© 2018 The Authors. Actin is one of the three major cytoskeletal components in eukaryotic cells. Myosin XI is an actin-based motor protein in plant cells. Organelles are attached to myosin XI and translocated along the actin filaments. This dynamic actin-myosin XI system plays a major role in subcellular organelle transport and cytoplasmic streaming. Previous studies have revealed that myosin-driven transport and the actin cytoskeleton play essential roles in plant cell growth. Recent data have indicated that the actin-myosin XI cytoskeleton is essential for not only cell growth but also reproductive processes and responses to the environment. In this review, we have summarized previous reports regarding the role of the actin-myosin XI cytoskeleton in cytoplasmic streaming and plant development and recent advances in the understanding of the functions of actin-myosin XI cytoskeleton in Arabidopsis thaliana.

Arabidopsis possesses 13 genes encoding class-XI myosins. Among these, myosin XI-I is phylogenetically distant. To examine the molecular properties of Arabidopsis thaliana myosin XI-I (At myosin XI-I), we performed in vitro mechanical and enzymatic analyses using recombinant constructs of At myosin XI-I. Unlike other biochemically studied class-XI myosins, At myosin XI-I showed extremely low actinactivated ATPase activity (V-max = 3.7 Pi s(-1) head(-1)). The actin-sliding velocity of At myosin XI-I was 0.25 mu m s(-1), >10 times lower than those of other class-XI myosins. The ADP dissociation rate from acto-At myosin XI-I was 17 s(-1), accounting for the low actin-sliding velocity. In contrast, the apparent affinity for actin in the presence of ATP, estimated from Kapp (0.61 mu M) of actin-activated ATPase, was extremely high. The equilibrium dissociation constant for actin was very low in both the presence and absence of ATP, indicating a high affinity for actin. To examine At myosin XI-I motility in vivo, green fluorescent protein-fused full-length At myosin XI-I was expressed in cultured Arabidopsis cells. At myosin XI-I localized not only on the nuclear envelope but also on small dots moving slowly (0.23 mu m s(-1)) along actin filaments. Our results show that the properties of At myosin XI-I differ from those of other Arabidopsis class-XI myosins. The data suggest that At myosin XI-I does not function as a driving force for cytoplasmic streaming but regulates the organelle velocity, supports processive organelle movement or acts as a tension generator.

Cytoplasmic streaming occurs widely in plants ranging from algae to angiosperms. However, the molecular mechanism and physiological role of cytoplasmic streaming have long remained unelucidated. Recent molecular genetic approaches have identified specific myosin members (XI-2 and XI-K as major and XI-1, XI-B, and XI-I as minor motive forces) for the generation of cytoplasmic streaming among 13 myosin XIs in Arabidopsis thaliana. Simultaneous knockout of these myosin XI members led to a reduced velocity of cytoplasmic streaming and marked defects of plant development. Furthermore, the artificial modifications of myosin XI-2 velocity changed plant and cell sizes along with the velocity of cytoplasmic streaming. Therefore, we assume that cytoplasmic streaming is one of the key regulators in determining plant size.

The 175-kDa myosin-11 from Nicotiana tabacum (Nt(175kDa)myosin-11) is exceptional in its mechanical activity as it is the fastest known processive actin-based motor, moving 10 times faster than the structurally related class 5 myosins. Although this ability might be essential for long-range organelle transport within larger plant cells, the kinetic features underlying the fast processive movement of Nt(175kDa)myosin-11 still remain unexplored. To address this, we generated a single-headed motor domain construct and carried out a detailed kinetic analysis. The data demonstrate that Nt(175kDa)myosin-11 is a highduty ratio motor, which remains associated with actin most of its enzymatic cycle. However, different from other processive myosins that establish a high duty ratio on the basis of a rate-limiting ADP-release step, Nt(175kDa)myosin-11 achieves a high duty ratio by a prolonged duration of the ATP-induced isomerization of the actin-bound states and ADP release kinetics, both of which in terms of the corresponding time constants approach the total ATPase cycle time. Molecular modeling predicts that variations in the charge distribution of the actin binding interface might contribute to the thermodynamic fine-tuning of the kinetics of this myosin. Our study unravels a new type of a high duty ratio motor and provides important insights into the molecular mechanism of processive movement of higher plant myosins.

Background: Molecular properties of class VIII myosin are not characterized. Results:Arabidopsis class VIII myosin, ATM1, has low enzymatic activity and high affinity for actin and is primarily localized at the cell cortex. Conclusion: Our data suggest that ATM1 functions as a tension sensor/generator. Significance: This is the first report of enzymatic and motile properties of class VIII myosin.
Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg2+-ATPase activity (V-max = 4 s(-1)), although their affinities for actin were high (K-actin = 4 m). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 m/s, respectively, from which the value for full-length ATM1 is calculated to be approximate to 0.2 m/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was approximate to 90%. ADP dissociation from the actinATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg2+ modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

Cytoplasmic streaming is active transport widely occurring in plant cells ranging from algae to angiosperms. Although it has been revealed that cytoplasmic streaming is generated by organelle-associated myosin XI moving along actin bundles, the fundamental function in plants remains unclear. We generated high- and low-speed chimeric myosin XI by replacing the motor domains of Arabidopsis thaliana myosin XI-2 with those of Chara corallina myosin XI and Homo sapiens myosin Vb, respectively. Surprisingly, the plant sizes of the transgenic Arabidopsis expressing high- and low-speed chimeric myosin XI-2 were larger and smaller, respectively, than that of the wild-type plant. This size change correlated with acceleration and deceleration, respectively, of cytoplasmic streaming. Our results strongly suggest that cytoplasmic streaming is a key determinant of plant size. Furthermore, because cytoplasmic streaming is a common system for intracellular transport in plants, our system could have applications in artificial size control in plants.

Plant myosin XI functions as a motor that generates cytoplasmic streaming in plant cells. Although cytoplasmic streaming is known to be regulated by intracellular Ca2+ concentration, the molecular mechanism underlying this control is not fully understood. Here, we investigated the mechanism of regulation of myosin XI by Ca2+ at the molecular level. Actin filaments were easily detached from myosin XI in an in vitro motility assay at high Ca2+ concentration (pCa 4) concomitant with the detachment of calmodulin light chains from the neck domains. Electron microscopic observations showed that myosin XI at pCa 4 shortened the neck domain by 30%. Single-molecule analysis revealed that the step size of myosin XI at pCa 4 was shortened to 27 nm under low load and to 22 nm under high load compared with 35 nm independent of the load for intact myosin XI. These results indicate that modulation of the mechanical properties of the neck domain is a key factor for achieving the Ca2+-induced regulation of cytoplasmic streaming.

Organelle motility, essential for cellular function, is driven by the cytoskeleton. In plants, actin filaments sustain the long-distance transport of many types of organelles, and microtubules typically fine-tune the motile behavior. In shoot epidermal cells of Arabidopsis thaliana seedlings, we show here that a type of RNA granule, the RNA processing body (P-body), is transported by actin filaments and pauses at cortical microtubules. Interestingly, removal of microtubules does not change the frequency of P-body pausing. Similarly, we show that Golgi bodies, peroxisomes, and mitochondria all pause at microtubules, and again the frequency of pauses is not appreciably changed after microtubules are depolymerized. To understand the basis for pausing, we examined the endoplasmic reticulum (ER), whose overall architecture depends on actin filaments. By the dual observation of ER and microtubules, we find that stable junctions of tubular ER occur mainly at microtubules. Removal of microtubules reduces the number of stable ER tubule junctions, but those remaining are maintained without microtubules. The results indicate that pausing on microtubules is a common attribute of motile organelles but that microtubules are not required for pausing. We suggest that pausing on microtubules facilitates interactions between the ER and otherwise translocating organelles in the cell cortex.

Actin plays fundamental roles in a wide array of plant functions, including cell division, cytoplasmic streaming, cell morphogenesis and organelle motility. Imaging the actin cytoskeleton in living cells is a powerful methodology for studying these important phenomena. Several useful probes for live imaging of filamentous actin (F-actin) have been developed, but new versatile probes are still needed. Here, we report the application of a new probe called Lifeact for visualizing F-actin in plant cells. Lifeact is a short peptide comprising 17 amino acids that was derived from yeast Abp140p. We used a LifeactVenus fusion protein for staining F-actin in Arabidopsis thaliana and were able to observe dynamic rearrangements of the actin meshwork in root hair cells. We also used LifeactVenus to visualize the actin cytoskeleton in the liverwort Marchantia polymorpha; this revealed unique and dynamic F-actin motility in liverwort cells. Our results suggest that Lifeact could be a useful tool for studying the actin cytoskeleton in a wide range of plant lineages.

Many aspects of plant development, including patterning and tropisms, are largely dependent on the asymmetric distribution of the plant signaling molecule auxin. Auxin transport inhibitors (ATIs), which interfere with directional auxin transport, have been essential tools in formulating this concept. However, despite the use of ATIs in plant research for many decades, the mechanism of ATI action has remained largely elusive. Using real-time live-cell microscopy, we show here that prominent ATIs such as 2,3,5-triiodobenzoic acid (TIBA) and 2-(1-pyrenoyl) benzoic acid (PBA) inhibit vesicle trafficking in plant, yeast, and mammalian cells. Effects on micropinocytosis, rab5-labeled endosomal motility at the periphery of HeLa cells and on fibroblast mobility indicate that ATIs influence actin cytoskeleton. Visualization of actin cytoskeleton dynamics in plants, yeast, and mammalian cells show that ATIs stabilize actin. Conversely, stabilizing actin by chemical or genetic means interferes with endocytosis, vesicle motility, auxin transport, and plant development, including auxin transport-dependent processes. Our results show that a class of ATIs act as actin stabilizers and advocate that actin-dependent trafficking of auxin transport components participates in the mechanism of auxin transport. These studies also provide an example of how the common eukaryotic process of actin-based vesicle motility can fulfill a plant-specific physiological role.

In many cases, actin filaments are arranged into bundles and serve as tracks for cytoplasmic streaming in plant cells. We have isolated an actin-filament bundling protein, which is composed of 115-kDa polypeptide (P-115-ABP), from the germinating pollen of lily, Lilium longiflorum [Nakayasu et al. (1998) Biochem. Biophys. Res. Commun. 249: 61]. P-115-ABP shared similar antigenicity with a plant 135-kDa actin-filament bundling protein (P-135-ABP), a plant homologue of villin. A full-length cDNA clone (ABP115; accession no. AB097407) was isolated from an expression cDNA library of lily pollen by immuno-screening using antisera against P-115-ABP and P-135-ABP. The amino acid sequence of P-115-ABP deduced from this clone showed high homology with those of P-135-ABP and four villin isoforms of Arabidopsis thaliana (AtVLN1, AtVLN2, AtVLN3 and AtVLN4), especially AtVLN4, indicating that P-115-ABP can also be classified as a plant villin. The P-115-ABP isolated biochemically from the germinating lily pollen was able to arrange F-actin filaments with uniform polarity into bundles and this bundling activity was suppressed by Ca2+-calmodulin (CaM), similar to the actin-filament bundling properties of P-135-ABP. The P-115-ABP type of plant villin was widely distributed in plant cells, from algae to land plants. In root hair cells of Hydrocharis dubia, this type of plant villin was co-localized with actin-filament bundles in the transvacuolar strands and the subcortical regions. Microinjection of the antiserum against P-115-ABP into living root hair cells caused the disappearance of transvaculor strands and alteration of the route of cytoplasmic streaming. In internodal cells of Chara corallina in which the P-135-ABP type of plant villin is lacking, the P-115-ABP type showed co-localization with actin-filament cables anchored on the intracellular surface of chloroplasts. These results indicated that plant villins are widely distributed and involved in the organization of actin filaments into bundles throughout the plant kingdom.

High velocity cytoplasmic streaming is found in various plant cells from algae to angiosperms. We characterized mechanical and enzymatic properties of a higher plant myosin purified from tobacco bright yellow-2 cells, responsible for cytoplasmic streaming, having a 175 kDa heavy chain and calmodulin light chains. Sequence analysis shows it to be a class XI myosin and a dimer with six IQ motifs in the light chain-binding domains of each heavy chain. Electron microscopy confirmed these predictions. We measured its ATPase characteristics, in vitro motility and, using optical trap nanometry, forces and movement developed by individual myosin XI molecules. Single myosin XI molecules move processively along actin with 35 nm steps at 7 mum/s, the fastest known processive motion. Processivity was confirmed by actin landing rate assays. Mean maximal force was similar to0.5 pN, smaller than for myosin IIs. Dwell time analysis of beads carrying single myosin XI molecules fitted the ATPase kinetics, with ADP release being rate limiting. These results indicate that myosin XI is highly specialized for generation of fast processive movement with concomitantly low forces.

Hormonal control of root growth was studied in Lemna minor. Although addition of gibberellic acid (GA(3)) to the culture medium did not promote the root growth, a gibberellin biosynthesis inhibitor, uniconazole P (Un-P), significantly inhibited root growth. Both length and diameter of roots in Un-P-treated plants were significantly smaller than those in control plants, mainly caused by inhibition of cell division. In epidermal cells, the length was slightly decreased and the width increased by Un-P treatment, indicating inhibition of elongation growth. GA(3) completely nullified the inhibition caused by Un-P. Transverse cortical microtubules (CMTs) of epidermal cells in the elongation zone were significantly fragmented by treatment with Un-P, but not by that in the presence of GA(3). The cellulose microfibril array in the Un-P-treated cells was more random and more oblique than that in the control cells. These results suggested that root growth in L. minor is regulated by endogenous gibberellin.

In many types of plant cell, bundles of actin filaments (AFs) are generally involved in cytoplasmic streaming and the organization of transvacuolar strands. Actin cross-linking proteins are believed to arrange AFs into the bundles. In root hair cells of Hydrocharis dubia (Blume) Baker, a 135-kDa polypeptide cross-reacted with an antiserum against a 135-kDa actin-bundling protein (135-ABP), a villin homologue, isolated from lily pollen tubes. Immunofluorescence microscopy revealed that the 135-kDa polypeptide co-localized with AF bundles in the transvacuolar strand and in the subcortical region of the cells. Microinjection of antiserum against 135-ABP into living root hair cells induced the disappearance of the transvacuolar strand. Concomitantly, thick AF bundles in the transvacuolar strand dispersed into thin bundles. In the root hair cells, AFs showed uniform polarity in the bundles, which is consistent with the in-vitro activity of 135-ABP. These results suggest that villin is a factor responsible for bundling AFs in root hair cells as well as in pollen tubes, and that it plays a key role in determining the direction of cytoplasmic streaming in these cells.

On the basis of the inhibition of myosin by 2,3-butanedione monoxime (BDM), the protein's involvement in various cell activities is discussed. However, it has not been established whether BDM inhibits plant myosin. In the present study, the effect of BDM on isolated plant myosin was analyzed in vitro. The sliding between myosin from lily (Lilium longiflorum) pollen tubes and actin filaments from skeletal muscle was inhibited to 25% at a concentration of 60 mM, indicating that BDM can be used as a myosin inhibitor for plant materials. Cytoplasmic streaming was completely inhibited by BDM at 30 mM in lily pollen tubes and at 70 mM in short root hair cells, and at 100 mM in long root hair cells of Hydrocharis dubia. However. BDM at high concentrations induced the disorganization of actin filament bundles in lily pollen tubes and short root hair cells. In addition, cortical microtubules were also fragmented in short root hair cells treated with BDM, suggesting a possible side effect of BDM.

In root hair cells of Limnobium stoloniferum, a protein phospha;ase inhibitor, calyculin A (CA), at concentrations higher than 50 nM inhibits cytoplasmic streaming and induces remarkable morphological changes in the cytoplasm: the transvacuolar strands disperse and spherical cytoplasmic bodies emerge. The mechanism of the morphological changes of the cytoplasm induced by CA was studied by pharmacological analyses. The formation of spherical bodies in cells treated with CA was suppressed by the actin-depolymerizing and -fragmenting drugs latrunculin B and cytochalasin D at concentrations higher than 100 nM and 5 mu M, respectively. In contrast, 100 mu M propyzamide, a microtubule-depolymerizing drug, did not affect the formation of spherical bodies by CA. Interestingly, 60 mM 2,3-butanedione monoxime, an inhibitor of myosin, also suppressed the CA-induced formation of cytoplasmic spherical bodies. These results indicate that the actin cytoskeleton is intimately involved in the morphological changes of the cytoplasm induced by CA.

It has been reported that auxin accelerates cytoplasmic streaming at low concentrations and inhibits it at high concentrations in several plant cells. In the present study, the mechanism of inhibition of cytoplasmic streaming by naphthalene acetic acid (NAA) at high concentrations was analyzed in root hair cells of Hydrocharis. Because the effective concentration of NAA inhibiting cytoplasmic streaming decreased when the extracellular pH (pHo) was lowered, it was hypothesized that cytoplasmic streaming is inhibited by NAA via acidification of the cytoplasm, This possibility was supported by the fact that acetic acid, propionic acid and decanoic acid also inhibited cytoplasmic streaming at low pHo. Acidification of the cytoplasm disturbed the orientation of actin filaments (AFs) and disrupted cortical microtubules (MTs). The effects of NAA were reversible; both cytoplasmic streaming and organization of the cytoskeleton were recovered upon removal of NAA. During the recovery, tracks of cytoplasmic streaming in the subcortical region temporarily showed a helical pattern along the longitudinal direction of the cell. Fluorescence staining of cytoskeletons revealed that both AFs and MTs aligned obliquely to the longitudinal axis of the cell. The helical streaming returned to the original reverse fountain streaming after several hours. The simultaneous changes in the organization of both cytoskeletons supported our previous report that the organization of AFs is regulated by MTs.

We studied the mechanism controlling the organization of actin filaments (AFs) in Hydrocharis root hair cells, in which reverse fountain streaming occurs. The distribution of AFs and microtubules (MTs) in root hair cells were analyzed by fluorescence microscopy and electron microscopy. AFs and MTs were found running in the longitudinal direction of the cell at the cortical region. AFs were observed in the transvacuolar strand, but not MTs. Ultrastructural studies revealed that AFs and MTs were colocalized and that MTs were closer to the plasma membrane than AFs. To examine if MTs regulate the organization of AFs, we carried out a double inhibitor experiment using cytochalasin B (CB) and propyzamide, which are inhibitors of AFs and MTs, respectively. CB reversibly inhibited cytoplasmic streaming while propyzamide alone had no effect on it. However, after treatment with both CB and propyzamide, removal of CB alone did not lead to recovery of cytoplasmic streaming. In these cells, AFs showed a meshwork structure. When propyzamide was also removed, cytoplasmic streaming and the original organization of AFs were recovered. These results strongly suggest that MTs are responsible for the organization of AFs in Hydrocharis root hair cells.