In vitro assays demonstrate that pollen tube organelles use kinesin-related motor proteins to move along microtubules

Mitigation of UV-B Radiation Stress in Tobacco Pollen by Expression of the Tardigrade Damage Suppressor Protein (Dsup)

Pollen, the male gametophyte of seed plants, is extremely sensitive to UV light, which may prevent fertilization. As a result, strategies to improve plant resistance to solar ultraviolet (UV) radiation are required. The tardigrade damage suppressor protein (Dsup) is a putative DNA-binding protein that enables tardigrades to tolerate harsh environmental conditions, including UV radiation, and was therefore considered as a candidate for reducing the effects of UV exposure on pollen. Tobacco pollen was genetically engineered to express Dsup and then exposed to UV-B radiation to determine the effectiveness of the protein in increasing pollen resistance. To establish the preventive role of Dsup against UV-B stress, we carried out extensive investigations into pollen viability, germination rate, pollen tube length, male germ unit position, callose plug development, marker protein content, and antioxidant capacity. The results indicated that UV-B stress has a significant negative impact on both pollen grain and pollen tube growth. However, Dsup expression increased the antioxidant levels and reversed some of the UV-B-induced changes to pollen, restoring the proper distance between the tip and the last callose plug formed, as well as pollen tube length, tubulin, and HSP70 levels. Therefore, the expression of heterologous Dsup in pollen may provide the plant male gametophyte with enhanced responses to UV-B stress and protection against harmful environmental radiation.

Microtubule cytoskeleton and mycorrhizal roots

For the establishment of the Arbuscular Mycorrhiza (AM) symbiosis it is essential that epidermis and cortical cells from plant roots suffer a strong reorganization to allow the penetration of intracellular fungal hyphae. In the same manner, the new formation of a periarbuscular membrane and a symbiotic interface with specific compositions are required for a functional symbiosis. It is believed that the cytoskeleton of the plant host plays an essential role in these processes, particularly the microtubule (MT) cytoskeleton, as huge modifications have been observed in the MT array of root cells accompanying the establishment of the AM symbiosis. Recent research has established a link between microtubule rearrangements and arbuscule functioning. However, further research is required to elucidate the specific functions of MT cytoskeleton along the different stages of the arbuscule life cycle and to unravel the signals triggering these changes.

The legacy of kinesins in the pollen tube thirty years later

The pollen tube is fundamental in the reproduction of seed plants. Particularly in angiosperms, we now have much information about how it grows, how it senses extracellular signals, and how it converts them into a directional growth mechanism. The expansion of the pollen tube is also related to dynamic cytoplasmic processes based on the cytoskeleton (such as polymerization/depolymerization of microtubules and actin filaments) or motor activity along with the two cytoskeletal systems and is dependent on motor proteins. While a considerable amount of information is available for the actomyosin system in the pollen tube, the role of microtubules in the transport of organelles or macromolecular structures is still quite uncertain despite that 30 years ago the first work on the presence of kinesins in the pollen tube was published. Since then, progress has been made in elucidating the role of kinesins in plant cells. However, their role within the pollen tube is still enigmatic. In this review, I will postulate some roles of kinesins in the pollen tube 30 years after their initial discovery based on information obtained in other plant cells in the meantime. The most concrete hypotheses predict that kinesins in the pollen tube enable the short movement of specific organelles or contribute to generative cell or sperm cell transport, as well as mediate specific steps in the process of endocytosis.

Let's shape again: the concerted molecular action that builds the pollen tube

The pollen tube is being subjected to control by a complex network of communication that regulates its shape and the misfunction of a single component causes specific deformations. In flowering plants, the pollen tube is a tubular extension of the pollen grain required for successful sexual reproduction. Indeed, maintaining the unique shape of the pollen tube is essential for the pollen tube to approach the embryo sac. Many processes and molecules (such as GTPase activity, phosphoinositides, Ca²⁺ gradient, distribution of reactive oxygen species and nitric oxide, nonuniform pH values, organization of the cytoskeleton, balance between exocytosis and endocytosis, and cell wall structure) play key and coordinated roles in maintaining the cylindrical shape of pollen tubes. In addition, the above factors must also interact with each other so that the cell shape is maintained while the pollen tube follows chemical signals in the pistil that guide it to the embryo sac. Any intrinsic changes (such as erroneous signals) or extrinsic changes (such as environmental stresses) can affect the above factors and thus fertilization by altering the tube morphology. In this review, the processes and molecules that enable the development and maintenance of the unique shape of pollen tubes in angiosperms are presented emphasizing their interaction with specific tube shape. Thus, the purpose of the review is to investigate whether specific deformations in pollen tubes can help us to better understand the mechanism underlying pollen tube shape.

Biochemical and cytological interactions between callose synthase and microtubules in the tobacco pollen tube

KEY MESSAGE

The article concerns the association between callose synthase and cytoskeleton by biochemical and ultrastructural analyses in the pollen tube. Results confirmed this association and immunogold labeling showed a colocalization. Callose is a cell wall polysaccharide involved in fundamental biological processes, from plant development to the response to abiotic and biotic stress. To gain insight into the deposition pattern of callose, it is important to know how the enzyme callose synthase is regulated through the interaction with the vesicle-cytoskeletal system. Actin filaments likely determine the long-range distribution of callose synthase through transport vesicles but the spatial/biochemical relationships between callose synthase and microtubules are poorly understood, although experimental evidence supports the association between callose synthase and tubulin. In this manuscript, we further investigated the association between callose synthase and microtubules through biochemical and ultrastructural analyses in the pollen tube model system, where callose is an essential component of the cell wall. Results by native 2-D electrophoresis, isolation of callose synthase complex and far-western blot confirmed that callose synthase is associated with tubulin and can therefore interface with cortical microtubules. In contrast, actin and sucrose synthase were not permanently associated with callose synthase. Immunogold labeling showed colocalization between the enzyme and microtubules, occasionally mediated by vesicles. Overall, the data indicate that pollen tube callose synthase exerts its activity in cooperation with the microtubular cytoskeleton.

Principles and Applications of Single Particle Tracking in Cell Research

It is a tough challenge for many decades to decipher the complex relationships between cell behaviors and cellular physical properties. Single particle tracking (SPT) with high spatial and temporal resolution has been applied extensively in cell research to understand physicochemical properties of cells and their bio-functions by tracking endogenous or exogenous probes. This review describes the fundamental principles of SPT as well as its applications in intracellular mechanics, membrane dynamics, organelles distribution, and processes of internalization and transport. Finally, challenges and future directions of SPT are also discussed.

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.

Overexpression of the KNOX gene Tkn4 affects pollen development and confers sensitivity to gibberellin and auxin in tomato

The knotted1-like homeobox genes not only regulate the formation and differentiation of meristems and vascular system but are also involved in biosynthesis and signal transduction of diverse plant hormones in tomato. Here, we showed that a knotted1-like homeobox gene Tkn4 is required for pollen and pollen tube growth when this gene is overexpressed in tomato. Pollen grains in the Tkn4 overexpressed plants (Tkn4-OX) germinated quicker than those in the wild-type (WT) plant cultured in vitro in germination media. The percentage of fruit set was higher in Tkn4-OX than in WT plants and the transgenic plants showed an ordered inflorescence. Tkn4-OX seedlings also exhibited sensitivity to gibberellins (GA) and auxins. RNA sequencing results showed that the expression of genes related to sugar, cell wall-modification, microtubule-associated vesicular transport for pollen growth, GA and auxin synthesis were significantly changed. Hence, Tkn4 contributes to a function in the development of pollen and pollen tube and the regulation of phytohormones to participate in plant growth. These results provided a potential application value for agricultural improvement to enhance the rate of fruit set in tomato.

Temperature-dependent signal transmission in chloroplast accumulation response

Chloroplast photorelocation movement, well-characterized light-induced response found in various plant species from alga to higher plants, is an important phenomenon for plants to increase photosynthesis efficiency and avoid photodamage. The signal for chloroplast accumulation movement connecting the blue light receptor, phototropin, and chloroplasts remains to be identified, although the photoreceptors and the mechanism of movement via chloroplast actin filaments have now been revealed in land plants. The characteristics of the signal have been found; the speed of signal transfer is about 1 µm min^-1 and that the signal for the accumulation response has a longer life and is transferred a longer distance than that of the avoidance response. Here, to collect the clues of the unknown signal substances, we studied the effect of temperature on the speed of signal transmission using the fern Adiantum capillus-veneris and found the possibility that the mechanism of signal transfer was not dependent on the simple diffusion of a substance; thus, some chemical reaction must also be involved. We also found new insights of signaling substances, such that microtubules are not involved in the signal transmission, and that the signal could even be transmitted through the narrow space between chloroplasts and the plasma membrane.

Emerging roles for microtubules in angiosperm pollen tube growth highlight new research cues

In plants, actin filaments have an important role in organelle movement and cytoplasmic streaming. Otherwise microtubules (MTs) have a role in restricting organelles to specific areas of the cell and in maintaining organelle morphology. In somatic plant cells, MTs also participate in cell division and morphogenesis, allowing cells to take their definitive shape in order to perform specific functions. In the latter case, MTs influence assembly of the cell wall, controlling the delivery of enzymes involved in cellulose synthesis and of wall modulation material to the proper sites. In angiosperm pollen tubes, organelle movement is generally attributed to the acto-myosin system, the main role of which is in distributing organelles in the cytoplasm and in carrying secretory vesicles to the apex for polarized growth. Recent data on membrane trafficking suggests a role of MTs in fine delivery and repositioning of vesicles to sustain pollen tube growth. This review examines the role of MTs in secretion and endocytosis, highlighting new research cues regarding cell wall construction and pollen tube-pistil crosstalk, that help unravel the role of MTs in polarized growth.

The Arabidopsis kinesin gene AtKin-1 plays a role in the nuclear division process during megagametogenesis

Atkin - 1 , the only Kinesin-1 member of Arabidopsis thaliana , plays a role during female gametogenesis through regulation of nuclear division cycles. Kinesins are microtubule-dependent motor proteins found in eukaryotic organisms. They constitute a superfamily that can be further classified into at least 14 families. In the Kinesin-1 family, members from animal and fungi play roles in long-distance transport of organelles and vesicles. Although Kinesin-1-like sequences have been identified in higher plants, little is known about their function in plant cells, other than in a recently identified Kinesin-1-like protein in a rice pollen semi-sterile mutant. In this study, the gene encoding the only Kinesin-1 member in Arabidopsis, AtKin-1 was found to be specifically expressed in ovules and anthers. AtKin-1 loss-of-function mutants showed substantially aborted ovules in siliques, and this finding was supported by complementation testing. Reciprocal crossing between mutant and wild-type plants indicated that a defect in AtKin-1 results in partially aborted megagametophytes, with no observable effects on pollen fertility. Further observation of ovule development in the mutant pistils indicated that the enlargement of the megaspore was blocked and nuclear division arrested at the one-nucleate stage during embryo sac formation. Our data suggest that AtKin-1 plays a role in the nuclear division cycles during megagametogenesis.

Reconstitution of microtubule-dependent organelle transport

Microtubule (MT)-based motor proteins transport many cellular factors to their functionally relevant locations within cells, and defects in transport are linked to human disease. Understanding the mechanism and regulation of this transport process in living cells is difficult because of the complex in vivo environment and limited means to manipulate the system. On the other hand, in vitro motility assays using purified motors attached to beads does not recapitulate the full complexity of cargo transport in vivo. Assaying motility of organelles in cell extracts is therefore attractive, as natural cargoes are being examined, but in an environment that is more amenable to manipulation. Here, we describe the purification and in vitro MT-based motility of phagosomes from Dictyostelium and lipid droplets from rat liver. These assays have the potential to address diverse questions related to endosome/phagosome maturation, fatty acid regulation, and could also serve as a starting point for reconstituting the motility of other types of organelles.

Microtubule depolymerization affects endocytosis and exocytosis in the tip and influences endosome movement in tobacco pollen tubes

Polarized organization of the cytoplasm of growing pollen tubes is maintained by coordinated function of actin filaments (AFs) and microtubules (MTs). AFs convey post-Golgi secretory vesicles to the tip where some fuse with specific domains of the plasma membrane (PM). Secretory activity is balanced by PM retrieval that maintains cell membrane economy and regulates the polarized composition of the PM, by dividing lipids/proteins between the shank and the tip. Although AFs play a key role in PM internalization in the shank, the role of MTs in exo-endocytosis needs to be characterized. The present results show that integrity of the MT cytoskeleton is necessary to control exo-endocytosis events in the tip. MT polymerization plays a role in promoting PM invagination in the apex of tobacco pollen tubes since nocodazole affected PM internalization in the tip and subsequent migration of endocytic vesicles from the apex for degradation. MT depolymerization in the apex and shank was associated with misallocation of a significantly greater amount of internalized PM to the Golgi apparatus and its early recycling to the secretory pathway. Fluorescence Recovery After Photobleaching (FRAP) experiments also showed that MT depolymerization in the tip region influenced the rate of exocytosis in the central domain of the apical PM.

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.

Endocytic Pathways and Recycling in Growing Pollen Tubes

Pollen tube growth is based on transport of secretory vesicles into the apical region where they fuse with a small area of the plasma membrane. The amount of secretion greatly exceeds the quantity of membrane required for growth. Mechanisms of membrane retrieval have recently been demonstrated and partially characterized using FM (Fei Mao) dyes or charged nanogold. Both these probes reveal that clathrin-dependent and -independent endocytosis occur in pollen tubes and are involved in distinct degradation pathways and membrane recycling. Exocytosis, internalization and sorting of PM proteins/lipids depend on the integrity of the actin cytoskeleton and are involved in actin filament organization. However, some kinds of endocytic and exocytic processes occurring in the central area of the tip still need to be characterized. Analysis of secretion dynamics and data derived from endocytosis highlight the complexity of events occurring in the tip region and suggest a new model of pollen tube growth.

Quantitative analysis of organelle distribution and dynamics in Physcomitrella patens protonemal cells

BACKGROUND

In the last decade, the moss Physcomitrella patens has emerged as a powerful plant model system, amenable for genetic manipulations not possible in any other plant. This moss is particularly well suited for plant polarized cell growth studies, as in its protonemal phase, expansion is restricted to the tip of its cells. Based on pollen tube and root hair studies, it is well known that tip growth requires active secretion and high polarization of the cellular components. However, such information is still missing in Physcomitrella patens. To gain insight into the mechanisms underlying the participation of organelle organization in tip growth, it is essential to determine the distribution and the dynamics of the organelles in moss cells.

RESULTS

We used fluorescent protein fusions to visualize and track Golgi dictyosomes, mitochondria, and peroxisomes in live protonemal cells. We also visualized and tracked chloroplasts based on chlorophyll auto-fluorescence. We showed that in protonemata all four organelles are distributed in a gradient from the tip of the apical cell to the base of the sub-apical cell. For example, the density of Golgi dictyosomes is 4.7 and 3.4 times higher at the tip than at the base in caulonemata and chloronemata respectively. While Golgi stacks are concentrated at the extreme tip of the caulonemata, chloroplasts and peroxisomes are totally excluded. Interestingly, caulonemata, which grow faster than chloronemata, also contain significantly more Golgi dictyosomes and fewer chloroplasts than chloronemata. Moreover, the motility analysis revealed that organelles in protonemata move with low persistency and average instantaneous speeds ranging from 29 to 75 nm/s, which are at least three orders of magnitude slower than those of pollen tube or root hair organelles.

CONCLUSIONS

To our knowledge, this study reports the first quantitative analysis of organelles in Physcomitrella patens and will make possible comparisons of the distribution and dynamics of organelles from different tip growing plant cells, thus enhancing our understanding of the mechanisms of plant polarized cell growth.

Are kinesins required for organelle trafficking in plant cells?

Plant cells exhibit active movement of membrane-bounded materials, which is more pronounced in large cells but is also appreciable in medium-sized cells and in tip-growing cells (such as pollen tubes and root hairs). Trafficking of organelles (such as Golgi bodies, endoplasmic reticulum, peroxisomes, and mitochondria) and vesicles is essential for plant cell physiology and allows a more or less homogeneous distribution of the cell content. It is well established that the long-range trafficking of organelles is dependent essentially on the network of actin filaments and is powered by the enzyme activity of myosins. However, some lines of evidence suggest that microtubules and members of the kinesin microtubule-based motor superfamily might have a role in the positioning and/or short-range movement of cell organelles and vesicles. Data collected in different cells (such as trichomes and pollen tubes), in specific stages of the plant cell life cycle (for example, during phragmoplast development) and for different organelle classes (mitochondria, Golgi bodies, and chloroplasts) encourage the hypothesis that microtubule-based motors might play subtle yet unclarified roles in organelle trafficking. In some cases, this function could be carried out in cooperation with actin filaments according to the model of "functional cooperation" by which motors of different families are associated with the organelle surface. Since available data did not provide an unambiguous conclusion with regard to the role of kinesins in organelle transport, here we want to debate such hypothesis.

Cytosolic proteins from tobacco pollen tubes that crosslink microtubules and actin filaments in vitro are metabolic enzymes

In plant cells, many processes require cooperative action of both microtubules and actin filaments, but proteins mediating interactions between these cytoskeletal members are mostly undiscovered. Here, we attempt to identify such proteins by affinity purification. Cytosol from Nicotiana tabacum (tobacco) pollen tubes was incubated first with actin filaments, and then proteins eluted from the actin were incubated with microtubules, and finally those microtubule-binding proteins were pooled in an active fraction. This fraction bundled actin filaments but not microtubules. However, when the fraction was added to both actin and microtubules, large bundles resulted, containing both polymers, regardless of the order of addition of components. Similar results were obtained when the order of affinity purification was reversed. The four most abundant bands from the fractions were identified from peptide fragments analyzed by mass spectrometry. The same four proteins were identified regardless of the order of affinity purification. The proteins are: homocysteine methyltransferase, phosphofructokinase, pyruvate decarboxylase, and glucan protein synthase (reversibly glycosylated protein). These results suggest the importance of structuring metabolism within the confines of the pollen tube cytoplasm.

Microtubule motors and pollen tube growth--still an open question

The growth of pollen tubes is supported by the continuous supply of secretory vesicles in the tip area. Movement and accumulation of vesicles is driven by the dynamic interplay between the actin cytoskeleton and motor proteins of the myosin family. A combination of the two protein systems is also responsible for the bidirectional movement of larger organelle classes. In contrast, the role of microtubules and microtubule-based motors is less clear and often ambiguous. Nevertheless, there is evidence which shows that the pollen tube contains a number of microtubule-based motors of the kinesin family. These motor proteins are likely to be associated with pollen tube organelles and, consequently, they have been hypothesized to participate in the distribution of organelles during pollen tube growth. Whether microtubule motor proteins take part in either the transport or positioning of organelles is not known for sure, but there is evidence for this second possibility. This review will discuss the current knowledge of microtubule-based motor proteins (including kinesins and hypothetical dyneins) and will make some hypothesis about their role in the pollen tube.

Spatial and temporal integration of signalling networks regulating pollen tube growth

The overall function of a cell is determined by its contingent of active signal transduction cascades interacting on multiple levels with metabolic pathways, cytoskeletal organization, and regulation of gene expression. Much work has been devoted to analysis of individual signalling cascades interacting with unique cellular targets. However, little is known about how cells integrate information across hierarchical signalling networks. Recent work on pollen tube growth indicates that several key signalling cascades respond to changes in cell hydrodynamics and apical volume. Combined with known effects on cytoarchitecture and signalling from other cell systems, hydrodynamics has the potential to integrate and synchronize the function of the broader signalling network in pollen tubes. This review will explore recent work on cell hydrodynamics in a variety of systems including pollen, and discuss hydrodynamic regulation of cell signalling and function including exocytosis and endocytosis, actin cytoskeleton reorganization, cell wall deposition and assembly, phospholipid and inositol polyphosphate signalling, ion flux, small G-proteins, fertilization, and self-incompatibility. The combined data support a newly emerging model of pollen tube growth.

Flexibility contra stiffness: the phragmoplast as a physical barrier for beads but not for vesicles

In plant cells, Golgi vesicles are transported to the division plane to fuse with each other, forming the cell plate, the initial membrane-bordered cell wall separating daughter cells. Vesicles, but not organelles, move through the phragmoplast, which consists of two opposing cylinders of microtubules and actin filaments, interlaced with endoplasmic reticulum membrane. To study physical aspects of this transport/inhibition process, we microinjected fluorescent synthetic 1,2-dioleoyl-sn-glycero-3-phospho-rac-1-glycerol (DOPG) vesicles and polystyrene beads into Tradescantia virginiana stamen hair cells. The phragmoplast was nonselective for DOPG vesicles of a size up to 150 nm in diameter but was a physical barrier for polystyrene beads having a diameter of 20 and 40 nm and also when beads were coated with the same DOPG membrane. We conclude that stiffness is a parameter for vesicle transit through the phragmoplast and discuss that cytoskeleton configurations can physically block such transit.

Comparative analyses of the proteomes of leaves and flowers at various stages of development reveal organ-specific functional differentiation of proteins in soybean

The functional differentiation of protein networks in individual organs and tissues of soybean at various developmental stages was investigated by proteomic approach. Protein extraction by Mg/NP-40 buffer followed by alkaline phenol-based method was optimized for proteomic analysis. Proteome analyses of leaves at various developmental stages showed 26 differentially expressed proteins, wherein proteins in translocon at the outer/inner envelope membrane of chloroplast protein-transport machineries increased significantly at the first trifoliate. Immunoblot analysis showed chaperonin-60 expressed abundantly in young leaves, whereas HSP 70 and ATP-synthase beta were constitutively expressed in all tissues. The net photosynthesis rate and chlorophyll content showed an age-dependent correlation in leaves. These results suggest that proteins involved in carbon assimilation, folding and assembly, and energy may work synchronously and show a linear correlation to photosynthesis at developmental stages of leaves. Comparison of flower bud and flower proteome reveals 29 differentially expressed proteins, wherein proteins involved in mitochondrial protein transport and assembly, secondary metabolism, and pollen-tube growth were up-regulated during flower development. Together, these results suggest that during developmental stages, each type of tissue is associated with a specific group of proteins; wherein proteins involved in energy, sugar metabolism, and folding, assembly, and destination may play pivotal roles in the maturation process of each organ or tissue.

Ectopic expression of S-RNase of Petunia inflata in pollen results in its sequestration and non-cytotoxic function

The specificity of S-RNase-based self-incompatibility (SI) is controlled by two S-locus genes, the pistil S-RNase gene and the pollen S-locus-F-box gene. S-RNase is synthesized in the transmitting cell; its signal peptide is cleaved off during secretion into the transmitting tract; and the mature "S-RNase", the subject of this study, is taken up by growing pollen tubes via an as-yet unknown mechanism. Upon uptake, S-RNase is sequestered in a vacuolar compartment in both non-self (compatible) and self (incompatible) pollen tubes, and the subsequent disruption of this compartment in incompatible pollen tubes correlates with the onset of the SI response. How the S-RNase-containing compartment is specifically disrupted in incompatible pollen tubes, however, is unknown. Here, we circumvented the uptake step of S-RNase by directly expressing S(2)-RNase, S(3)-RNase and non-glycosylated S(3)-RNase of Petunia inflata, with green fluorescent protein (GFP) fused at the C-terminus of each protein, in self (incompatible) and non-self (compatible) pollen of transgenic plants. We found that none of these ectopically expressed S-RNases affected the viability or the SI behavior of their self or non-self-pollen/pollen tubes. Based on GFP fluorescence of in vitro-germinated pollen tubes, all were sequestered in both self and non-self-pollen tubes. Moreover, the S-RNase-containing compartment was dynamic in living pollen tubes, with movement dependent on the actin-myosin-based molecular motor system. All these results suggest that glycosylation is not required for sequestration of S-RNase expressed in pollen tubes, and that the cytosol of pollen is the site of the cytotoxic action of S-RNase in SI.

Chapter 3. New insights into plant vacuolar structure and dynamics

The plant vacuole is a multifunctional organelle and is essential for plant development and growth. The most distinctive feature of the plant vacuole is its size, which usually occupies over 80-90% of the cell volume in well-developed somatic cells, and is therefore highly involved in cell growth and plant body size. Recent progress in the visualization of the vacuole, together with developments in image analysis, has revealed the highly organized and complex morphology of the vacuole, as well as its dynamics. The plant vacuolar membrane (VM) forms not only a typically large vacuole but also other structures, such as tubular structures, transvacuolar strands, bulbs, and sheets. In higher plant cells, actin microfilaments are mainly located near the VM and are involved in vacuolar shape changes with the actin-myosin systems. Most recently, microtubule-dependent regulation of vacuolar structures in moss plant cells was reported, suggesting a diversity of mechanisms regulating vacuolar morphogenesis.

Organelle motility in the pollen tube: a tale of 20 years

Organelle movement is an evident feature of pollen tubes and is essential for the process of tube growth because it enables the proper distribution of organelles and the accumulation of secretory vesicles in the tube apex. Organelles move along the actin filaments through dynamic interactions with myosin but other proteins are probably responsible for control of this activity. The role of microtubules and microtubule-based motors is less clear and somewhat enigmatic. Nevertheless, the pollen tube is an excellent cell model in which to study and analyse the molecular mechanisms that drive and control organelle motility in relation to plant cell expansion. Current knowledge and the main scientific discoveries in this field of research over the last 20 years are summarized here. Future prospects in the study of the molecular mechanisms that mediate organelle transport and vesicle accumulation during pollen tube elongation are also discussed.

Overexpression of PwTUA1, a pollen-specific tubulin gene, increases pollen tube elongation by altering the distribution of alpha-tubulin and promoting vesicle transport

Tubulin genes are intimately associated with cell division and cell elongation, which are central to plant secondary cell wall development. However, their roles in pollen tube polar growth remain elusive. Here, a TUA1 gene from Picea wilsonii, which is specifically expressed in pollen, was isolated. Semi-quantitative RT-PCR analysis showed that the amount of PwTUA1 transcript varied at each stage of growth of the pollen tube and was induced by calcium ions and boron. Transient expression analysis in P. wilsonii pollen indicated that PwTUA1 improved pollen germination and pollen tube growth. The pollen of transgenic Arabidopsis overexpressing PwTUA1 also showed a higher percentage of germination and faster growth than wild-type plants not only in optimal germination medium, but also in medium supplemented with elevated levels of exogenous calcium ions or boron. Immunofluorescence and electron microscopy showed alpha-tubulin to be enriched and more vesicles accumulated in the apex region in germinating transgenic Arabidopsis pollen compared with wild-type plants. These results demonstrate that PwTUA1 up-regulated by calcium ions and boron contributes to pollen tube elongation by altering the distribution of alpha-tubulin and regulating the deposition of pollen cell wall components during the process of tube growth. The possible role of PwTUA1 in microtubule dynamics and organization was discussed.

The dynamic pollen tube cytoskeleton: live cell studies using actin-binding and microtubule-binding reporter proteins

Pollen tubes elongate within the pistil to transport sperm cells to the embryo sac for fertilization. Growth occurs exclusively at the tube apex, rendering pollen tube elongation a most dramatic polar cell growth process. A hallmark pollen tube feature is its cytoskeleton, which comprises elaborately organized and dynamic actin microfilaments and microtubules. Pollen tube growth is dependent on the actin cytoskeleton; its organization and regulation have been examined extensively by various approaches, including fluorescent protein labeled actin-binding proteins in live cell studies. Using the previously described GFP-NtADF1 and GFP-LlADF1, and a new actin reporter protein NtPLIM2b-GFP, we re-affirm that the predominant actin structures in elongating tobacco and lily pollen tubes are long, streaming actin cables along the pollen tube shank, and a subapical structure comprising shorter actin cables. The subapical collection of actin microfilaments undergoes dynamic changes, giving rise to the appearance of structures that range from basket- or funnel-shaped, mesh-like to a subtle ring. NtPLIM2b-GFP is used in combination with a guanine nucleotide exchange factor for the Rho GTPases, AtROP-GEF1, to illustrate the use of these actin reporter proteins to explore the linkage between the polar cell growth process and its actin cytoskeleton. Contrary to the actin cytoskeleton, microtubules appear not to play a direct role in supporting the polar cell growth process in angiosperm pollen tubes. Using a microtubule reporter protein based on the microtubule end-binding protein from Arabidopsis AtEB1, GFP-AtEB1, we show that the extensive microtubule network in elongating pollen tubes displays varying degrees of dynamics. These reporter proteins provide versatile tools to explore the functional connection between major structural and signaling components of the polar pollen tube growth process.

Structural and signaling networks for the polar cell growth machinery in pollen tubes

Pollen tubes elongate within the pistil to transport sperms to the female gametophytes for fertilization. Pollen tubes grow at their tips through a rapid and polarized cell growth process. This tip growth process is supported by an elaborate and dynamic actin cytoskeleton and a highly active membrane trafficking system that together provide the driving force and secretory activities needed for growth. A polarized cytoplasm with an abundance of vesicles and tip-focused Ca(2+) and H(+) concentration gradients are important for the polar cell growth process. Apical membrane-located Rho GTPases regulate Ca(2+) concentration and actin dynamics in the cytoplasm and are crucial for maintaining pollen tube polarity. Pollen tube growth is marked by periods of rapid and slow growth phases. Activities that regulate and support this tip growth process also show oscillatory fluctuations. How these activities correlate with the rapid, polar, and oscillatory pollen tube growth process is discussed.

Distinct tubulin genes are differentially expressed during barley grain development

Tubulins, as major components involved in the organization of microtubules, play an important role in plant development. We describe here the expression profiles of all known alpha-tubulin (TUA), beta-tubulin (TUB) and gamma-tubulin (TUG) genes of barley (Hordeum vulgare), involving eight newly identified TUB sequences, five established TUA genes and one TUG gene. Macroarray and Northern blot-based expression patterns in the pericarp, endosperm and embryo were obtained over the course of the development of the grain between anthesis and maturation. These revealed that the various tubulin genes differed in their levels of expression, and to some extent were tissue specific. Two expression peaks were detected in the developing endosperm. The first and more prominent peak, at 2 days after flowering, included expression of almost all the tubulin genes. These tubulins are thought to be involved in mitoses during the formation of the syncytial endosperm. The second, less pronounced but more extended, peak included only some of the tubulin genes (HvTUA3, HvTUB1 and HvTUG) and might be associated with the cell wall organization in aleurone and starchy endosperm. The HvTUA5 gene is expressed only in embryo of the developing grain and may be associated with shoot establishment. The expression profiles of the tubulin folding cofactors HvTFC A and HvTFC B as well as small G-protein HvArl2 genes were almost perfectly correlated with the global levels of tubulin mRNA, implying that they have a role in the control of the polymerization of alpha/beta-tubulin heterodimers.

Differential expansion and expression of alpha- and beta-tubulin gene families in Populus

Microtubule organization is intimately associated with cellulose microfibril deposition, central to plant secondary cell wall development. We have determined that a relatively large suite of eight alpha-TUBULIN (TUA) and 20 beta-TUBULIN (TUB) genes is expressed in the woody perennial Populus. A number of features, including gene number, alpha:beta gene representation, amino acid changes at the C terminus, and transcript abundance in wood-forming tissue, distinguish the Populus tubulin suite from that of Arabidopsis thaliana. Five of the eight Populus TUAs are unusual in that they contain a C-terminal methionine, glutamic acid, or glutamine, instead of the more typical, and potentially regulatory, C-terminal tyrosine. Both C-terminal Y-type (TUA1) and M-type (TUA5) TUAs were highly expressed in wood-forming tissues and pollen, while the Y-type TUA6 and TUA8 were abundant only in pollen. Transcripts of the disproportionately expanded TUB family were present at comparatively low levels, with phylogenetically distinct classes predominating in xylem and pollen. When tension wood induction was used as a model system to examine changes in tubulin gene expression under conditions of augmented cellulose deposition, xylem-abundant TUA and TUB genes were up-regulated. Immunolocalization of TUA and TUB in xylem and phloem fibers of stems further supported the notion of heavy microtubule involvement during cellulose microfibril deposition in secondary walls. The high degree of sequence diversity, differential expansion, and differential regulation of Populus TUA and TUB families may confer flexibility in cell wall formation that is of adaptive significance to the woody perennial growth habit.

Moving mitochondria: establishing distribution of an essential organelle

Mitochondria form a dynamic network responsible for energy production, calcium homeostasis and cell signaling. Appropriate distribution of the mitochondrial network contributes to organelle function and is essential for cell survival. Highly polarized cells, including neurons and budding yeast, are particularly sensitive to defects in mitochondrial movement and have emerged as model systems for studying mechanisms that regulate organelle distribution. Mitochondria in multicellular eukaryotes move along microtubule tracks. Actin, the primary cytoskeletal component used for transport in yeast, has more subtle functions in other organisms. Kinesin, dynein and myosin isoforms drive motor-based movement along cytoskeletal tracks. Milton and syntabulin have recently been identified as potential organelle-specific adaptor molecules for microtubule-based motors. Miro, a conserved GTPase, may function with Milton to regulate transport. In yeast, Mmr1p and Ypt11p, a Rab GTPase, are implicated in myosin V-based mitochondrial movement. These potential adaptors could regulate motor activity and therefore determine individual organelle movements. Anchoring of stationary mitochondria also contributes to organelle retention at specific sites in the cell. Together, movement and anchoring ultimately determine mitochondrial distribution throughout the cell.

SB401, a pollen-specific protein from Solanum berthaultii, binds to and bundles microtubules and F-actin

We characterize a novel, pollen-specific, microtubule-associated protein, SB401, found in Solanum berthaultii. This protein binds to and bundles taxol-stabilized microtubules and enhances tubulin polymerization in a concentration-dependent manner, particularly at lower temperatures. Electron microscopy revealed that the protein decorates the entire length of microtubules. Cross-linking and electrophoresis studies showed that SB401 protein forms dimers, and suggest that dimerization could account for bundling. Double immunofluorescent staining of pollen tubes of S. berthaultii showed that SB401 protein co-localized with cortical microtubule bundles. SB401 protein also binds to and bundles actin filaments, and could connect actin filaments to microtubules. SB401 protein had a much higher affinity for microtubules than for actin filaments. In the presence of both cytoskeletal elements, the protein preferentially bound microtubules to form bundles. These results demonstrate that SB401 protein may have important roles in organizing the cytoskeleton in pollen tubes.

How pollen tubes grow

Sexual reproduction of flowering plants depends on delivery of the sperm to the egg, which occurs through a long, polarized projection of a pollen cell, called the pollen tube. The pollen tube grows exclusively at its tip, and this growth is distinguished by very fast rates and reaches extended lengths. Thus, one of the most fascinating aspects of pollen biology is the question of how enough cell wall material is produced to accommodate such rapid extension of pollen tube, and how the cell wall deposition and structure are regulated to allow for rapid changes in the direction of growth. This review discusses recent advances in our understanding of the mechanism of pollen tube growth, focusing on such basic cellular processes as control of cell shape and growth by a network of cell wall-modifying enzymes, molecular motor-mediated vesicular transport, and intracellular signaling by localized gradients of second messengers.

Microtubule- and Actin Filament-Dependent Motors are Distributed on Pollen Tube Mitochondria and Contribute Differently to Their Movement

The pollen tube exhibits cytoplasmic streaming of organelles, which is dependent on the actin-myosin system. Although microtubule-based motors have also been identified in the pollen tube, many uncertainties exist regarding their role in organelle transport. As part of our attempt to understand the role of microtubule-based movement in the pollen tube of tobacco, we investigated the cooperation between microtubules and actin filaments in the transport of mitochondria and Golgi vesicles, which are distributed differently in the growing pollen tube. The analysis was performed using in vitro motility assays in which organelles move along both microtubules and actin filaments. The results indicated that the movement of mitochondria and Golgi vesicles is slow and continuous along microtubules but fast and irregular along actin filaments. In addition, the presence of microtubules in the motility assays forces organelles to use lower velocities. Actin- and tubulin-binding tests, immunoblotting and immunogold labeling indicated that different organelles bind to identical myosins but associate with specific kinesins. We found that a 90 kDa kinesin (previously known as 90 kDa ATP-MAP) is associated with mitochondria but not with Golgi vesicles, whereas a 170 kDa myosin is distributed on mitochondria and other organelle classes. In vitro and in vivo motility assays indicate that microtubules and kinesins decrease the speed of mitochondria, thus contributing to their positioning in the pollen tube.

The sliding theory of cytoplasmic streaming: fifty years of progress

Fifty years ago, an important paper appeared in Botanical Magazine Tokyo. Kamiya and Kuroda proposed a sliding theory for the mechanism of cytoplasmic streaming. This pioneering study laid the basis for elucidation of the molecular mechanism of cytoplasmic streaming--the motive force is generated by the sliding of myosin XI associated with organelles along actin filaments, using the hydrolysis energy of ATP. The role of the actin-myosin system in various plant cell functions is becoming evident. The present article reviews progress in studies on cytoplasmic streaming over the past 50 years.

Tobacco mosaic virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP), which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in the intercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory have provided evidence that the vRNA movement process is correlated with the ability of the MP to interact with microtubules, although the exact role of this interaction during infection is not known. Here, we have used a variety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associated protein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MP in whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against the hypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes with kinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driven transport processes during infection.

Signalling pathways in pollen germination and tube growth

Signalling is an integral component in the establishment and maintenance of cellular identity. In plants, tip-growing cells represent an ideal system to investigate signal transduction mechanisms, and among these, pollen tubes (PTs) are one of the favourite models. Many signalling pathways have been identified during germination and tip growth, namely, Ca(2+), calmodulin, phosphoinositides, protein kinases, cyclic AMP, and GTPases. These constitute a large and complex web of signalling networks that intersect at various levels such as the control of vesicle targeting and fusion and the physical state of the actin cytoskeleton. Here we discuss some of the most recent advances made in PT signal transduction cascades and their implications for our future research. For reasons of space, emphasis was given to signalling mechanisms that control PT reorientation, so naturally many other relevant works have not been cited.

A methionine synthase homolog is associated with secretory vesicles in tobacco pollen tubes

Seven isoforms of 85 kDa polypeptides (p85) were identified as methionine synthase (MetE) homologs by partial aminoacid sequencing in tobacco pollen tube extracts. Immunocytochemistry data showed a localization of the antigen on the surface of tip-focussed post-Golgi secretory vesicles (SVs), that appear to be partially associated with microtubules (Mts). The chemical dissection of pollen tube high speed supernatant (HSS) showed that two distinct pools of MetE are present in pollen tubes, one being the more acidic isoforms sedimenting at 15S and the remaining at 4S after zonal centrifugation through a sucrose density gradient. The identification of the MetE within the pollen tube and its possible participation as methyl donor in a wide range of metabolic reactions, makes it a good subject for studies on pollen tube growth regulation.

Identification and characterization of plasma membrane proteins that bind to microtubules in pollen tubes and generative cells of tobacco

The organization and function of microtubules in plant cells are important in many developmental stages. Connections between microtubules and the endomembrane system of plant cells have been discovered by microscopy, but the molecular characteristics of these relationships are mostly unknown except for a few cases. Using two antibodies raised against microtubule-associated proteins (MAPs) from maize, we have identified two polypeptides that share properties of the MAP family in the pollen tube of Nicotiana tabacum. The two polypeptides (with an apparent Mr of 161 and 90 kDa) bind efficiently to animal and plant microtubules and are found in association with the cellular membranes of the pollen tube, from which they can be solubilized with a zwitterionic detergent. One of these proteins has been purified and shown to promote the assembly of tubulin and, to a lesser extent, the bundling of microtubules. Subcellular fractionation indicated that the two proteins are associated with the plasma membrane compartment. The two proteins are found to co-localize in situ with cortical microtubules in the vegetative cytoplasm of tobacco pollen tubes; co-localization is also evident in the generative cell. According to these data, both the 161 and 90 kDa polypeptides are likely to mediate the interactions between the plasma membrane and microtubules in pollen tubes. In addition, functional data indicate that these MAP-like proteins take part in the process of microtubule assembly and reorganization occurring during cell growth. The evidence that both proteins associate with different cellular compartments also suggests a broad-spectrum role in mediating the dynamic relationships between microtubules and plant cell membranes.

Cytoplasmic streaming in plants

Plant cells are surrounded by a cell wall composed of polysaccharides and hence can change neither their form nor their position. However, active movement of organelles (cytoplasmic streaming or protoplasmic streaming) is observed in plant cells, and involvement of the actin/myosin system in these processes has been suggested. Successful biochemical and biophysical approaches to studying myosins have extensively promoted the understanding of the molecular mechanism underlying these phenomena.

Microtubules and microfilaments coordinate to direct a fountain streaming pattern in elongating conifer pollen tube tips

This study investigates how microtubules and microfilaments control organelle motility within the tips of conifer pollen tubes. Organelles in the 30-microm-long clear zone at the tip of Picea abies (L.) Karst. (Pinaceae) pollen tubes move in a fountain pattern. Within the center of the tube, organelles move into the tip along clearly defined paths, move randomly at the apex, and then move away from the tip beneath the plasma membrane. This pattern coincides with microtubule and microfilament organization and is the opposite of the reverse fountain seen in angiosperm pollen tubes. Application of latrunculin B, which disrupts microfilaments, completely stops growth and reduces organelle motility to Brownian motion. The clear zone at the tip remains intact but fills with thin tubules of endoplasmic reticulum. Applications of amiprophosmethyl, propyzamide or oryzalin, which all disrupt microtubules, stop growth, alter organelle motility within the tip, and alter the organization of actin microfilaments. Amiprophosmethyl inhibits organelle streaming and collapses the clear zone of vesicles at the extreme tip together with the disruption of microfilaments leading into the tip, leaving the plasma membrane intact. Propyzamide and oryzalin cause the accumulation of membrane tubules or vacuoles in the tip that reverse direction and stream in a reverse fountain. The microtubule disruption caused by propyzamide and oryzalin also reorganizes microfilaments from a fibrillar network into pronounced bundles in the tip cytoplasm. We conclude that microtubules control the positioning of organelles into and within the tip and influence the direction of streaming by mediating microfilament organization.

Developmental and cell-specific expression of ZWICHEL is regulated by the intron and exon sequences of its gene

Functional studies with ZWICHEL ( ZWI ), which encodes a Ca(2+)-calmodulin-regulated kinesin, have shown its involvement in trichome morphogenesis and cell division. To identify regulatory regions that control the ZWI expression pattern, we generated transgenic Arabidopsis plants with a GUS reporter driven by different lengths of the ZWI gene 5' region alone or 5' and 3' regions together. The 5' fusions contain varying lengths of the coding and non-coding regions of beta - HYDROXYISOBUTYRYL-CoA HYDROLASE 1 ( CHY1 ), which is upstream of ZWI, and a 162 bp intergenic region. In transgenic plants with 5' 460::GUS, GUS activity was observed primarily in the root hairs whereas transgenic plants with an additional 5' 266 bp region from the CHY1 gene (5' 726::GUS) showed strong GUS accumulation in the entire root including root hairs and root tip, calli and at various developmental stages in trichomes and pollen. However, very little GUS accumulation was detected in roots of dark-grown or root tips of cold-treated seedlings with 5' ZWI constructs. These results were further confirmed by quantifying GUS enzyme activity and transcripts in these seedlings. Calli and pollen transformed with the 5' distal 268 bp fused in antisense orientation to the proximal 460 bp did not show GUS expression. Further, IAA-treated dark-grown seedlings with 726::GUS, but not with 460::GUS, showed high GUS expression in specific regions (outer layer 2a cells) at the base of the lateral roots. The ZWI 3' region (3 kb) did not influence the GUS expression pattern driven by the 5' 726 bp. The absence of CHY1 transcripts in the chy1-2 mutant did not alter either ZWI expression or ZWI-mediated trichome morphogenesis. Thus, our data suggest that the 3' part of the CHY1 gene contains regulatory elements that control ZWI gene expression in dividing cells and other cells that exhibit polarized growth such as root hairs, pollen and trichomes. This is the first evidence that the regulatory regions conferring developmental and cell-specific expression of a gene reside in the introns and exons of its upstream protein-coding gene.

Dynamics and regulation of plant interphase microtubules: a comparative view

Microtubule and actin cytoskeletons are fundamental to a variety of cellular activities within eukaryotic organisms. Extensive information on the dynamics and functions of microtubules, as well as on their regulatory proteins, have been revealed in fungi and animals, and corresponding pictures are now slowly emerging in plants. During interphase, plant cells contain highly dynamic cortical microtubules that organize into ordered arrays, which are apparently regulated by distinct groups of microtubule regulators. Comparison with fungal and animal microtubules highlights both conserved and unique mechanisms for the regulation of the microtubule cytoskeleton in plants.

Isolation and analyses of genes preferentially expressed during early cotton fiber development by subtractive PCR and cDNA array

Cotton fibers are differentiated epidermal cells originating from the outer integuments of the ovule. To identify genes involved in cotton fiber elongation, we performed subtractive PCR using cDNA prepared from 10 days post anthesis (d.p.a.) wild-type cotton fiber as tester and cDNA from a fuzzless-lintless (fl) mutant as driver. We recovered 280 independent cDNA fragments including most of the previously published cotton fiber-related genes. cDNA macroarrays showed that 172 genes were significantly up-regulated in elongating cotton fibers as confirmed by in situ hybridization in representative cases. Twenty-nine cDNAs, including a putative vacuolar (H+)-ATPase catalytic subunit, a kinesin-like calmodulin binding protein, several arabinogalactan proteins and key enzymes involved in long chain fatty acid biosynthesis, accumulated to greater than 50-fold in 10 d.p.a. fiber cells when compared to that in 0 d.p.a. ovules. Various upstream pathways, such as auxin signal transduction, the MAPK pathway and profilin- and expansin-induced cell wall loosening, were also activated during the fast fiber elongation period. This report constitutes the first systematic analysis of genes involved in cotton fiber development. Our results suggest that a concerted mechanism involving multiple cellular pathways is responsible for cotton fiber elongation.