Localized endocytosis in tobacco pollen tubes: visualisation and dynamics of membrane retrieval by a fluorescent phospholipid

Bioorthogonal click chemistry for fluorescence imaging of choline phospholipids in plants

BACKGROUND

Phospholipids are important structural and signaling molecules in plant membranes. Some fluorescent dyes can stain general lipids of membranes, but labeling and visualization of specific lipid classes have yet to be developed for most components of the membrane. New techniques for visualizing membrane lipids are needed to further delineate their dynamic structural and signaling roles in plant cells. In this study we examined whether propargylcholine, a bioortholog of choline, can be used to label the major membrane lipid, phosphatidylcholine, and other choline phospholipids in plants. We established that propargylcholine is readily taken up by roots, and that its incorporation is not detrimental to plant growth. After plant tissue is harvested and fixed, a click-chemistry reaction covalently links the alkyne group of propargylcholine to a fluorescently-tagged azide, resulting in specific labeling of choline phospholipids.

RESULTS

Uptake of propargylcholine, followed by click chemistry with fluorescein or Alexa Fluor 594 azide was used to visualize choline phospholipids in cells of root, leaf, stem, silique and seed tissues from Arabidopsis thaliana. Co-localization with various subcellular markers indicated coinciding fluorescent signals in cell membranes, such as the tonoplast and the ER. Among different cell types in the leaf epidermis, guard cells displayed strong labeling. Mass spectrometry-based lipidomic analysis of the various plant tissues revealed that incorporation of propargylcholine was strongest in roots with approximately 50% of total choline phospholipids being labeled, but it was also incorporated in the other tissues including seeds. Phospholipid profiling confirmed that, in each tissue analyzed, incorporation of the bioortholog had little impact on the pool of choline plus choline-like phospholipids or other lipid species.

CONCLUSION

We developed and validated a click-chemistry based method for fluorescence imaging of choline phospholipids using a bioortholog of choline, propargylcholine, in various cell-types and tissues from Arabidopsis. This click-chemistry method provides a direct way to metabolically tag and visualize specific lipid molecules in plant cells. This work paves the way for future studies addressing in situ localization of specific lipids in plants.

Secretion and Endocytosis in Pollen Tubes: Models of Tip Growth in the Spot Light

Pollen tube tip growth is a widely used model ideally suited to study cellular processes underlying polarized cell expansion. Local secretion supplying material for plasma membrane (PM) and cell wall extension is essential for this process. Cell wall biogenesis requires fusion of secretory vesicles with the PM at an about 10× higher rate than PM extension. Excess material is therefore incorporated into the PM, which needs to be reinternalized through endocytosis. The classical model of tip growth proposes that exocytosis occurs at the apex and that newly incorporated PM material is transported to adjacent lateral regions, where excess material is endocytically recycled. This model was recently challenged based on studies indicating that lateral exocytosis may be balanced by apical endocytosis. This review provides an overview of published data pertaining to exocytosis, endocytosis and vesicular trafficking in pollen tubes. Its key aim is to present classical and alternative models of tip growth in the light of available experimental data. By necessity, the review focusses on pollen tubes of angiosperm models (Nicotiana tabacum, Arabidopsis, Lilium longiflorum), which have been studied far more extensively and grow much faster than structurally strikingly different gymnosperm pollen tubes. Only major transport pathways are considered, which substantially contribute to the mass-flow of membrane material at the pollen tube tip. Growth oscillation, which may be displayed in particular by fast-growing pollen tubes, are not discussed as their influence on the spatial organization of apical membrane traffic is not understood.

Plant ER geometry and dynamics: biophysical and cytoskeletal control during growth and biotic response

The endoplasmic reticulum (ER) is an intricate and dynamic network of membrane tubules and cisternae. In plant cells, the ER 'web' pervades the cortex and endoplasm and is continuous with adjacent cells as it passes through plasmodesmata. It is therefore the largest membranous organelle in plant cells. It performs essential functions including protein and lipid synthesis, and its morphology and movement are linked to cellular function. An emerging trend is that organelles can no longer be seen as discrete membrane-bound compartments, since they can physically interact and 'communicate' with one another. The ER may form a connecting central role in this process. This review tackles our current understanding and quantification of ER dynamics and how these change under a variety of biotic and developmental cues.

NtGNL1a ARF-GEF acts in endocytosis in tobacco cells

BACKGROUND

Processes of anterograde and retrograde membrane trafficking play an important role in cellular homeostasis and dynamic rearrangements of the plasma membrane (PM) in all eukaryotes. These processes depend on the activity of adenosine ribosylation factors (ARFs), a family of GTP-binding proteins and their guanine exchange factors (GEFs). However, knowledge on the function and specificity of individual ARF-GEFs for individual steps of membrane trafficking pathways is still limited in plants.

RESULTS

In this work, treatments with various trafficking inhibitors showed that the endocytosis of FM 4-64 is largely dynamin-dependent and relies on proteins containing endocytic tyrosine-based internalization motif and intact cytoskeleton. Interestingly, brefeldin A (BFA), reported previously as an inhibitor of anterograde membrane trafficking in plants, appeared to be the most potent inhibitor of endocytosis in tobacco. In concert with this finding, we demonstrate that the point mutation in the Sec7 domain of the GNOM-LIKE protein1a (NtGNL1a) confers intracellular trafficking pathway-specific BFA resistance. The internalization of FM 4-64 and trafficking of PIN-FORMED1 (PIN1) auxin efflux carrier in BY-2 tobacco cells were studied to reveal the function of the ARF-GEF NtGNL1a in these.

CONCLUSIONS

Altogether, our observations uncovered the role of NtGNL1a in endocytosis, including endocytosis of PM proteins (as PIN1 auxin efflux carrier). Moreover these data emphasize the need of careful evaluation of mode of action of non-native inhibitors in various species. In addition, they demonstrate the potential of tobacco BY-2 cells for selective mapping of ARF-GEF-regulated endomembrane trafficking pathways.

Osmotic Stress Modulates the Balance between Exocytosis and Clathrin-Mediated Endocytosis in Arabidopsis thaliana

The sessile life style of plants creates the need to deal with an often adverse environment, in which water availability can change on a daily basis, challenging the cellular physiology and integrity. Changes in osmotic conditions disrupt the equilibrium of the plasma membrane: hypoosmotic conditions increase and hyperosmotic environment decrease the cell volume. Here, we show that short-term extracellular osmotic treatments are closely followed by a shift in the balance between endocytosis and exocytosis in root meristem cells. Acute hyperosmotic treatments (ionic and nonionic) enhance clathrin-mediated endocytosis simultaneously attenuating exocytosis, whereas hypoosmotic treatments have the opposite effects. In addition to clathrin recruitment to the plasma membrane, components of early endocytic trafficking are essential during hyperosmotic stress responses. Consequently, growth of seedlings defective in elements of clathrin or early endocytic machinery is more sensitive to hyperosmotic treatments. We also found that the endocytotic response to a change of osmotic status in the environment is dominant over the presumably evolutionary more recent regulatory effect of plant hormones, such as auxin. These results imply that osmotic perturbation influences the balance between endocytosis and exocytosis acting through clathrin-mediated endocytosis. We propose that tension on the plasma membrane determines the addition or removal of membranes at the cell surface, thus preserving cell integrity.

Increased endocytosis of fluorescent phospholipid in tobacco pollen in microgravity and inhibition by verapamil

Gravity sensing in plants occurs in specialised tissues, like in the columella in root tips or the endodermis in shoots. Generally, dense organelles, acting as statoliths, are thought to interact with the cytosekeleton and ion channels in gravitropism. We examined the possibility that tobacco pollen tubes (Nicotiana sylvestris) having an elaborate cytoskeleton could perceive gravity through interaction of the cytoskeleton and the endomembrane system and organelles. Using lipid endocytosis as a quantitative parameter, we show that endocytosis is increased transiently in microgravity within 3 min. This increase is inhibited by the calcium blocker verapamil, suggesting that calcium is lowered in the tip, which is known to increase endocytosis in the pollen tube.

Calcium: The Missing Link in Auxin Action

Due to their sessile lifestyles, plants need to deal with the limitations and stresses imposed by the changing environment. Plants cope with these by a remarkable developmental flexibility, which is embedded in their strategy to survive. Plants can adjust their size, shape and number of organs, bend according to gravity and light, and regenerate tissues that were damaged, utilizing a coordinating, intercellular signal, the plant hormone, auxin. Another versatile signal is the cation, Ca(2+), which is a crucial second messenger for many rapid cellular processes during responses to a wide range of endogenous and environmental signals, such as hormones, light, drought stress and others. Auxin is a good candidate for one of these Ca(2+)-activating signals. However, the role of auxin-induced Ca(2+) signaling is poorly understood. Here, we will provide an overview of possible developmental and physiological roles, as well as mechanisms underlying the interconnection of Ca(2+) and auxin signaling.

Cell wall assembly and intracellular trafficking in plant cells are directly affected by changes in the magnitude of gravitational acceleration

Plants are able to sense the magnitude and direction of gravity. This capacity is thought to reside in selected cell types within the plant body that are equipped with specialized organelles called statoliths. However, most plant cells do not possess statoliths, yet they respond to changes in gravitational acceleration. To understand the effect of gravity on the metabolism and cellular functioning of non-specialized plant cells, we investigated a rapidly growing plant cell devoid of known statoliths and without gravitropic behavior, the pollen tube. The effects of hyper-gravity and omnidirectional exposure to gravity on intracellular trafficking and on cell wall assembly were assessed in Camellia pollen tubes, a model system with highly reproducible growth behavior in vitro. Using an epi-fluorescence microscope mounted on the Large Diameter Centrifuge at the European Space Agency, we were able to demonstrate that vesicular trafficking is reduced under hyper-gravity conditions. Immuno-cytochemistry confirmed that both in hyper and omnidirectional gravity conditions, the characteristic spatial profiles of cellulose and callose distribution in the pollen tube wall were altered, in accordance with a dose-dependent effect on pollen tube diameter. Our findings suggest that in response to gravity induced stress, the pollen tube responds by modifying cell wall assembly to compensate for the altered mechanical load. The effect was reversible within few minutes demonstrating that the pollen tube is able to quickly adapt to changing stress conditions.

EHD1 functions in endosomal recycling and confers salt tolerance

Endocytosis is a crucial process in all eukaryotic organisms including plants. We have previously shown that two Arabidopsis proteins, AtEHD1 and AtEHD2, are involved in endocytosis in plant systems. Knock-down of EHD1 was shown to have a delayed recycling phenotype in mammalians. There are many works in mammalian systems detailing the importance of the various domains in EHDs but, to date, the domains of plant EHD1 that are required for its activity have not been characterized. In this work we demonstrate that knock-down of EHD1 causes a delayed recycling phenotype and reduces Brefeldin A sensitivity in Arabidopsis seedlings. The EH domain of EHD1 was found to be crucial for the localization of EHD1 to endosomal structures. Mutant EHD1 lacking the EH domain did not localize to endosomal structures and showed a phenotype similar to that of EHD1 knock-down seedlings. Mutants lacking the coiled-coil domain, however, showed a phenotype similar to wild-type or EHD1 overexpression seedlings. Salinity stress is a major problem in current agriculture. Microarray data demonstrated that salinity stress enhances the expression of EHD1, and this was confirmed by semi quantitative RT-PCR. We demonstrate herein that transgenic plants over expressing EHD1 possess enhanced tolerance to salt stress, a property which also requires an intact EH domain.

Roles for actin assembly in endocytosis

Endocytosis includes a number of processes by which cells internalize segments of their plasma membrane, enclosing a wide variety of material from outside the cell. Endocytosis can contribute to uptake of nutrients, regulation of signaling molecules, control of osmotic pressure, and function of synapses. The actin cytoskeleton plays an essential role in several of these processes. Actin assembly can create protrusions that encompass extracellular materials. Actin can also support the processes of invagination of a membrane segment into the cytoplasm, elongation of the invagination, scission of the new vesicle from the plasma membrane, and movement of the vesicle away from the membrane. We briefly discuss various types of endocytosis, including phagocytosis, macropinocytosis, and clathrin-independent endocytosis. We focus mainly on new findings on the relative importance of actin in clathrin-mediated endocytosis (CME) in yeast versus mammalian cells.

Pollen tube reuses intracellular components of nucellar cells undergoing programmed cell death in Pinus densiflora

Through the process known as programmed cell death (PCD), nucelli of Pinus densiflora serve as the transmitting tissue for growth of the pollen tube. We sought to clarify the processes of degradation of nucellar cell components and their transport to the pollen tube during PCD in response to pollen tube penetration of such nucelli. Stimulated by pollination, synthesis of large amounts of starch grains occurred in cells in a wide region of the nucellus, but as the pollen tube penetrated the nucellus, starch grains were degraded in amyloplasts of nucellar cells. In cells undergoing PCD, electron-dense vacuoles with high membrane contrast appeared, assumed a variety of autophagic structures, expanded, and ultimately collapsed and disappeared. Vesicles and electron-dense amorphous materials were released inside the thickened walls of cells undergoing PCD, and those vesicles and materials reaching the pollen tube after passing through the extracellular matrix were taken into the tube by endocytosis. These results show that in PCD of nucellar cells, intracellular materials are degraded in amyloplasts and vacuoles, and some of the degraded material is supplied to the pollen tube by vesicular transport to support tube growth.

Lead deposited in the cell wall of Funaria hygrometrica protonemata is not stable--a remobilization can occur

The hypothesis that lead (Pb) can be uptake or remobilized from the cell wall (CW) by internalization withlow-esterified pectins (up to 40%--JIM5-P), was studied in tip-growing apical cell of Funaria hygrometrica protonemata. Treatment 4h with 1mM PbCl(2) caused marked vesicular traffic intensification and the common internalization of JIM5-P from the CW. Lead bound to JIM5-P was internalized from the CW, together with this compound and entered the protoplast. It showed that Pb deposited in CW is not as safe for plant cell as previously believed. However, pulse-chase experiments (recovering 4 h and 24 h) indicated that CW and its thickenings can function as the final sequestration compartments. In Pb deposition sites, a callose layer occurred. It was localized from the protoplast site, next to Pb deposits separating sequestrated to CW and its thickenings Pb from plasma membrane almost certainly protecting the plant cell from its returning into the protoplast.

Fluorescent phosphocholine--a specific marker for the endoplasmic reticulum and for lipid droplets in Chara internodal cells

The staining pattern of 1,2-bis(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-undecanoyl)-sn-glycero-3-phosphocholine (Bodipy PC) was investigated in internodal cells of the green alga Chara corallina. Ten minutes after dye addition, Bodipy-PC-derived fluorescence appeared in lipid droplets and after 1 h in the cortical endoplasmic reticulum (ER) and in the inner ER tubes. Staining of the ER required energy but was independent of an intact actin or microtubule cytoskeleton and independent of vesicular endocytosis. The size of the lipid droplets varied between 0.25 microm in elongating cells and 3.2 microm in senescent internodes. They moved together with or along the cortical ER cisternae in a cytoskeleton-independent manner or remained immobile up to several minutes. Detachment of lipid droplets from the cortical ER or fusion of lipid droplets was never observed. The results of this study suggest that Bodipy PC is a valuable, less toxic alternative to 3,3'-dihexyloxacarbocyanine iodide (DiOC6) staining of the ER in Chara. They confirm an earlier report about microtubule-dependent cortical ER morphology and dynamics in elongating internodes and offer new perspectives for the study of organelle interactions.

BFA-induced compartments from the Golgi apparatus and trans-Golgi network/early endosome are distinct in plant cells

Brefeldin A (BFA) is a useful tool for studying protein trafficking and identifying organelles in the plant secretory and endocytic pathways. At low concentrations (5-10 microg ml(-1)), BFA caused both the Golgi apparatus and trans-Golgi network (TGN), an early endosome (EE) equivalent in plant cells, to form visible aggregates in transgenic tobacco BY-2 cells. Here we show that these BFA-induced aggregates from the Golgi apparatus and TGN are morphologically and functionally distinct in plant cells. Confocal immunofluorescent and immunogold electron microscope (EM) studies demonstrated that BFA-induced Golgi- and TGN-derived aggregates are physically distinct from each other. In addition, the internalized endosomal marker FM4-64 co-localized with the TGN-derived aggregates but not with the Golgi aggregates. In the presence of the endocytosis inhibitor tyrphostin A23, which acts in a dose- and time-dependent manner, SCAMP1 (secretory carrier membrane protein 1) and FM4-64 are mostly excluded from the SYP61-positive BFA-induced TGN aggregates, indicating that homotypic fusion of the TGN rather than de novo endocytic trafficking is important for the formation of TGN/EE-derived BFA-induced aggregates. As the TGN also serves as an EE, continuously receiving materials from the plasma membrane, our data support the notion that the secretory Golgi organelle is distinct from the endocytic TGN/EE in terms of its response to BFA treatment in plant cells. Thus, the Golgi and TGN are probably functionally distinct organelles in plants.

The coiled-coil domain of EHD2 mediates inhibition of LeEix2 endocytosis and signaling

Endocytosis has been suggested to be crucial for the induction of plant immunity in several cases. We have previously shown that two Arabidopsis proteins, AtEHD1 and AtEHD2, are involved in endocytosis in plant systems. AtEHD2 has an inhibitory effect on endocytosis of transferrin, FM-4-64, and LeEix2. There are many works in mammalian systems detailing the importance of the various domains in EHDs but, to date, the domains of plant EHD2 that are required for its inhibitory activity on endocytosis remained unknown. In this work we demonstrate that the coiled-coil domain of EHD2 is crucial for the ability of EHD2 to inhibit endocytosis in plants, as mutant EHD2 forms lacking the coiled-coil lost the ability to inhibit endocytosis and signaling of LeEix2. The coiled-coil was also required for binding of EHD2 to the LeEix2 receptor. It is therefore probable that binding of EHD2 to the LeEix2 receptor is required for inhibition of LeEix2 internalization. We also show herein that the P-loop of EHD2 is important for EHD2 to function properly. The EH domain of AtEHD2 does not appear to be involved in inhibition of endocytosis. Moreover, AtEHD2 influences actin organization and may exert its inhibitory effect on endocytosis through actin re-distribution. The coiled-coil domain of EHD2 functions in inhibition of endocytosis, while the EH domain does not appear to be involved in inhibition of endocytosis.

Hypergravity prevents seed production in Arabidopsis by disrupting pollen tube growth

How tightly land plants are adapted to the gravitational force (g) prevailing on Earth has been of interest because unlike many other environmental factors, g presents as a constant force. Ontogeny of mature angiosperms begins with an embryo that is formed after tip growth by a pollen tube delivers the sperm nucleus to the egg. Because of the importance to plant fitness, we have investigated how gravity affects these early stages of reproductive development. Arabidopsis thaliana (L.) Heynh. plants were grown for 13 days prior to being transferred to growth chambers attached to a large diameter rotor, where they were continuously exposed to 2-g or 4-g for the subsequent 11 days. Plants began flowering 1 day after start of the treatments, producing hundreds of flowers for analysis of reproductive development. At 4-g, Arabidopsis flowers self-pollinated normally but did not produce seeds, thus derailing the entire life cycle. Pollen viability and stigma esterase activity were not compromised by hypergravity; however, the growth of pollen tubes into the stigmas was curtailed at 4-g. In vitro pollen germination assays showed that 4-g average tube length was less than half that for 1-g controls. Closely related Brassica rapa L., which produces seeds at 4-g, required forces in excess of 6-g to slow in vitro tube growth to half that at 1-g. The results explain why seed production is absent in Arabidopsis at 4-g and point to species differences with regard to the g-sensitivity of pollen tube growth.

Actin and endocytosis: mechanisms and phylogeny

The regulated assembly of actin filament networks is a crucial part of endocytosis, with crucial temporal and spatial relationships between proteins of the endocytic and actin assembly machinery. Of particular importance has been a wealth of studies in budding and fission yeast. Cell biology approaches, combined with molecular genetics, have begun to uncover the complexity of the regulation of actin dynamics during the endocytic process. In a wide range of organisms, clathrin-mediated endocytosis appears to be linked to Arp2/3-mediated actin assembly. The conservation of the components, across a wide range eukaryotic species, suggests that the partnership between endocytosis and actin may be evolutionarily ancient.

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.

Cell polarity signaling in Arabidopsis

Cell polarization is intimately linked to plant development, growth, and responses to the environment. Major advances have been made in our understanding of the signaling pathways and networks that regulate cell polarity in plants owing to recent studies on several model systems, e.g., tip growth in pollen tubes, cell morphogenesis in the leaf epidermis, and polar localization of PINs. From these studies we have learned that plant cells use conserved mechanisms such as Rho family GTPases to integrate both plant-specific and conserved polarity cues and to coordinate the cytoskeketon dynamics/reorganization and vesicular trafficking required for polarity establishment and maintenance. This review focuses upon signaling mechanisms for cell polarity formation in Arabidopsis, with an emphasis on Rho GTPase signaling in polarized cell growth and how these mechanisms compare with those for cell polarity signaling in yeast and animal systems.

Tip growth: signaling in the apical dome

Signaling molecules, such as ROP/RAC GTPases and their regulators, reactive oxygen species (ROS) and phospholipids, play pivotal roles in the control of tip growth in pollen tubes and root hairs. They are often localized to the apical growing region of these cells, where their functions are tightly interconnected with cytoskeletal rearrangement and polar vesicle trafficking, which participate in tip growth as well as affect the generation and maintenance of the apical growing region. Recent advances in our understanding of the interface between these cellular activities and signaling in tip growth will be discussed.

AtEHDs, novel Arabidopsis EH-domain-containing proteins involved in endocytosis

SUMMARY

Endocytosis is an essential process by which the eukaryotic cell internalizes exogenous material. Studies in yeast and mammalian cells have revealed that endocytosis is a complex molecular process depending on regulated interactions between a variety of proteins and lipids through specific modules. One such module is the Eps15 homology (EH) domain, a conserved modular protein-interaction domain found in several endocytic proteins. The EH-domain-containing proteins function as regulators of endocytosis through their ability to interact with other proteins involved in this process. Here we describe the isolation and characterization of two Arabidopsis EH-domain-containing proteins (AtEHD1 and AtEHD2). We show that the two proteins are involved in endocytosis in plant systems and demonstrate that the Arabidopsis EHD proteins function similarly to mammalian EHDs. Similarly to hEHD2, over-expression of AtEHD2 has an inhibitory effect on endocytosis. While transgenic plants over-expressing AtEHD1 had no detectable phenotype, downregulation of AtEHD1 caused retardation of entry of endocytosed material into plant cells.

Intermediate organelles of the plant secretory pathway: identity and function

The secretory pathway of eukaryotic cells comprises a network of organelles that connects three large membranes, the plasma membrane, the vacuole and the endoplasmic reticulum. The Golgi apparatus and the various post-Golgi organelles that control vacuolar sorting, secretion and endocytosis can be regarded as intermediate organelles of the endocytic and biosynthetic routes. Many processes in the secretory pathway have evolved differently in plants and cannot be studied using yeast or mammalian cells as models. The best characterized organelles are the Golgi apparatus and the prevacuolar compartment, but recent work has shed light on the role of the trans Golgi network, which has to be regarded as a separate organelle in plants. In this study, we wish to highlight recent findings regarding the late secretory pathway and its crosstalk with the early secretory pathway as well as the endocytic route in plants. Recently published findings and suggested models are discussed within the context of known features of the equivalent pathway in other eukaryotes.

Vesicle trafficking dynamics and visualization of zones of exocytosis and endocytosis in tobacco pollen tubes

Pollen tubes are one of the fastest growing eukaryotic cells. Rapid anisotropic growth is supported by highly active exocytosis and endocytosis at the plasma membrane, but the subcellular localization of these sites is unknown. To understand molecular processes involved in pollen tube growth, it is crucial to identify the sites of vesicle localization and trafficking. This report presents novel strategies to identify exocytic and endocytic vesicles and to visualize vesicle trafficking dynamics, using pulse-chase labelling with styryl FM dyes and refraction-free high-resolution time-lapse differential interference contrast microscopy. These experiments reveal that the apex is the site of endocytosis and membrane retrieval, while exocytosis occurs in the zone adjacent to the apical dome. Larger vesicles are internalized along the distal pollen tube. Discretely sized vesicles that differentially incorporate FM dyes accumulate in the apical, subapical, and distal regions. Previous work established that pollen tube growth is strongly correlated with hydrodynamic flux and cell volume status. In this report, it is shown that hydrodynamic flux can selectively increase exocytosis or endocytosis. Hypotonic treatment and cell swelling stimulated exocytosis and attenuated endocytosis, while hypertonic treatment and cell shrinking stimulated endocytosis and inhibited exocytosis. Manipulation of pollen tube apical volume and membrane remodelling enabled fine-mapping of plasma membrane dynamics and defined the boundary of the growth zone, which results from the orchestrated action of endocytosis at the apex and along the distal tube and exocytosis in the subapical region. This report provides crucial spatial and temporal details of vesicle trafficking and anisotropic growth.