of the cell. The section has passed through more internal regions of the lower cells. (B) Cartoon showing cellulose synthases moving through the plasma membrane. Microtubules below the plasma membrane guide the synthases and the wall microfibrils are positioned parallel with the underlying microtubules. When the cell expands adjacent cellulose microfibrils move apart from one another and elongation occurs in a direction that is perpendicular to the microtubule/microfibril alignment (indicated by double-headed arrows in A and B). Microtubules in (A) were visualized in a transgenic Arabidopsis plant expressing a microtubule binding domain – green fluorescent protein fusion [144]. 750 S.R. Bisgrove / Plant Science 175 (2008) 747–755 together are disrupted, the wall becomes more extensible and cell 2.2. Microtubule reorientations and tropisms expansion increases. Thus, the orientation of cellulose microfibrils in the wall and the degree of cross-linking largely determine the The concept that microtubules determine the orientation of direction and the rates at which cells expand [61–68]. cellulose microfibrils in the wall has been put forth as an explanation for how bend formation might be regulated [76,77]. 2.1. Cellulose deposition Microtubule reorientations have been observed in the cells that are involved in bend formation in plants that are responding to tropic Cellulose microfibrils are produced by cellulose synthase cues. Microtubules on the inner flanks of bending organs become complexes embedded in the plasma membrane. The synthases oriented parallel with the long axis of the organ while micro- travel laterally in the membrane, extruding microfibrils to the tubules on the outer sides are transversely oriented [76–81].If outside as they move [64–66]. This means that the arrangement of cellulose synthase complexes follow microtubule tracks, then microfibrils in the wall is determined by the paths that are taken by microfibrils will be deposited in the same direction as the the synthase complexes during cellulose synthesis. Microtubules reoriented microtubules. Since cells with transverse microfibrils appear to be involved in guiding synthase movement, but the elongate more than cells with longitudinal microfibrils, the end mechanism is not completely understood. Cortical microtubules in result will be organ bending [41,76,77,79,80]. In line with this idea elongating cells are closely associated with the plasma membrane is the observation that microtubules reorient in response to auxin and co-aligned with the microfibrils in the wall (Fig. 2). Based on [82–88]. Hence, the formation of an auxin gradient across a this observation, the microtubule/microfibril co-alignment model tropically stimulated root or stem could trigger microtubule/ postulates that cortical microtubules act as guides for the cellulose microfibril reorientations that lead to bending [41,77,80]. These synthase complexes as they move through the plasma membrane microtubule reorientations must also be coordinated across all of (as reviewed in refs. [53,69,70]). This co-alignment model was the cells in the bend. accepted for many years as the textbook version [71] for how Although a model in which auxin-induced microtubule ordered arrays of microfibrils are deposited into the wall, even reorientations are responsible for organ bending has been though direct evidence linking microtubules with cellulose suggested (see ref. [41] for a review) attempts to validate this synthase movement was lacking [69]. hypothesis have produced conflicting results [41]. One set of A direct connection between synthase movements and micro- experiments involves treating seedlings with agents that disrupt tubules in elongating cells was finally established in 2006. microtubule organization or function. In some cases tropic bending Synthase complexes moving in paths that coincided with cortical occurred while in other instances it was inhibited [84,89,90]. For microtubules were visualized in transgenic Arabidopsis plants example, in some experiments maize roots pretreated with expressing cellulose synthase – yellow fluorescent protein fusions microtubule depolymerizing agents were able to bend in response [72]. Synthases were seen tracking along curved microtubules and to a gravity stimulus [89,84], although the total amount of when microtubules were reoriented the synthases altered their curvature was less than in untreated controls [84]. However, in movement according to new microtubule positions. other experiments treating maize coleoptiles with herbicides that Although these observations clearly link synthase movement to depolymerized microtubules inhibited gravitropic bending even at microtubules, there is also evidence that suggests this is not the low concentrations that only partially eliminated microtubules whole story. When microtubules are disassembled either by [90]. In contrast, phototropism proceeded even after complete treatment with pharmacological or herbicidal agents, or by removal of cortical microtubules [90]. mutation, synthase complexes continue to move in tracks that Imposing mechanical strain on an organ can also cause are roughly parallel with each other and aligned arrays of microtubules to reorient [91]. This raises the possibility that the microfibrils continue to be deposited into the wall [50,72]. How reorientations during tropic responses result from the mechanical synthase movements might be guided in the absence of micro- strain produced when the organ bends. When mechanical tubules is not clear, but the findings do suggest that there is an counterforces were used to prevent or reverse tropic curvatures alternative or additional guidance mechanism [53,69,73]. in maize coleoptiles and bean epicotyls, microtubules reoriented in Cells with compromised microtubules lose the ability to response to the mechanical strain rather than the tropic stimuli elongate and they swell by expanding radially even though [91–93]. On the other hand, microtubules in maize coleoptiles did ordered arrays of cellulose are deposited into their walls [50]. This reorient in response to gravity when bending was physically suggests that organized cellulose arrays alone are not sufficient for prevented, suggesting that tropic stimuli can induce microtubule directional control of cell expansion and that microtubules may be reorientations in the absence of a bend [80]. influencing the wall in additional ways [50,53,73]. One explanation Issues have also been raised regarding the timing of the is that microtubule disruption leads to the deposition of short microtubule reorientations that are proposed to result in cellulose microfibrils [53]. According to the microfibril length differential growth and organ bending. In some cases microtubule regulation model, walls with shorter microfibrils will be less able reorientations that preceded organ bending were observed. For to restrict cell expansion to one direction [53]. One could also example, in gravity stimulated maize coleoptiles [80] and cut speculate that microtubules control the spatial arrangement of snapdragon spikes [94] microtubule reorientations occurred additional molecules involved in cell wall modification and/or before the organs bent. However, microtubule reorientations do synthesis. If so, microtubule disruption could result in the loss or not always precede organ bending. For example, in gravity aberrant localization of molecules that stabilize or loosen the wall stimulated maize roots organ bending was initiated before leading to altered cell expansion. Microtubules have been microtubule reorientations occurred [78]. In addition, microtubule implicated in the localization of two other proteins involved in reorientations that preceded organ bending were not consistently cellulose synthesis, KORRIGAN (an endo-1,4-b-D-glucanase observed when maize coleoptiles responding to light and gravity involved in cellulose synthesis [74]) and COBRA (a glycosyl- stimuli were analyzed [95]. Another issue that has been raised is phosphatidyl inositol (GPI) containing protein [75]). Both KORRI- based on the idea that changes in cell elongation rates due to GAN and COBRA are distributed in linear arrays along the plasma microtubule reorientations are likely to be time-consuming as they membrane and treatment with the microtubule depolymerizing involve reinforcing the cell wall through cellulose synthesis after herbicide oryzalin disrupts this localization pattern [74,75]. the microtubule reorientations occur [96]. In support of this idea, S.R. Bisgrove / Plant Science 175 (2008) 747–755 751 correlations of cell elongation rates and microtubule/microfibril mechanism by which cellulose synthase complexes are guided alignments in elongating Arabidopsis roots indicate that micro- during cellulose deposition (Fig. 3A and B). In such a scenario, fibrils remain transversely aligned until well after microtubules microtubules would influence cell elongation and organ bending have reoriented [97]. Since the changes in cell elongation rates that by coordinating the positions of signaling molecules in the plasma occur during plant tropisms are initiated quickly, it has been membrane with the structural components of the wall that are argued that microtubule reorientation is unlikely to play a role in modified during the response (Fig. 3B). the early stages of bend formation even if the reorientations occur In animal and fungal cells the rapidly growing or plus-ends of before the bend forms [96]. However, another suggestion is that microtubules also interact with proteins localized in the cortex of microtubule reorientations play a role later in bend formation, the cell next to the plasma membrane (reviewed in refs. [113– perhaps as a mechanism to reinforce bending once it is initiated 115]) and there is evidence that suggests microtubule plus-ends [78]. have roles in plant tropisms. Arabidopsis plants carrying mutations Given all of the results discussed above, a simple model in in the microtubule plus-end binding protein END BINDING 1 (EB1) which auxin-induced microtubule reorientations lead to differ- are slower to initiate bends after touch and/or gravity stimuli ential growth and organ bending is unlikely, although micro- [116]. The delay appears to be specific for the initiation of tubules could influence cell expansion and organ bending in other differential growth as mutants grow at the same rate as wild type ways. In addition to affecting microtubule organization, auxin also plants. Mutant roots have a tendency to grow in loops on tilted agar alters gene expression and ion homeostasis in cells [33–36,38,98– surfaces, indicating that they also have difficulty turning off 102]. According to the ‘‘Acid Growth Theory’’ [38,98] auxin differential growth [116]. stimulates cell expansion by triggering the extrusion of protons How might EB1 influence differential growth during tropic into the wall. Acidification activates pH-sensitive enzymes and responses? Although the mechanism is unknown, EB1 proteins have these proteins increase cell expansion by cleaving the linkages been the object of intense scrutiny in animal and fungal cells and between polysaccharide components in the wall [38,39,98]. results from these studies could provide insight into the roles of EB1 Another well-known effect of auxin is its ability to alter gene in plants. EB1 accumulates preferentially on the rapidly growing (or transcription and it follows that a gradient of auxin across a stem or plus) ends of microtubules in all of the organisms in which it has root would result in the expression of different sets of genes on been studied [113,115,117–120]. While bound to the microtubule each side of the organ. Proteins that enhance cell expansion are end EB1 proteins usually make microtubules more dynamic [113]. synthesized preferentially on the faster growing flank where they The increase in dynamics is thought to help microtubules search can stimulate growth. The cell wall loosening proteins known as the cytoplasm for ‘‘capture’’ sites. When an appropriate site is expansins localize preferentially to the more rapidly expanding encountered EB1 mediates interactions between the microtubule outer flank of gravistimulated maize roots [103]. Expression end and other proteins localized at the site [113,121]. Capture sites analyses in Brassica oleracea seedlings have identified a set of often contain actin and EB1 is known to mediate interactions tropically stimulated genes that are preferentially expressed in the between microtubules and actin in some cell types (reviewed in ref. cells where auxin levels are the highest. Among the up-regulated [113]). The interactions of microtubule ends with capture sites genes were enzymes that have been associated with roles in cell serves several functions including the targeted delivery of vesicles, wall expansion [33,68]. These newly synthesized proteins must be signaling molecules, and ion channels to specific places in the cell transported to their sites of action in the plasma membrane or cell [113,114,122–131]. Whether a similar system operates in plant cells wall. Although speculative, it is possible that microtubules are is not known. However, one could speculate that the interaction of involved in positioning these molecules in the proper places at the microtubule ends with specific sites in the cortex of the cell plays a appropriate times (see Fig. 3 and discussion below). Note, in this role in tropic responses (see Fig. 3C). A captured microtubule could scenario microtubule disruption would not necessarily prevent direct the insertion, removal, or position of proteins and/or localization to the plasma membrane or cell wall, although it could complexes at specific sites in the cortex of the cell by serving as a affect placements with respect to space and/or time. track along which cargo-bearing microtubule-based motors like kinesins would travel. Alternatively, interactions between micro- 3. Beyond cellulose deposition: new roles for microtubules? tubule ends and capture sites could also be transient in nature. Microtubule plus-end binding proteins like EB1 appear to serve as Several lines of evidence suggest that microtubules could be platforms for recruiting signaling molecules to the active ends of involved in organizational events associated with the plasma microtubules (reviewed in ref. [113]). In this case a brief encounterat membrane [104]. As mentioned above, the localization of both an appropriate capture site would be sufficient to deliver a specific COBRA and KORRIGAN, two proteins with roles in cell wall factor. biosynthesis, are microtubule-dependent [74,75]. Although these The idea that microtubules might be involved in the spatial proteins could be part of cellulose synthase complexes, their integration of endo- and exocytotic events has been proposed localization in the plasma membrane is microtubule-dependent. A [104]. In addition to a role in the localization of the secreted link between microtubules and the localization of an arabinoga- proteins COBRA, KORRIGAN, and the arabinogalactan protein lactan cell surface GPI-linked protein from tomato has also been discussed above, microtubules also appear to have roles in reported [105]. Microtubule disruption caused a relocalization of endocytosis in plant cells [132,133]. During preprophase band this protein [105]. GPI-anchored proteins have been associated formation in cultured tobacco BY-2 cells endocytotic vesicles are with protein-containing membrane microdomains or lipid rafts in internalized in trans-vacuolar cytoplasmic strands that form the plasma membrane [106–108]. Evidence from the animal connections between the nucleus and the cortex of the cell. These literature suggests that lipid rafts may have roles in signaling cytoplasmic strands contain microtubules and treatment of cells processes and it is thought that they could serve as centers for with an herbicide that depolymerizes microtubules disrupts signaling cascades [109,110]. It has been proposed that similar vesicle internalization [132]. Ligand-mediated endocytosis of a membrane microdomains may also be present in plant cells receptor involved in plant defense responses (the flagellin receptor [111,112]. Although speculative, one possibility is that micro- FLS2) also utilizes microtubules [133]. Endocytosis of FLS2 is tubules play a role in positioning signaling proteins and/or thought to be a regulatory step in the signaling pathway [133]. One complexes in the plasma membrane in a manner similar to the could speculate that these microtubule-dependant endocytotic 752 S.R. Bisgrove / Plant Science 175 (2008) 747–755

Fig. 3. Possible roles for microtubules in plant tropisms. (A) According to the microtubule/microfibril co-alignment model microtubules serve as guides that influence the movement of cellulose synthase complexes during cellulose deposition. In response to tropic cues, there is a microtubule reorientation that has been proposed to change the direction of cellulose deposition and reduce the amount of cell elongation on the inner flank of a bending organ. Although the change in cell elongation rate may be too slow to play a role in bend initiation, the reorientations could be involved at later stages, perhaps to reinforce the bend once it is initiated. (B) Although speculative, it is possible that microtubules could constrain the positions of signaling proteins and/or complexes in the plasma membrane in a way that is similar to the mechanism postulated in the microtubule/microfibril co-alignment model. This would enable the plant to spatially coordinate structural components of the wall with the signaling molecules that trigger changes in wall extensibility during tropic responses. (C) Another way that microtubules could influence tropism is through interactions of their ends with specific sites in the cortex of the cell. Proteins that preferentially bind the more rapidly growing or plus-ends of microtubules, such as EB1, can mediate both microtubule dynamics and interactions of the microtubule end with other proteins or structures in the cell. EB1 tends to make microtubules more dynamic and this increase in dynamics is thought to facilitate microtubule searching of the cytoplasm for specific ‘‘capture’’ sites. In animal and fungal cells EB1 is also known to interact with several other proteins including signaling molecules. When the microtubule end encounters a suitable site proteins associated with the microtubule end (yellow balls) and proteins localized at the capture site (blue balls) can interact with one another. Capture sites often contain actin and EB1 is sometimes involved in crosstalk between the two cytoskeletal structures (for simplicity actin is not shown in A or B). Transient encounters would be sufficient for the release of factors associated with the microtubule end while stably connected microtubules could serve as tracks that direct the movement of cargo-bearing motor proteins (not shown) to and from the capture site. These interactions could play a role in tropisms by delivering signals or materials that are needed to regulate organ bending to the appropriate sites in the cortex of the cell. They could also be involved in signal perception either via connections to mechanosensory ion channels in the plasma membrane or by mediating the removal or insertion of signaling molecules such as receptors into or out of the plasma membrane. events involve the interactions of microtubule ends with cortical Microtubules have been implicated with roles in the perception capture sites. Other plasma membrane proteins, including of touch and gravity signals [16,26,41,90,95,137–139]. There are hormone receptors and ion channels, are also internalized in reports that microtubule inhibitors modify the rate of amyloplast plant cells, although the cytoskeletal requirements for many of sedimentation and inhibit auxin transport during gravitropism these endocytotic events have not been determined [134–136]. (reviewed in ref. [41]). The mechanism by which gravity is converted S.R. Bisgrove / Plant Science 175 (2008) 747–755 753 into a biochemical signal is unknown, although one possibility is that hold the potential to yield new insights into how microtubules falling amyloplasts could be sensed by mechanosensitive ion function in plant cells. channels [26]. Touch sensing is also thought to be mediated by mechanosensitive ion channels, a hypothesis that is based in part on Acknowledgement analyses conducted on touch receptor neurons in the nematode worm Caenorhabditis elegans [16,26]. Genetic analyses indicate that S.R.B. is funded by a Natural Sciences and Engineering Research mutations in tubulins associated with these neurons cause loss of Council of Canada Discovery Grant (Application 331017). mechanoresponse [140]. These microtubules are thought to form tethers that are linked, either directly or indirectly, to ion channels in References the plasma membrane [141,142]. Force from a touch stimulus is thought to displace the microtubule tethers and this induces a [1] G.I. Cassab, Other tropisms and their relationship to gravitropism, in: S. Gilroy, conformational change in the channel that activates it [26,140,141]. P.H. Masson (Eds.), Plant Tropisms, Blackwell Publishing Ltd., Ames, Iowa, 2008, 2+ pp. 123–139. Ca channels have been implicated in touch responses in plants [2] J. Braam, In touch: plant responses to mechanical stimuli, New Phytol. 165 [9,16,26] and patch-clamping analyses have detected mechan- (2005) 373–389. osensitive calcium channels that are affected by microtubule [3] D. Eapen, M.L. Barroso, M.E. Campos, G. Ponce, G. Corkidi, J.G. Dubrovsky, G.I. Cassab, A no hydrotropic response root mutant that responds positively to inhibitors [143]. However, the molecular identity of these channels gravitropism in Arabidopsis, Plant Physiol. 131 (2003) 536–546. is unknown. Proteins that function as mechanosensitive ion [4] T. Kaneyasu, A. Kobayashi, M. Nakayama, N. Fujii, H. Takahashi, Y. Miyazawa, channels in plants have recently been identified [7,10]. Whether Auxin response, but not its polar transport, plays a role in hydrotropism of Arabidopsis roots, J. Exp. Bot. (2007) 1143–1150. these channels are involved in touch or gravity sensing and how [5] A. Kobayashi, A. Takahashi, Y. Kakimoto, Y. Miyazawa, N. Fujii, A. Higashitani, H. their activities are regulated is unknown. Takahashi, A gene essential for hydrotropism in roots, Proc. Natl. Acad. Sci. U.S.A. 104 (2007) 4724–4729. [6] G. Ponce, F.A. Rasgado, G.I. Cassab, Roles of amyloplasts and water deficit in root 4. Summary and future directions tropisms, Plant Cell Environ. 31 (2008) 205–217. [7] E.S. Haswell, R. Peyronnet, H. Barbier-Brygoo, E.M. Meyerowitz, J.-M. Frachisse, Although microtubules have been associated with plant Two MscS homologs provide mechanosensitive channel activities in the Arabi- tropisms for many years our understanding of how they participate dopsis root, Curr. Biol. 18 (2008) 730–734. [8] J.M. Kimbrough, R. Salinas-Mondragon, W.F. Boss, C.S. Brown, H.W. Sederoff, The in these growth responses is far from complete. One proposal is fast and transient transcriptional network of gravity and mechanical stimulation that microtubules function during the differential growth response in the Arabidopsis root apex, Plant Physiol. 136 (2004) 2790–2805. that leads to organ bending [41,76,77,79,80]. This idea is based on [9] E. McCormack, L. Velasquez, N.A. Delk, J. Braam, Touch-responsive behaviors and gene expression in plants, in: S.M. Frantisek Baluska, Dieter Volkmann (Eds.), the concept that microtubules determine the direction of cell Communication in Plants, Springer, 2006. elongation by guiding the deposition of cellulose microfibrils into [10] Y. Nakagawa, T. Katagiri, K. Shinozaki, Z. Qi, H. Tatsumi, T. Furuichi, A. Kishigami, the cell wall. However, the hypothesis is controversial. In particular M. Sokabe, I. Kojima, S. Sato, T. Kato, S. Tabata, K. Iida, A. Terashima, M. Nakano, M. Ikeda, T. Yamanaka, H. Iida, Arabidopsis plasma membrane protein crucial for there are questions surrounding the timing of microtubule/ Ca2+ influx and touch sensing in roots, Proc. Natl. Acad. Sci. U.S.A. 104 (2007) microfibril reorientations with respect to the onset of differential 3639–3644. growth [78,80,94,96,97]. Several recent reports indicate that [11] S. Gilroy, Plant tropisms, Curr. Biol. 18 (2008) R275–R277. [12] J.M. Christie, Phototropin blue-light receptors, Annu. Rev. Plant Biol. 58 (2007) microtubules also have roles in organizational events associated 21–45. with the plasma membrane [74,75,105,132,133]. In addition, [13] C.A. Esmon, U.V. Pedmale, E. Liscum, Plant tropisms: providing the power of plants carrying mutations in the microtubule plus-end binding movement to a sessile organism, Int. J. Dev. Biol. 49 (2005) 665–674. [14] B.R. Harrison, M.T. Morita, P.H. Masson, M. Tasaka, Signal transduction in protein EB1 exhibit delayed responses to touch/gravity signals gravitropism, in: S. Gilroy, P.H. Masson (Eds.), Plant Tropisms, Blackwell Publish- [116]. Although speculative, these studies raise the possibility that ing, Ames, Iowa, 2008, pp. 21–45. microtubules could influence the wall in ways that are indepen- [15] J.Z. Kiss, Where’s the water? Hydrotropism in plants, Proc. Natl. Acad. Sci. U.S.A. dent of a role in guiding cellulose deposition. 104 (2007) 4247–4248. [16] G.B. Monshausen, S.J. Swanson, S. Gilroy, Touch sensing and thigmotropism, in: Another possibility is that microtubules function in the S. Gilroy, P.H. Masson (Eds.), Plant Tropisms, Blackwell Publishing, Ames, Iowa, perception of touch and gravity signals [16,26,41,90,95,137– 2008, pp. 91–122. 139]. There is evidence that indicates microtubules are involved [17] J.L. Mullen, J.Z. Kiss, Phototropism and its relationship to gravitropism, in: S. Gilroy, P.H. Masson (Eds.), Plant Tropisms, Blackwell Publishing Ltd., Ames, Iowa, in the activation of mechanosensory ion channels in animal cells 2008, pp. 79–90. [140,141,142] and microtubules appear to affect the activity of [18] A. Valster, E.B. Blancaflor, Mechanisms of gravity perception in higher plants, in: mechanosensitive Ca2+ channels in plant membranes [143]. S. Gilroy, P.H. Masson (Eds.), Plant Tropisms, Blackwell Publishing, Ames, Iowa, 2008, pp. 3–19. However, mechanistic links between microtubules and the [19] C.W. Whippo, R.P. Hangarter, Phototropism: bending towards enlightenment, mechanoreceptors that might be involved in touch/gravity sensing Plant Cell 18 (2006) 1110–1119. are speculative. [20] S. Tokutomi, D. Matsuoka, K. Zikihara, Molecular structure and regulation of phototropin kinase by blue light, Biochim. Biophys. Acta 1784 (2008) 133–142. One way to assess how microtubules function during tropisms [21] B. Kang, N. Grancher, V. KoyVmann, D. Lardemer, S. Burney, M. Ahmad, Multiple is to analyze proteins that interact with microtubules. Several interactions between cryptochrome and phototropin blue-light signalling path- microtubule-associated proteins have been identified in plants ways in Arabidopsis thaliana, Planta 227 (2008) 1091–1099. [22] S. Inoue, T. Kinoshita, M. Matsumoto, K. Nakayama, M. Doi, K. Shimazaki, Blue (see refs. [43,46] for reviews). Many of these proteins affect the light-induced autophosphorylation of phototropin is a primary step for signal- stability or organization of microtubule arrays [46] and the ing, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 5626–5631. corresponding mutants often have cell expansion defects that [23] M.T. Morita, M. Tasaka, Gravity sensing and signaling, Curr. Opin. Plant Biol. 7 make it difficult to measure tropic responses. Because plants (2004) 712–718. [24] R. Wayne, M.P. Staves, A down to earth model of gravisensing or Newton’s Law of carrying mutations in EB1 genes have defects in touch/gravity Gravitation from the apple’s perspective, Physiol. Plant. 98 (1996) 917–921. responses they hold promise for providing further insights into the [25] R.M. Perrin, L.-S. Young, N. Murthy, B.R. Harrison, Y.A.N. Wang, J.L. Will, P.H. role of microtubules in tropisms. Biochemical screens for proteins Masson, Gravity signal transduction in primary roots, Ann. Bot. 96 (2005) 737–743. [26] P. Nick, Microtubules as sensors for abiotic stimuli, in: P. Nick (Ed.), Plant that interact directly with EB1 could identify proteins that are Microtubules, Springer-Verlag, Berlin/Heidelberg, 2008, pp. 175–203. important for either sensing touch/gravity cues or initiating [27] N. Takahashi, N. Goto, K. Okada, H. Takahashi, Hydrotropism in abscisic acid, wavy, differential growth. Genetic screens for enhancers or suppressors and gravitropic mutants of Arabidopsis thaliana, Planta 216 (2002) 203–211. [28] N. Takahashi, Y. Yamazaki, A. Kobayashi, A. Higashitani, H. Takahashi, Hydro- of the eb1 mutant phenotype could identify genes that function at tropism interacts with gravitropism by degrading amyloplasts in seedling roots different steps in the same or parallel pathways. These analyses of Arabidopsis and radish, Plant Physiol. 132 (2003) 805–810. 754 S.R. Bisgrove / Plant Science 175 (2008) 747–755

[29] D. Eapen, M.L. Barroso, G. Ponce, M.E. Campos, G.I. Cassab, Hydrotropism: root [62] D.J. Cosgrove, Growth of the plant cell wall, Nat. Rev. Mol. Cell Biol. 6 (2005) 850– growth responses to water, Trends Plant Sci. 10 (2005) 44–50. 861. [30] G.K. Muday, A. Rahman, Auxin transport and the integration of gravitropic [63] I. Burgert, P. Fratzl, Mechanics of the expanding cell wall, in: J.-P. Verbelen, K. growth, in: S. Gilroy, P.H. Masson (Eds.), Plant Tropisms, Blackwell Publishing Vissenberg (Eds.), The Expanding Cell, Springer-Verlag, Berlin/Heidelberg, 2007, Ltd., Ames, Iowa, 2008, pp. 47–77. pp. 191–215. [31] K.T. Yamamoto, Happy end in sight after 70 years of controversy, Trends Plant [64] K. Hematy, H. Hofte, Cellulose and cell elongation, in: J.-P. Verbelen, K. Vissen- Sci. 8 (2003) 359–360. berg (Eds.), The Expanding Cell, Springer-Verlag, Berlin/Heidelberg, 2007, pp. [32] L. Abas, R. Benjamins, N. Malenica, T. Paciorek, J. Wigniewska, J.C. Moulinier- 33–56. Anzola, T. Sieberer, J. Friml, C. Luschnig, Intracellular trafficking and proteolysis [65] O. Lerouxel, D.M. Cavalier, A.H. Liepman, K. Keegstra, Biosynthesis of plant cell of the Arabidopsis auxin-efflux facilitator PIN2 are involved in root gravitropism, wall polysaccharides – a complex process, Curr. Opin. Plant Biol. 9 (2006) 621– Nat. Cell Biol. 8 (2006) 249–256. 630. [33] C.A. Esmon, A.G. Tinsley, K. Ljung, G. Sandberg, L.B. Hearne, E. Liscum, A gradient [66] C. Somerville, Cellulose synthesis in higher plants, Annu. Rev. Cell Dev. Biol. 22 of auxin and auxin-dependent transcription precedes tropic growth responses, (2006) 53–78. Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 236–241. [67] F. Marga, M. Grandbois, D.J. Cosgrove, T.I. Baskin, Cell wall extension results in [34] I. Fuchs, K. Philippar, R. Hedrich, Ion channels meet auxin action, Plant Biol. 8 the coordinate separation of parallel microfibrils: evidence from scanning (2006) 353–359. electrons microscopy and atomic force microscopy, Plant J. 43 (2005) 181–190. [35] K. Palme, A. Dovzhenko, F.A. Ditengou, Auxin transport and gravitational [68] S.J. McQueen-Mason, N.T. Le, D. Brocklehurst, Expansins, in: J.-P. Verbelen, K. research: perspectives, Protoplasma 229 (2006) 175–181. Vissenberg (Eds.), The Expanding Cell, Springer-Verlag, Berlin/Heidelberg, 2007, [36] S. Philosoph-Hadas, H. Friedman, S. Meir, L. Gerald, Gravitropic bending and pp. 117–138. plant hormones, Vitam. Horm. 72 (2005) 31–78. [69] A.M.C. Emons, H. Hofte, B.M. Mulder, Microtubules and cellulose microfibrils: [37] A. Vieten, M. Sauer, P.B. Brewer, J. Friml, Molecular and cellular aspects of auxin- how intimate is their relationship? Trends Plant Sci. 12 (2007) 279–281. transport-mediated development, Trends Plant Sci. 12 (2007) 160–168. [70] C. Lloyd, Microtubules make tracks for cellulose, Science 312 (2006) 1482–1483. [38] A. Hager, Role of the plasma membrane H+-ATPase in auxin-induced elongation [71] B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts, J.D. Watson, Molecular Biology of growth: historical and new aspects, J. Plant Res. 116 (2003) 483–505. the Cell, Garland Publishing Inc., New York, 1994. [39] M. Grebe, Growth by auxin: when a weed needs acid, Science 310 (2005) 60–61. [72] A.R. Paredez, C.R. Somerville, D.W. Ehrhardt, Visualization of cellulose synthase [40] C.S. Buer, P. Sukumar, G.K. Muday, Ethylene modulates flavonoid accumulation demonstrates functional association with microtubules, Science 312 (2006) and gravitropic responses in roots of Arabidopsis, Plant Physiol. 140 (2006) 1384– 1491–1495. 1396. [73] A. Paradez, A. Wright, D.W. Ehrhardt, Microtubule cortical array organization [41] E.B. Blancaflor, The cytoskeleton and gravitropism in higher plants, J. Plant and plant cell morphogenesis, Curr. Opin. Plant Biol. 9 (2006) 571–578. Growth Reg. 21 (2002) 120–136. [74] S. Robert, A. Bichet, O. Grandjean, D. Kierzkowski, B. Satiat-Jeunemaitre, S. [42] D.W. Ehrhardt, Straighten up and fly right—microtubule dynamics and organi- Pelletier, M.-T. Hauser, H. Hofte, S. Vernhettes, An Arabidopsis endo-1,4-b-D- zation of non-centrosomal arrays in higher plants, Curr. Opin. Cell Biol. 20 (2008) glucanase involved in cellulose synthesis undergoes regulated intracellular 107–116. cycling, Plant Cell 17 (2005) 3378–3389. [43] T. Hamada, Microtubule-associated proteins in higher plants, J. Plant Res. 120 [75] F. Roudier, A.G. Fernandez, M. Fujita, R. Himmelspach, G.H.H. Borner, G. Schin- (2007) 79–98. delman, S. Song, T.I. Baskin, P. Dupree, G.O. Wasteneys, P.N. Benfey, COBRA, an [44] J. Lucas, S.L. Shaw, Cortical microtubule arrays in the Arabidopsis seedling, Curr. Arabidopsis extracellular glycosyl-phosphatidyl inositol-anchored protein, spe- Opin. Plant Biol. 11 (2008) 94–98. cifically controls highly anisotropic expansion through its involvement in cel- [45] M. Pastuglia, D. Bouchez, Molecular encounters at microtubule ends in the plant lulose microfibril orientation, Plant Cell 17 (2005) 1749–1763. cell cortex, Curr. Opin. Plant Biol. 10 (2007) 557–563. [76] E.B. Blancaflor, K.H. Hasenstein, Organization of cortical microtubules in grav- [46] J.C. Sedbrook, D. Kaloriti, Microtubules, MAPs and plant directional cell expan- iresponding maize roots, Planta 191 (1993) 231–237. sion, Trends Plant Sci. 13 (2008) 303–310. [77] P. Nick, R. Bergfeld, E. Schafer, P. Schopfer, Unilateral reorientation of micro- [47] J.C. Ambrose, T. Shoji, A.M. Kotzer, J.A. Pighin, G.O. Wasteneys, The Arabidopsis tubules at the outer epidermal wall during photo- and gravitropic curvature of CLASP gene encodes a microtubule-associated protein involved in cell expansion maize coleoptiles and sunflower hypocotyls, Planta 181 (1990) 162–168. and division, Plant Cell 19 (2007) 2763–2775. [78] E.B. Blancaflor, K.H. Hasenstein, Time course and auxin sensitivity of cortical [48] T.I. Baskin, J.E. Wilson, A. Cork, R.E. Williamson, Morphology and microtubule microtubule reorientation in maize roots, Protoplasma 185 (1995) 72–82. organization in Arabidopsis roots exposed to oryzalin or taxol, Plant Cell Physiol. [79] R. Himmelspach, C.L. Wymer, C.W. Lloyd, P. Nick, Gravity-induced reorientation 35 (1994) 935–942. of cortical microtubules observed in vivo, Plant J. 18 (1999) 449–453. [49] R.S. McClinton, R. Sung, Organization of cortical microtubules at the plasma [80] R. Himmelspach, P. Nick, Gravitropic microtubule reorientation can be membrane in Arabidopsis, Planta 201 (1997) 252–260. uncoupled from growth, Planta 212 (2001) 184–189. [50] K. Sugimoto, R. Himmelspach, R.E. Williamson, G.O. Wasteneys, Mutation or [81] M. Saiki, H. Fujita, K. Soga, K. Wakabayashi, S. Kamisaka, M. Yamashita, T. Hoson, drug-dependent microtubule disruption causes radial swelling without altering Cellular basis for the automorphic curvature of rice coleoptiles on a three- parallel cellulose microfibril deposition in Arabidopsis root cells, Plant Cell 15 dimensional clinostat: possible involvement of reorientation of cortical micro- (2003) 1414–1429. tubules, J. Plant Res. 118 (2005) 199–205. [51] A.T. Whittington, O. Vugrek, K.J. Wei, N.G. Hasenbein, K. Sugimoto, M.C. Rash- [82] F. Baluska, P.W. Barlow, D. Volkmann, Complete disintegration of the micro- brooke, G.O. Wasteneys, MOR1 is essential for organizing cortical microtubules tubular cytoskeleton precedes its auxin-mediated reconstruction in postmitotic in plants, Nature 411 (2001) 610–613. maize root cells, Plant Cell Physiol. 37 (1996) 1013–1021. [52] G. Wasteneys, M. Fujita, Establishing and maintaining axial growth: wall [83] K. Fischer, P. Schopfer, Interaction of auxin, light, and mechanical stress in mechanical properties and the cytoskeleton, J. Plant Res. 119 (2006) 5–10. orienting microtubules in relation to tropic curvature in the epidermis of maize [53] G.O. Wasteneys, D.A. Collings, The cytoskeleton and co-ordination of directional coleoptiles, Protoplasma 196 (1997) 108–116. expansion in a multicellular context, in: J.-P. Verbelen, K. Vissenberg (Eds.), The [84] K.H. Hasenstein, E.B. Blancaflor, J.S. Lee, The microtubule cytoskeleton does not Expanding Cell, Springer-Verlag, Berlin/Heidelberg, 2007, pp. 217–248. integrate auxin transport and gravitropism in maize roots, Physiol. Plant. 105 [54] I. Furutani, Y. Watanabe, R. Prieto, M. Masukawa, K. Suzuki, K. Naoi, S. Thita- (1999) 729–738. madee, T. Shikanai, T. Hashimoto, The SPIRAL genes are required for directional [85] K. Mayumi, H. Shibaoka, The cyclic reorientation of cortical microtubules on control of cell elongation in Arabidopsis thaliana, Development 127 (2000) 4443– walls with a crossed polylamellate structure: effects of plant hormones and an 4453. inhibitor of protein kinases on the progression of the cycle, Protoplasma 195 [55] K. Nakajima, I. Furutani, H. Tachimoto, H. Matsubara, T. Hashimoto, SPIRAL1 (1996) 112–122. encodes a plant-specific microtubule-localized protein required for directional [86] P. Nick, E. Schafer, M. Furuya, Auxin redistribution during first positive photo- control of rapidly expanding Arabidopsis cells, Plant Cell 16 (2004) 1178–1190. tropism in corn coleoptiles: microtubule reorientation and the Cholodny-Went [56] J.C. Sedbrook, D.W. Ehrhardt, S.E. Fisher, W.-R. Scheible, C. Somerville, The Theory, Plant Physiol. 99 (1992) 1302–1308. Arabidopsis SKU6/SPIRAL gene encodes a plus end-localized microtubule-inter- [87] K. Vissenberg, A.-H. Quelo, K. Van Gestel, G. Olyslaegers, J.-P. Verbelen, From acting protein involved in directional cell expansion, Plant Cell 16 (2004) 1506– hormone signal, via the cytoskeleton, to cell growth in single cells of tobacco, Cell 1520. Biol. Int. 24 (2000) 343–349. [57] S. Thitamadee, K. Tuchihara, T. Hashimoto, Microtubule basis for left-handed [88] B. Wiesler, Q.-Y. Wang, P. Nick, The stability of cortical microtubules depends on helical growth in Arabidopsis, Nature 417 (2002) 193–196. their orientation, Plant J. 32 (2002) 1023–1032. [58] T. Ishida, Y. Kaneko, M. Iwano, T. Hashimoto, Helical microtubule arrays in a [89] F. Baluska, M. Hauskrecht, P. Barlow, A. Sievers, Gravitropism of the primary root collection of twisting tubulin mutants of Arabidopsis thaliana, Proc. Natl. Acad. of maize: a complex pattern of differential cellular growth in the cortex inde- Sci. U.S.A. 104 (2007) 8544–8549. pendent of the microtubular cytoskeleton, Planta 198 (1996) 310–318. [59] M. Oliva, C. Dunand, Waving and skewing: how gravity and the surface of growth [90] P. Nick, E. Schafer, R. Hertel, M. Furuya, On the putative role of microtubules in media affect root development in Arabidopsis, New Phytol. 176 (2007) 37–43. gravitropism of maize coleoptiles, Plant Cell Physiol. 32 (1991) 873–880. [60] A. Bannigan, A.M.D. Wiedemeier, R.E. Williamson, R.L. Overall, T.I. Baskin, [91] K. Zandomeni, P. Schopfer, Mechanosensory microtubule reorientation in the Cortical microtubule arrays lose uniform alignment between cells and are epidermis of maize coleoptiles subjected to bending stress, Protoplasma 182 oryzalin resistant in the Arabidopsis mutant, radially swollen 6, Plant Cell Physiol. (1994) 96–101. 47 (2006) 949–958. [92] K. Fischer, P. Schopfer, Physical strain mediated microtubule reorientation in the [61] T.I. Baskin, Anisotropic expansion of the plant cell wall, Annu. Rev. Cell Dev. Biol. epidermis of gravitropically or phototropically stimulated maize coleoptiles, 21 (2005) 203–222. Plant J. 15 (1998) 119–123. S.R. Bisgrove / Plant Science 175 (2008) 747–755 755

[93] T. Ikushima, T. Shimmen, Mechano-sensitive orientation of cortical microtu- [119] J. Mathur, N. Mathur, B. Kernebeck, B.P. Srinivas, M. Hulskamp, A novel localiza- bules during gravitropism in azuki bean epicotyls, J. Plant Res. 118 (2005) 19–26. tion pattern for an EB1-like protein links microtubule dynamics to endomem- [94] Z. Zhang, H. Friedman, S. Meir, I. Rosenberger, A.H. Halevy, S. Philosoph-Hadas, brane organization, Curr. Biol. 13 (2003) 1991–1997. Microtubule reorientation in shoots precedes bending during the gravitropic [120] D. Van Damme, K. Van Poucke, E. Boutant, C. Ritzenthaler, D. Inze, D. Geelen, In response of cut snapdragon spikes, J. Plant Physiol. 165 (2008) 289–296. vivo dynamics and differential microtubule-binding activities of MAP65 pro- [95] P. Nick, M. Furuya, E. Schafer, Do microtubules control growth in tropism? teins, Plant Physiol. 136 (2004) 3956–3967. Experiments with maize coleoptiles, Plant Cell Physiol. 32 (1991) 999–1006. [121] Y. Mimori-Kiyosue, S. Tsukita, Search-and-Capture’’ of microtubules through [96] H. Shibaoka, Plant hormone-induced changes in the orientation of cortical plus-end-binding proteins (+TIPs), J. Biochem. 134 (2003) 321–326. microtubules: alterations in the cross-linking between microtubules and the [122] A.I.M. Barth, K.A. Siemers, W.J. Nelson, Dissecting interactions between EB1, plasma membrane, Annu. Rev. Plant Physiol. Plant Mol. Biol. 45 (1994) 527–544. microtubules and APC in cortical clusters at the plasma membrane, J. Cell Sci. [97] K. Sugimoto, R.E. Williamson, G.O. Wasteneys, New techniques enable compara- 115 (2002) 1583–1590. tive analysis of microtubule orientation, wall texture, and growth rate in intact [123] I. Grigoriev, S.M. Gouveia, B. van der Vaart, J. Demmers, J.T. Smyth, S. Honnappa, roots of Arabidopsis, Plant Physiol. 124 (2000) 1493–1506. D. Splinter, M.O. Steinmetz, J.W. Putney Jr., C.C. Hoogenraad, A. Akhmanova, [98] D.L. Rayle, R.E. Cleland, The acid growth theory of auxin-induced cell elongation STIM1 is a MT-plus-end-tracking protein involved in remodeling of the ER, Curr. is alive and well, Plant Physiol. 99 (1992) 1271–1274. Biol. 18 (2008) 177–182. [99] E.B. Blancaflor, P.H. Masson, Plant gravitropism. Unraveling the ups and downs [124] C. Gu, W. Zhou, M.A. Puthenveedu, M. Xu, Y.N. Jan, L.Y. Jan, The microtubule plus- of a complex process, Plant Physiol. 133 (2003) 1677–1690. end tracking protein EB1 is required for Kv1 voltage-gated K+ channel axonal [100] M. Christian, D. Schenck, M. Bottger, H. Luthen, B. Steffens, New insight into targeting, Neuron 52 (2006) 803–816. auxin perception, signal transduction, and transport, in: U.L.K. Esser, W. Beys- [125] S. Honnappa, O. Okhrimenko, R. Jaussi, H. Jawhari, I. Jelesarov, F.K. Winkler, M.O. chlag, J. Murata (Eds.), Progress in Botany, Springer, Berlin/Heidelberg, 2006, pp. Steinmetz, Key interaction modes of dynamic +TIP networks, Mol. Cell 23 (2006) 219–247. 663–671. [101] K. Philippar, I. Fuchs, H. Luthen, S. Hoth, C.S. Bauer, K. Haga, G. Thiel, K. Ljung, G. [126] S.L. Rogers, U. Wiedemann, U. Hacker, C. Turck, R.D. Vale, Drosophila RhoGEF2 Sandberg, M. Bottger, D. Becker, R. Hedrich, Auxin-induced K+ channel expres- associates with microtubule plus ends in an EB1-dependent manner, Curr. Biol sion represents an essential step in coleoptile growth and gravitropism, Proc. 14 (2004) 1827–1833. Natl. Acad. Sci. U.S.A. 96 (1999) 12186–12191. [127] R.M. Shaw, A.J. Fay, M.A. Puthenveedu, M. von Zastrow, Y.-N. Jan, L.Y. Jan, [102] W.D. Teale, I.A. Paponov, K. Palme, Auxin in action: signalling, transport and Microtubule plus-end-tracking proteins target gap junctions directly from the the control of plant growth and development, Nat. Rev. Mol. Cell Biol. 7 (2006) cell interior to adherens junctions, Cell 128 (2007) 547–560. 847–859. [128] L. Sun, J. Gao, X. Dong, M. Liu, D. Li, X. Shi, J.-T. Dong, X. Lu, C. Liu, J. Zhou, EB1 [103] N. Zhang, K.H. Hasenstein, Distribution of expansins in graviresponding maize promotes Aurora-B kinase activity through blocking its inactivation by protein roots, Plant Cell Physiol. 41 (2000) 1305–1312. phosphatase 2A, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 7153–7158. [104] Y. Boutte, S. Vernhettes, B. Satiat-Jeunemaitre, Involvement of the cytoskeleton [129] K.T. Vaughan, TIP maker and TIP marker; EB1 as a master controller of micro- in the secretory pathway and plasma membrane organisation of higher plant tubule plus ends, J. Cell Biol. 171 (2005) 197–200. cells, Cell Biol. Int. 31 (2007) 649–654. [130] Y. Wen, C.H. Eng, J. Schmoranzer, N. Cabrera-Poch, E.J.S. Morris, M. Chen, B.J. [105] H.S. Sardar, J. Yang, A.M. Showalter, Molecular interactions of arabinogalactan Wallar, A.S. Alberts, G.G. Gundersen, EB1 and APC bind to mDia to stabilize proteins with cortical microtubules and F-actin in bright yellow-2 tobacco microtubules downstream of Rho and promote cell migration, Nat. Cell Biol. 6 cultured cells, Plant Physiol. 142 (2006) 1469–1479. (2004) 820–830. [106] G.H.H. Borner, D.J. Sherrier, T. Weimar, L.V. Michaelson, N.D. Hawkins, A. [131] X.S. Wu, G.L. Tsan, J.A. Hammer III, Melanophilin and myosin Va track the MacAskill, J.A. Napier, M.H. Beale, K.S. Lilley, P. Dupree, Analysis of detergent- microtubule plus end on EB1, J. Cell Biol. 171 (2005) 201–207. resistant membranes in Arabidopsis. Evidence for plasma membrane lipid rafts, [132] P. Dhonukshe, J. Mathur, M. Hulskamp, T.J. Gadella, Microtubule plus-ends Plant Physiol. 137 (2005) 104–116. reveal essential links between intracellular polarization and localized modula- [107] A.K. Grennan, Lipid rafts in plants, Plant Physiol. 143 (2007) 1083–1085. tion of endocytosis during division-plane establishment in plant cells, BMC Biol. [108] G.J. Seifert, K. Roberts, The biology of arabinogalactan proteins, Annu. Rev. Plant 3 (2005) 11–25. Biol. 58 (2007) 137–161. [133] S. Robatzek, D. Chinchilla, T. Boller, Ligand-induced endocytosis of the pattern [109] F. Hullin-Matsuda, T. Kobayashi, Monitoring the distribution and dynamics of recognition receptor FLS2 in Arabidopsis, Genes Dev. 20 (2006) 537–542. signaling microdomains in living cells with lipid-specific probes, Cell. Mol. Life [134] M.L. Gifford, F.C. Robertson, D.C. Soares, G.C. Ingram, Arabidopsis CRINKLY4 Sci. 64 (2007) 2492–2504. function, internalization, and turnover are dependent on the extracellular [110] P. Sengupta, B. Baird, D. Holowka, Lipid rafts, fluid/fluid phase separation, and crinkly repeat domain, Plant Cell 17 (2005) 1154–1166. their relevance to plasma membrane structure and function, Sem. Cell Dev. Biol. [135] E. Russinova, J.-W. Borst, M. Kwaaitaal, A. Cano-Delgado, Y. Yin, J. Chory, S.C. de 18 (2007) 583–590. Vries, Heterodimerization and endocytosis of Arabidopsis brassinosteroid recep- [111] B. Lefebvre, F. Furt, M.-A. Hartmann, L.V. Michaelson, J.-P. Carde, F. Sargueil- tors BRI1 and AtSERK3 (BAK1), Plant Cell 16 (2004) 3216–3229. Boiron, M. Rossignol, J.A. Napier, J. Cullimore, J.-J. Bessoule, S. Mongrand, [136] J.-U. Sutter, C. Sieben, A. Hartel, C. Eisenach, G. Thiel, M.R. Blatt, Abscisic acid Characterization of lipid rafts from Medicago truncatula root plasma mem- triggers the endocytosis of the Arabidopsis KAT1 K+ channel and its recycling to branes: a proteomic study reveals the presence of a raft-associated redox the plasma membrane, Curr. Biol. 17 (2007) 1396–1402. system, Plant Physiol. 144 (2007) 402–418. [137] F. Baluska, K.H. Hasenstein, Root cytoskeleton: its role in perception of and [112] H.S. Sardar, A.M. Showalter, A cellular networking model involving interactions response to gravity, Planta 203 (1997) S69–S78. among glycosyl-phosphatidylinositol (GPI)-anchored plasma membrane arabi- [138] P. Nick, R. Godbole, Q.Y. Wang, Probing rice gravitropism with cytoskeletal drugs doglactan proteins (AGPs), microtubules and F-actin in tobacco BY-2 Cells, Plant and cytoskeletal mutants, Biol. Bull. 192 (1997) 141–143. Signal. Behav. 2 (2007) 8–9. [139] C. Gutjahr, P. Nick, Acrylamide inhibits gravitropism and affects microtubules in [113] A. Akhmanova, M.O. Steinmetz, Tracking the ends: a dynamic protein network rice coleoptiles, Protoplasma 227 (2006) 211–222. controls the fate of microtubule tips, Nat. Rev. Mol. Cell Biol. 9 (2008) 309–322. [140] N. Tavernarakis, M. Driscoll, Molecular modeling of mechanotransduction in the [114] G. Lansbergen, A. Akhmanova, Microtubule plus end: a hub of cellular activities, nematode Caenorhabditis elegans, Annu. Rev. Physiol. 59 (1997) 659–689. Traffic 7 (2006) 499–507. [141] J.G. Cueva, A. Mulholland, M.B. Goodman, Nanoscale organization of the MEC-4 [115] S.R. Bisgrove, W.E. Hable, D.L. Kropf, +TIPs and microtubule regulation. The DEG/ENaC sensory mechanotransduction channel in Caenorhabditis elegans beginning of the plus end in plants, Plant Physiol. 136 (2004) 3855–3863. touch receptor neurons, J. Neurosci. 27 (2007) 14089–14098. [116] S.R. Bisgrove, Y.-R.J. Lee, B. Liu, N.T. Peters, D.L. Kropf, The microtubule plus-end [142] G.G. Ernstrom, M. Chalfie, Genetics of sensory mechanotransduction, Annu. Rev. binding protein EB1 functions in root responses to touch and gravity signals in Genet. 36 (2002) 411–453. Arabidopsis, Plant Cell 20 (2008) 396–410. [143] J.P. Ding, B.G. Pickard, Mechanosensory calcium-selective cation channels in [117] J. Chan, G.M. Calder, J.H. Doonan, C.W. Lloyd, EB1 reveals mobile microtubule epidermal cells 3 (1993) 83–110. nucleation sites in Arabidopsis, Nat. Cell Biol. 5 (2003) 967–971. [144] C.L. Granger, R.J. Cyr, Spatiotemporal relationships between growth and micro- [118] R. Dixit, E. Chang, R. Cyr, Establishment of polarity during organization of tubule orientation as revealed in living root cells of Arabidopsis thaliana trans- the acentrosomal plant cortical microtubule array, Mol. Biol. Cell 17 (2006) formed with green-fluorescent-protein gene construct GFP-MBD, Protoplasma 1298–1305. 216 (2001) 201–214.