Actin filament organization and polarity in pollen tubes revealed by myosin II subfragment 1 decoration

Actin cytoskeleton in the control of vesicle transport, cytoplasmic organization, and pollen tube tip growth

Pollen tubes extend rapidly via tip growth. This process depends on a dynamic actin cytoskeleton, which has been implicated in controlling organelle movements, cytoplasmic streaming, vesicle trafficking, and cytoplasm organization in pollen tubes. In this update review, we describe the progress in understanding the organization and regulation of the actin cytoskeleton and the function of the actin cytoskeleton in controlling vesicle traffic and cytoplasmic organization in pollen tubes. We also discuss the interplay between ion gradients and the actin cytoskeleton that regulates the spatial arrangement and dynamics of actin filaments and the organization of the cytoplasm in pollen tubes. Finally, we describe several signaling components that regulate actin dynamics in pollen tubes.

RNAi-Mediated Knockdown of Calreticulin3a Impairs Pollen Tube Growth in Petunia

Pollen tube growth depends on several complex processes, including exo/endocytosis, cell wall biogenesis, intracellular transport, and cell signaling. Our previous results provided evidence that calreticulin (CRT)—a prominent calcium (Ca²⁺)-buffering molecular chaperone in the endoplasmic reticulum (ER) lumen—is involved in pollen tube formation and function. We previously cloned and characterized the CRT gene belonging to the CRT1/2 subgroup from Petunia hybrida (PhCRT1/2), and found that post-transcriptional silencing of PhCRT1/2 expression strongly impaired pollen tube growth in vitro. Here, we report cloning of a new PhCRT3a homolog; we identified the full-length cDNA sequence and described its molecular characteristics and phylogenetic relationships to other plant CRT3 genes. Using an RNA interference (RNAi) strategy, we found that knockdown of PhCRT3a gene expression caused numerous defects in the morphology and ultrastructure of cultivated pollen tubes, including disorganization of the actin cytoskeleton and loss of cytoplasmic zonation. Elongation of siPhCRT3a pollen tubes was disrupted, and some of them ruptured. Our present data provide the first evidence that PhCRT3a expression is required for normal pollen tube growth. Thus, we discuss relationships between diverse CRT isoforms in several interdependent processes driving the apical growth of the pollen tube, including actomyosin-dependent cytoplasmic streaming, organelle positioning, vesicle trafficking, and cell wall biogenesis.

A Tug-of-War Model Explains the Saltatory Sperm Cell Movement in Arabidopsis thaliana Pollen Tubes by Kinesins With Calponin Homology Domain

Upon pollination, two sperm cells are transported inside the growing pollen tube toward the apex. One sperm cell fertilizes the egg cell to form the zygote, while the other fuses with the two polar nuclei to form the triploid endosperm. In Arabidopsis thaliana, the transport of the two sperm cells is characterized by sequential forward and backward movements with intermediate pauses. Until now, it is under debate which components of the plant cytoskeleton govern this mechanism. The sperm cells are interconnected and linked to the vegetative nucleus via a cytoplasmic projection, thus forming the male germ unit. This led to the common hypothesis that the vegetative nucleus is actively transported via myosin motors along actin cables while pulling along the sperm cells as passive cargo. In this study, however, we show that upon occasional germ unit disassembly, the sperm cells are transported independently and still follow the same bidirectional movement pattern. Moreover, we found that the net movement of sperm cells results from a combination of both longer and faster runs toward the pollen tube apex. We propose that the observed saltatory movement can be explained by the function of kinesins with calponin homology domain (KCH). This subgroup of the kinesin-14 family actively links actin filaments and microtubules. Based on KCH's specific properties derived from in vitro experiments, we built a tug-of-war model that could reproduce the characteristic sperm cell movement in pollen tubes.

Control of the Actin Cytoskeleton Within Apical and Subapical Regions of Pollen Tubes

In flowering plants, sexual reproduction involves a double fertilization event, which is facilitated by the delivery of two non-motile sperm cells to the ovule by the pollen tube. Pollen tube growth occurs exclusively at the tip and is extremely rapid. It strictly depends on an intact actin cytoskeleton, and is therefore an excellent model for uncovering the molecular mechanisms underlying dynamic actin cytoskeleton remodeling. There has been a long-term debate about the organization and dynamics of actin filaments within the apical and subapical regions of pollen tube tips. By combining state-of-the-art live-cell imaging with the usage of mutants which lack different actin-binding proteins, our understanding of the origin, spatial organization, dynamics and regulation of actin filaments within the pollen tube tip has greatly improved. In this review article, we will summarize the progress made in this area.

Mechanics of Pollen Tube Elongation: A Perspective

Pollen tube (PT) serves as a vehicle that delivers male gametes (sperm cells) to a female gametophyte during double fertilization, which eventually leads to the seed formation. It is one of the fastest elongating structures in plants. Normally, PTs traverse through the extracellular matrix at the transmitting tract after penetrating the stigma. While the endeavor may appear simple, the molecular processes and mechanics of the PT elongation is yet to be fully resolved. Although it is the most studied "tip-growing" structure in plants, several features of the structure (e.g., Membrane dynamics, growth behavior, mechanosensing etc.) are only partially understood. In many aspects, PTs are still considered as a tissue rather than a "unique cell." In this review, we have attempted to discuss mainly on the mechanics behind PT-elongation and briefly on the molecular players involved in the process. Four aspects of PTs are particularly discussed: the PT as a cell, its membrane dynamics, mechanics of its elongation, and the potential mechanosensors involved in its elongation based on relevant findings in both plant and non-plant models.

Remarkable structural transformations of actin bundles are driven by their initial polarity, motor activity, crosslinking, and filament treadmilling

Bundled actin structures play a key role in maintaining cellular shape, in aiding force transmission to and from extracellular substrates, and in affecting cellular motility. Recent studies have also brought to light new details on stress generation, force transmission and contractility of actin bundles. In this work, we are primarily interested in the question of what determines the stability of actin bundles and what network geometries do unstable bundles eventually transition to. To address this problem, we used the MEDYAN mechano-chemical force field, modeling several micron-long actin bundles in 3D, while accounting for a comprehensive set of chemical, mechanical and transport processes. We developed a hierarchical clustering algorithm for classification of the different long time scale morphologies in our study. Our main finding is that initially unipolar bundles are significantly more stable compared with an apolar initial configuration. Filaments within the latter bundles slide easily with respect to each other due to myosin activity, producing a loose network that can be subsequently severely distorted. At high myosin concentrations, a morphological transition to aster-like geometries was observed. We also investigated how actin treadmilling rates influence bundle dynamics, and found that enhanced treadmilling leads to network fragmentation and disintegration, while this process is opposed by myosin and crosslinking activities. Interestingly, treadmilling bundles with an initial apolar geometry eventually evolve to a whole gamut of network morphologies based on relative positions of filament ends, such as sarcomere-like organization. We found that apolar bundles show a remarkable sensitivity to environmental conditions, which may be important in enabling rapid cytoskeletal structural reorganization and adaptation in response to intracellular and extracellular cues.

The Balance between Actin-Bundling Factors Controls Actin Architecture in Pollen Tubes

How actin-bundling factors cooperatively regulate shank-localized actin bundles remains largely unexplored. Here we demonstrate that FIM5 and PLIM2a/PLIM2b decorate shank-localized actin bundles and that loss of function of PLIM2a and/or PLIM2b suppresses phenotypes associated with fim5 mutants. Specifically, knockout of PLIM2a and/or PLIM2b partially suppresses the disorganized actin bundle and intracellular trafficking phenotype in fim5 pollen tubes. PLIM2a/PLIM2b generates thick but loosely packed actin bundles, whereas FIM5 generates thin but tight actin bundles that tend to be cross-linked into networks in vitro. Furthermore, PLIM2a/PLIM2b and FIM5 compete for binding to actin filaments in vitro, and PLIM2a/PLIM2b decorate disorganized actin bundles in fim5 pollen tubes. These data together suggest that the disorganized actin bundles in fim5 mutants are at least partially due to gain of function of PLIM2a/PLIM2b. Our data suggest that the balance between FIM5 and PLIM2a/PLIM2b is crucial for the normal bundling and organization of shank-localized actin bundles in pollen tubes.

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The transcription of PLIM2a and PLIM2b is upregulated in fim5 pollen tubes

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Downregulation of PLIM2a and/or PLIM2b suppresses the defects in fim5 pollen tubes

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Both FIM5 and PLIM2a/PLIM2b decorate shank-localized actin filaments

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FIM5 can inhibit the binding of PLIM2a and PLIM2b to actin filaments

Actin Bundles in The Pollen Tube

The angiosperm pollen tube delivers two sperm cells into the embryo sac through a unique growth strategy, named tip growth, to accomplish fertilization. A great deal of experiments have demonstrated that actin bundles play a pivotal role in pollen tube tip growth. There are two distinct actin bundle populations in pollen tubes: the long, rather thick actin bundles in the shank and the short, highly dynamic bundles near the apex. With the development of imaging techniques over the last decade, great breakthroughs have been made in understanding the function of actin bundles in pollen tubes, especially short subapical actin bundles. Here, we tried to draw an overall picture of the architecture, functions and underlying regulation mechanism of actin bundles in plant pollen tubes.

Electromechanics of polarized cell growth

One of the most challenging questions in cell and developmental biology is how molecular signals are translated into mechanical forces that ultimately drive cell growth and motility. Despite an impressive body of literature demonstrating the importance of cytoskeletal and motor proteins as well as osmotic stresses for cell developmental mechanics, a host of dissenting evidence strongly suggests that these factors per se cannot explain growth mechanics even at the level of a single tip-growing cell. The present study addresses this issue by exploring fundamental interrelations between electrical and mechanical fields operating in cells. In the first instance, we employ a simplified but instructive model of a quiescent cell to demonstrate that even in a quasi-equilibrium state, ion transport processes are conditioned principally by mechanical tenets. Then we inquire into the electromechanical conjugacy in growing pollen tubes as biologically relevant and physically tractable developmental systems owing to their extensively characterized growth-associated ionic fluxes and strikingly polarized growth and morphology. A comprehensive analysis of the multifold stress pattern in the growing apices of pollen tubes suggests that tip-focused ionic fluxes passing through the polyelectrolyte-rich apical cytoplasm give rise to electrokinetic flows that actualize otherwise isotropic intracellular turgor into anisotropic stress field. The stress anisotropy can be then imparted from the apical cytoplasm to the abutting frontal cell wall to induce its local extension and directional cell growth. Converging lines of evidence explored in the concluding sections attest that tip-focused ionic fluxes and associated interfacial transport phenomena are not specific for pollen tubes but are also employed by a vast variety of algal, plant, fungal and animal cells, rendering their cytoplasmic stress fields essentially anisotropic and ultimately instrumental in cell shaping, growth and motility.

Fimbrins 4 and 5 Act Synergistically During Polarized Pollen Tube Growth to Ensure Fertility in Arabidopsis

The germination and polar growth of pollen are prerequisite for double fertilization in plants. The actin cytoskeleton and its binding proteins play pivotal roles in pollen germination and pollen tube growth. Two homologs of the actin-bundling protein fimbrin, AtFIM4 and AtFIM5, are highly expressed in pollen in Arabidopsis and can form distinct actin architectures in vitro, but how they co-operatively regulate pollen germination and pollen tube growth in vivo is largely unknown. In this study, we explored their functions during pollen germination and polar growth. Histochemical analysis demonstrated that AtFIM4 was expressed only after pollen grain hydration and, in the early stage of pollen tube growth, the expression level of AtFIM4 was low, indicating that it functions mainly during polarized tube growth, whereas AtFIM5 had high expression levels in both pollen grains and pollen tubes. Atfim4/atfim5 double mutant plants had fertility defects including reduced silique length and seed number, which were caused by severe defects in pollen germination and pollen tube growth. When the atfim4/atfim5 double mutant was complemented with the AtFIM5 protein, the polar growth of pollen tubes was fully rescued; however, AtFIM4 could only partially restore these defects. Fluorescence labeling showed that loss of function of both AtFIM4 and AtFIM5 decreased the extent of actin filament bundling throughout pollen tubes. Collectively, our results revealed that AtFIM4 acts co-ordinately with AtFIM5 to organize and maintain normal actin architecture in pollen grains and pollen tubes to fulfill double fertilization in plants.

Calreticulin is required for calcium homeostasis and proper pollen tube tip growth in Petunia

MAIN CONCLUSION

Calreticulin is involved in stabilization of the tip-focused Ca 2+ gradient and the actin cytoskeleton arrangement and function that is required for several key processes driving Petunia pollen tube tip growth. Although the precise mechanism is unclear, stabilization of a tip-focused calcium (Ca2+) gradient seems to be critical for pollen germination and pollen tube growth. We hypothesize that calreticulin (CRT), a Ca2+-binding/buffering chaperone typically residing in the lumen of the endoplasmic reticulum (ER) of eukaryotic cells, is an excellent candidate to fulfill this role. We previously showed that in Petunia pollen tubes growing in vitro, CRT is translated on ER membrane-bound ribosomes that are abundant in the subapical zone of the tube, where CRT's Ca2+-buffering and chaperone activities might be particularly required. Here, we sought to determine the function of CRT using small interfering RNA (siRNA) to, for the first time in pollen tubes growing in vitro, knockdown expression of a gene. We demonstrate that siRNA-mediated post-transcriptional silencing of Petunia hybrida CRT gene (PhCRT) expression strongly impairs pollen tube growth, cytoplasmic zonation, actin cytoskeleton organization, and the tip-focused Ca2+ gradient. Moreover, reduction of CRT alters the localization and disturbs the structure of the ER in abnormally elongating pollen tubes. Finally, cytoplasmic streaming is inhibited, and most of the pollen tubes rupture. Our data clearly show an interplay between CRT, Ca2+ gradient, actin-dependent cytoplasmic streaming, organelle positioning, and vesicle trafficking during pollen tube elongation. Thus, we suggest that CRT functions in Petunia pollen tube growth by stabilizing Ca2+ homeostasis and acting as a chaperone to assure quality control of glycoproteins passing through the ER.

LlFH1-mediated interaction between actin fringe and exocytic vesicles is involved in pollen tube tip growth

Pollen tube tip growth is an extreme form of polarized cell growth, which requires polarized exocytosis based on a dynamic actin cytoskeleton. However, the molecular basis for the connection between actin filaments and exocytic vesicles is unclear. Here, we identified a Lilium longiflorum pollen-specific formin (LlFH1) and found that it localized at the apical vesicles and plasma membrane (PM). Overexpression of LlFH1 induced excessive actin cables in the tube tip region, and downregulation of LlFH1 eliminated the actin fringe. Fluorescence recovery after photobleaching (FRAP) analysis revealed that LlFH1-labeled exocytic vesicles exhibited an initial accumulation at the shoulder of the apex and coincided with the leading edge of the actin fringe. Time-lapse analysis suggested that nascent actin filaments followed the emergence of the apical vesicles, implying that LlFH1 at apical vesicles could initiate actin polymerization. Biochemical assays showed that LlFH1 FH1FH2 could nucleate actin polymerization, but then capped the actin filament at the barbed end and inhibited its elongation. However, in the presence of lily profilins, LlFH1 FH1FH2 could accelerate barbed-end actin elongation. In addition, LlFH1 FH1FH2 was able to bundle actin filaments. Thus, we propose that LlFH1 and profilin coordinate the interaction between the actin fringe and exocytic vesicle trafficking during pollen tube growth of lily.

High precision, localized proton gradients and fluxes generated by a microelectrode device induce differential growth behaviors of pollen tubes

Pollen tubes are tip-growing plant cells that deliver the sperm cells to the ovules for double fertilization of the egg cell and the endosperm. Various directional cues can trigger the reorientation of pollen tube growth direction on their passage through the female tissues. Among the external stimuli, protons serve an important, regulatory role in the control of pollen tube growth. The generation of local guidance cues has been challenging when investigating the mechanisms of perception and processing of such directional triggers in pollen tubes. Here, we developed and characterized a microelectrode device to generate a local proton gradient and proton flux through water electrolysis. We confirmed that the cytoplasmic pH of pollen tubes varied with environmental pH change. Depending on the position of the pollen tube tip relative to the proton gradient, we observed alterations in the growth behavior, such as bursting at the tip, change in growth direction, or complete growth arrest. Bursting and growth arrest support the hypothesis that changes in the extracellular H⁺ concentration may interfere with cell wall integrity and actin polymerization at the growing tip. A change in growth direction for some pollen tubes implies that they can perceive the local proton gradient and respond to it. We also showed that the growth rate is directly correlated with the extracellular pH in the tip region. Our microelectrode approach provides a simple method to generate protons and investigate their effect on plant cell growth.

Perturbation Analysis of Calcium, Alkalinity and Secretion during Growth of Lily Pollen Tubes

Pollen tubes grow by spatially and temporally regulated expansion of new material secreted into the cell wall at the tip of the tube. A complex web of interactions among cellular components, ions and small molecule provides dynamic control of localized expansion and secretion. Cross-correlation studies on oscillating lily (Lilium formosanum Wallace) pollen tubes showed that an increase in intracellular calcium follows an increase in growth, whereas the increase in the alkaline band and in secretion both anticipate the increase in growth rate. Calcium, as a follower, is unlikely to be a stimulator of growth, whereas the alkaline band, as a leader, may be an activator. To gain further insight herein we reversibly inhibited growth with potassium cyanide (KCN) and followed the re-establishment of calcium, pH and secretion patterns as growth resumed. While KCN markedly slows growth and causes the associated gradients of calcium and pH to sharply decline, its removal allows growth and vital processes to fully recover. The calcium gradient reappears before growth restarts; however, it is preceded by both the alkaline band and secretion, in which the alkaline band is slightly advanced over secretion. Thus the pH gradient, rather than the tip-focused calcium gradient, may regulate pollen tube growth.

Male gametophyte development and function in angiosperms: a general concept

Overview of pollen development. Male gametophyte development of angiosperms is a complex process that requires coordinated activity of different cell types and tissues of both gametophytic and sporophytic origin and the appropriate specific gene expression. Pollen ontogeny is also an excellent model for the dissection of cellular networks that control cell growth, polarity, cellular differentiation and cell signaling. This article describes two sequential phases of angiosperm pollen ontogenesis-developmental phase leading to the formation of mature pollen grains, and a functional or progamic phase, beginning with the impact of the grains on the stigma surface and ending at double fertilization. Here we present an overview of important cellular processes in pollen development and explosive pollen tube growth stressing the importance of reserves accumulation and mobilization and also the mutual activation of pollen tube and pistil tissues, pollen tube guidance and the communication between male and female gametophytes. We further describe the recent advances in regulatory mechanisms involved such as posttranscriptional regulation (including mass transcript storage) and posttranslational modifications to modulate protein function, intracellular metabolic signaling, ionic gradients such as Ca(2+) and H(+) ions, cell wall synthesis, protein secretion and intercellular signaling within the reproductive tissues.

The pollen tube clear zone: clues to the mechanism of polarized growth

Pollen tubes usually exhibit a prominent region at their apex called the "clear zone" because it lacks light refracting amyloplasts. A robust, long clear zone often associates with fast growing pollen tubes, and thus serves as an indicator of pollen tube health. Nevertheless we do not understand how it arises or how it is maintained. Here we review the structure of the clear zone, and attempt to explain the factors that contribute to its formation. While amyloplasts and vacuolar elements are excluded from the clear zone, virtually all other organelles are present including secretory vesicles, mitochondria, Golgi dictyosomes, and the endoplasmic reticulum (ER). Secretory vesicles aggregate into an inverted cone appressed against the apical plasma membrane. ER elements move nearly to the extreme apex, whereas mitochondria and Golgi dictyosomes move less far forward. The cortical actin fringe assumes a central position in the control of clear zone formation and maintenance, given its role in generating cytoplasmic streaming. Other likely factors include the tip-focused calcium gradient, the apical pH gradient, the influx of water, and a host of signaling factors (small G-proteins). We think that the clear zone is an emergent property that depends on the interaction of several factors crucial for polarized growth.

Organelle trafficking, the cytoskeleton, and pollen tube growth

The pollen tube is fundamental for the reproduction of seed plants. Characteristically, it grows relatively quickly and uni-directionally ("polarized growth") to extend the male gametophyte to reach the female gametophyte. The pollen tube forms a channel through which the sperm cells move so that they can reach their targets in the ovule. To grow quickly and directionally, the pollen tube requires an intense movement of organelles and vesicles that allows the cell's contents to be distributed to sustain the growth rate. While the various organelles distribute more or less uniformly within the pollen tube, Golgi-released secretory vesicles accumulate massively at the pollen tube apex, that is, the growing region. This intense movement of organelles and vesicles is dependent on the dynamics of the cytoskeleton, which reorganizes differentially in response to external signals and coordinates membrane trafficking with the growth rate of pollen tubes.

Live cell imaging reveals actin-cytoskeleton-induced self-association of the actin-bundling protein WLIM1

Crosslinking of actin filaments into bundles is essential for the assembly and stabilization of specific cytoskeletal structures. However, relatively little is known about the molecular mechanisms underlying actin bundle formation. The two LIM-domain-containing proteins define a novel and evolutionarily conserved family of actin-bundling proteins whose actin-binding and -crosslinking activities primarily rely on their LIM domains. Using TIRF microscopy, we describe real-time formation of actin bundles induced by tobacco NtWLIM1 in vitro. We show that NtWLIM1 binds to single filaments and subsequently promotes their interaction and zippering into tight bundles of mixed polarity. NtWLIM1-induced bundles grew by both elongation of internal filaments and addition of preformed fragments at their extremities. Importantly, these data are highly consistent with the modes of bundle formation and growth observed in transgenic Arabidopsis plants expressing a GFP-fused Arabidopsis AtWLIM1 protein. Using two complementary live cell imaging approaches, a close relationship between NtWLIM1 subcellular localization and self-association was established. Indeed, both BiFC and FLIM-FRET data revealed that, although unstable NtWLIM1 complexes can sporadically form in the cytosol, stable complexes concentrate along the actin cytoskeleton. Remarkably, disruption of the actin cytoskeleton significantly impaired self-association of NtWLIM1. In addition, biochemical analyses support the idea that F-actin facilitates the switch of purified recombinant NtWLIM1 from a monomeric to a di- or oligomeric state. On the basis of our data, we propose a model in which actin binding promotes the formation and stabilization of NtWLIM1 complexes, which in turn might drive the crosslinking of actin filaments.

Control of cell wall extensibility during pollen tube growth

In this review, we address the question of how the tip-growing pollen tube achieves its rapid rate of elongation while maintaining an intact cell wall. Although turgor is essential for growth to occur, the local expansion rate is controlled by local changes in the viscosity of the apical wall. We focus on several different structures and underlying processes that are thought to be major participants including exocytosis, the organization and activity of the actin cytoskeleton, calcium and proton physiology, and cellular energetics. We think that the actin cytoskeleton, in particular the apical cortical actin fringe, directs the flow of vesicles to the apical domain, where they fuse with the plasma membrane and contribute their contents to the expanding cell wall. While pH gradients, as generated by a proton-ATPase located on the plasma membrane along the side of the clear zone, may regulate rapid actin turnover and new polymerization in the fringe, the tip-focused calcium gradient biases secretion towards the polar axis. The recent data showing that exocytosis of new wall material precedes and predicts the process of cell elongation provide support for the idea that the intussusception of newly secreted pectin contributes to decreases in apical wall viscosity and to cell expansion. Other prime factors will be the localization and activity of the enzyme pectin methyl-esterase, and the chelation of calcium by pectic acids. Finally, we acknowledge a role for reactive oxygen species in the control of wall viscosity.

Transport logistics in pollen tubes

Cellular organelles move within the cellular volume and the effect of the resulting drag forces on the liquid causes bulk movement in the cytosol. The movement of both organelles and cytosol leads to an overall motion pattern called cytoplasmic streaming or cyclosis. This streaming enables the active and passive transport of molecules and organelles between cellular compartments. Furthermore, the fusion and budding of vesicles with and from the plasma membrane (exo/endocytosis) allow for transport of material between the inside and the outside of the cell. In the pollen tube, cytoplasmic streaming and exo/endocytosis are very active and fulfill several different functions. In this review, we focus on the logistics of intracellular motion and transport processes as well as their biophysical underpinnings. We discuss various modeling attempts that have been performed to understand both long-distance shuttling and short-distance targeting of organelles. We show how the combination of mechanical and mathematical modeling with cell biological approaches has contributed to our understanding of intracellular transport logistics.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Fungal lectin of Peltigera canina induces chemotropism of compatible Nostoc cells by constriction-relaxation pulses of cyanobiont cytoskeleton

A glycosylated arginase acting as a fungal lectin from Peltigera canina is able to produce recruitment of cyanobiont Nostoc cells and their adhesion to the hyphal surface. This implies that the cyanobiont would develop organelles to motility towards the chemoattractant. However when visualized by transmission electron microscopy, Nostoc cells recently isolated from P. canina thallus do not reveal any motile, superficial organelles, although their surface was covered by small spindles and serrated layer related to gliding. The use of S-(3,4-dichlorobenzyl)isothiourea, blebbistatin, phalloidin and latrunculin A provide circumstantial evidence that actin microfilaments rather than MreB, the actin-like protein from prokaryota, and, probably, an ATPase which develops contractile function similar to that of myosin II, are involved in cell motility. These experimental facts, the absence of superficial elements (fimbriae, pili or flagellum) related to cell movement, and the appearance of sunken cells during of after movement verified by scanning electron microscopy, support the hypothesis that the motility of lichen cyanobionts could be achieved by contraction-relaxation episodes of the cytoskeleton induced by fungal lectin act as a chemoattractant.

Spatial and temporal expression of actin depolymerizing factors ADF7 and ADF10 during male gametophyte development in Arabidopsis thaliana

The actin cytoskeleton plays a crucial role in many aspects of plant cell development. During male gametophyte development, the actin arrays are conspicuously remodeled both during pollen maturation in the anther and after pollen hydration on the receptive stigma and pollen tube elongation. Remodeling of actin arrays results from the highly orchestrated activities of numerous actin binding proteins (ABPs). A key player in actin remodeling is the actin depolymerizing factor (ADF), which increases actin filament treadmilling rates. We prepared fluorescent protein fusions of two Arabidopsis pollen-specific ADFs, ADF7 and ADF10. We monitored the expression and subcellular localization of these proteins during male gametophyte development, pollen germination and pollen tube growth. ADF7 and ADF10 were differentially expressed with the ADF7 signal appearing in the microspore stage and that of ADF10 only during the polarized microspore stage. ADF7 was associated with the microspore nucleus and the vegetative nucleus of the mature grain during less metabolically active stages, but in germinating pollen grains and elongating pollen tubes, it was associated with the subapical actin fringe. On the other hand, ADF10 was associated with filamentous actin in the developing gametophyte, in particular with the arrays surrounding the apertures of the mature pollen grain. In the shank of elongating pollen tubes, ADF10 was associated with thick actin cables. We propose possible specific functions of these two ADFs based on their differences in expression and localization.

Distribution of callose synthase, cellulose synthase, and sucrose synthase in tobacco pollen tube is controlled in dissimilar ways by actin filaments and microtubules

Callose and cellulose are fundamental components of the cell wall of pollen tubes and are probably synthesized by distinct enzymes, callose synthase and cellulose synthase, respectively. We examined the distribution of callose synthase and cellulose synthase in tobacco (Nicotiana tabacum) pollen tubes in relation to the dynamics of actin filaments, microtubules, and the endomembrane system using specific antibodies to highly conserved peptide sequences. The role of the cytoskeleton and membrane flow was investigated using specific inhibitors (latrunculin B, 2,3-butanedione monoxime, taxol, oryzalin, and brefeldin A). Both enzymes are associated with the plasma membrane, but cellulose synthase is present along the entire length of pollen tubes (with a higher concentration at the apex) while callose synthase is located in the apex and in distal regions. In longer pollen tubes, callose synthase accumulates consistently around callose plugs, indicating its involvement in plug synthesis. Actin filaments and endomembrane dynamics are critical for the distribution of callose synthase and cellulose synthase, showing that enzymes are transported through Golgi bodies and/or vesicles moving along actin filaments. Conversely, microtubules appear to be critical in the positioning of callose synthase in distal regions and around callose plugs. In contrast, cellulose synthases are only partially coaligned with cortical microtubules and unrelated to callose plugs. Callose synthase also comigrates with tubulin by Blue Native-polyacrylamide gel electrophoresis. Membrane sucrose synthase, which expectedly provides UDP-glucose to callose synthase and cellulose synthase, binds to actin filaments depending on sucrose concentration; its distribution is dependent on the actin cytoskeleton and the endomembrane system but not on microtubules.

Arabidopsis FIMBRIN5, an actin bundling factor, is required for pollen germination and pollen tube growth

Actin cables in pollen tubes serve as molecular tracks for cytoplasmic streaming and organelle movement and are formed by actin bundling factors like villins and fimbrins. However, the precise mechanisms by which actin cables are generated and maintained remain largely unknown. Fimbrins comprise a family of five members in Arabidopsis thaliana. Here, we characterized a fimbrin isoform, Arabidopsis FIMBRIN5 (FIM5). Our results show that FIM5 is required for the organization of actin cytoskeleton in pollen grains and pollen tubes, and FIM5 loss-of-function associates with a delay of pollen germination and inhibition of pollen tube growth. FIM5 decorates actin filaments throughout pollen grains and tubes. Actin filaments become redistributed in fim5 pollen grains and disorganized in fim5 pollen tubes. Specifically, actin cables protrude into the extreme tips, and their longitudinal arrangement is disrupted in the shank of fim5 pollen tubes. Consequently, the pattern and velocity of cytoplasmic streaming were altered in fim5 pollen tubes. Additionally, loss of FIM5 function rendered pollen germination and tube growth hypersensitive to the actin-depolymerizing drug latrunculin B. In vitro biochemical analyses indicated that FIM5 exhibits actin bundling activity and stabilizes actin filaments. Thus, we propose that FIM5 regulates actin dynamics and organization during pollen germination and tube growth via stabilizing actin filaments and organizing them into higher-order structures.

A transmembrane formin nucleates subapical actin assembly and controls tip-focused growth in pollen tubes

Pollen tubes are highly polarized plant cells specialized in delivering sperm for fertilization. Pollen tube growth is rapid, occurs exclusively at the tip, and can reach distances thousands of times the diameter of the pollen grain without cell division, thus representing an excellent model system for studying asymmetric cell growth. In flowering plants, pollen tube growth is dependent on the actin cytoskeleton, which supports an efficient vesicle trafficking system to deliver membrane and cell-wall materials to the tube tip. A highly dynamic subapical actin structure and an apical vesicular zone are known to be critical for the tip-growth process. How this apical organization is maintained, how the subapical actin structure is assembled, and direct evidence for its functional coupling with tip growth remain to be established. Here, we show that a tip-located, cell membrane-anchored actin-nucleating protein, the Arabidopsis formin homology5 (FH5), stimulates actin assembly from the subapical membrane, provides actin filaments for vesicular trafficking to the apical dome, and mediates assembly of the subapical actin structure. Moreover, FH5-expressing pollen tubes provided a unique opportunity to demonstrate that assembly of the subapical actin structure is concomitant with the acquisition of rapid tip growth, providing further support for their functional coupling. Together, our results show that FH5 plays a pivotal role in establishing the subapical actin and apical vesicular organization critical for tip-focused growth in pollen tubes.

Microfilament orientation constrains vesicle flow and spatial distribution in growing pollen tubes

The dynamics of cellular organelles reveals important information about their functioning. The spatio-temporal movement patterns of vesicles in growing pollen tubes are controlled by the actin cytoskeleton. Vesicle flow is crucial for morphogenesis in these cells as it ensures targeted delivery of cell wall polysaccharides. Remarkably, the target region does not contain much filamentous actin. We model the vesicular trafficking in this area using as boundary conditions the expanding cell wall and the actin array forming the apical actin fringe. The shape of the fringe was obtained by imposing a steady state and constant polymerization rate of the actin filaments. Letting vesicle flux into and out of the apical region be determined by the orientation of the actin microfilaments and by exocytosis was sufficient to generate a flux that corresponds in magnitude and orientation to that observed experimentally. This model explains how the cytoplasmic streaming pattern in the apical region of the pollen tube can be generated without the presence of actin microfilaments.

Arabidopsis formin3 directs the formation of actin cables and polarized growth in pollen tubes

Cytoplasmic actin cables are the most prominent actin structures in plant cells, but the molecular mechanism underlying their formation is unknown. The function of these actin cables, which are proposed to modulate cytoplasmic streaming and intracellular movement of many organelles in plants, has not been studied by genetic means. Here, we show that Arabidopsis thaliana formin3 (AFH3) is an actin nucleation factor responsible for the formation of longitudinal actin cables in pollen tubes. The Arabidopsis AFH3 gene encodes a 785-amino acid polypeptide, which contains a formin homology 1 (FH1) and a FH2 domain. In vitro analysis revealed that the AFH3 FH1FH2 domains interact with the barbed end of actin filaments and have actin nucleation activity in the presence of G-actin or G actin-profilin. Overexpression of AFH3 in tobacco (Nicotiana tabacum) pollen tubes induced excessive actin cables, which extended into the tubes' apices. Specific downregulation of AFH3 eliminated actin cables in Arabidopsis pollen tubes and reduced the level of actin polymers in pollen grains. This led to the disruption of the reverse fountain streaming pattern in pollen tubes, confirming a role for actin cables in the regulation of cytoplasmic streaming. Furthermore, these tubes became wide and short and swelled at their tips, suggesting that actin cables may regulate growth polarity in pollen tubes. Thus, AFH3 regulates the formation of actin cables, which are important for cytoplasmic streaming and polarized growth in pollen tubes.

Calreticulin expression and localization in plant cells during pollen-pistil interactions

In this report, the distributions of calreticulin (CRT) and its transcripts in Haemanthus pollen, pollen tubes, and somatic cells of the hollow pistil were studied. Immunoblot analysis of protein extracts from mature anthers, dry and germinated pollen, growing pollen tubes, and unpollinated/pollinated pistils revealed a strong expression of CRT. Both in vitro and in situ studies confirmed the presence of CRT mRNA and protein in pollen/pollen tubes and somatic cells of the pistil transmitting tract. The co-localization of these molecules in ER of these cells suggests that the rough ER is a site of CRT translation. In the pistil, accumulation of the protein in pollen tubes, transmitting tract epidermis (tte), and micropylar cells of the ovule (mc) was correlated with the increased level of exchangeable calcium. Therefore, CRT as a Ca(2+)-binding/buffering protein, may be involved in mechanism of regulation calcium homeostasis in these cells. The functional role of the protein in pollen-pistil interactions, apart from its postulated function in cellular Ca(2+) homeostasis, is discussed.

How to shape a cylinder: pollen tube as a model system for the generation of complex cellular geometry

Expansive growth in plant cells is a formidable problem for biophysical studies, and the mechanical principles governing the generation of complex cellular geometries are still poorly understood. Pollen, the male gametophyte stage of the flowering plants, is an excellent model system for the investigation of the mechanics of complex growth processes. The initiation of pollen tube growth requires first of all, the spatially confined formation of a protuberance. This process must be controlled by the mechanical properties of the cell wall, since turgor is a non-vectorial force. In the elongating tube, cell wall expansion is confined to the apex of the cell, requiring the tubular region to be stabilized against turgor-induced tensile stress. Tip focused surface expansion must be coordinated with the supply of cell wall material to this region requiring the precise, logistical control of intracellular transport processes. The advantage of such a demanding mechanism is the high efficiency it confers on the pollen tube in leading an invasive way of life.