Distinct endocytic pathways identified in tobacco pollen tubes using charged nanogold

Nanomaterials in plant physiology: Main effects in normal and under temperature stress

Global climate change and high population growth rates lead to problems of food security and environmental pollution, which require new effective methods to increase yields and stress tolerance of important crops. Nowadays the question of using artificial chemicals is very relevant in theoretical and practical terms. It is important that such substances in low concentrations protect plants under stress conditions, but at the same time inflict minimal damage on the environment and human health. Nanotechnology, which allows the production of a wide range of nanomaterials (NM), provides novel techniques in this direction. NM include structures less than 100 nm. The review presents data on the methods of NM production, their properties, pathways for arrival in plants and their use in human life. It is shown that NM, due to their unique physical and chemical properties, can cross biological barriers and accumulate in cells of live organisms. The influence of NM on plant organism can be both positive and negative, depending on the NM chemical nature, their size and dose, the object of study, and the environmental conditions. This review provides a comparative analysis of the effect of artificial metal nanoparticles (NPm), the commonly employed NMs in plant physiology, on two important aspects of plant life: photosynthetic apparatus activity and antioxidant system function. According to studies, NM affect not only the functional activity of photosynthetic apparatus, but also structural organization of chloroplats. In addition, the literature analysis reflects the dual action of NM on oxidative processes, and antioxidant status of plants. These facts considerably complicate the ideas about possible mechanisms and further use of NPm in biology. In this regard, data on the effects of NM on plants under abiotic stressors are of great interest. Separate section is devoted to the use of NM as adaptogens that increase plant stress tolerance to unfavorable temperatures. Possible mechanisms of NM effects on plants are discussed, as well as the strategies for their further use in basic science and sustainable agriculture.

Rho of Plants patterning: linking mathematical models and molecular diversity

ROPs (Rho of Plants) are plant specific small GTPases involved in many membrane patterning processes and play important roles in the establishment and communication of cell polarity. These small GTPases can produce a wide variety of patterns, ranging from a single cluster in tip-growing root hairs and pollen tubes to an oriented stripe pattern controlling protoxylem cell wall deposition. For an understanding of what controls these various patterns, models are indispensable. Consequently, many modelling studies on small GTPase patterning exist, often focusing on yeast or animal cells. Multiple patterns occurring in plants, however, require the stable co-existence of multiple active ROP clusters, which does not occur with the most common yeast/animal models. The possibility of such patterns critically depends on the precise model formulation. Additionally, different small GTPases are usually treated interchangeably in models, even though plants possess two types of ROPs with distinct molecular properties, one of which is unique to plants. Furthermore, the shape and even the type of ROP patterns may be affected by the cortical cytoskeleton, and cortex composition and anisotropy differ dramatically between plants and animals. Here, we review insights into ROP patterning from modelling efforts across kingdoms, as well as some outstanding questions arising from these models and recent experimental findings.

Spatiotemporal organization and correlation of tip-focused exocytosis and endocytosis in regulating pollen tube tip growth

Pollen tube polar growth is a key cellular process during plant fertilization and is regulated by tip-focused exocytosis and endocytosis. However, the spatiotemporal dynamics and localizations of apical exocytosis and endocytosis in the tip region are still a matter of debate. Here, we use a refined spinning-disk confocal microscope coupled with fluorescence recovery after photobleaching for sustained live imaging and quantitative analysis of rapid vesicular activities in growing pollen tube tips. We traced and analyzed the occurrence site of exocytic plasma membrane-targeting of Arabidopsis secretory carrier membrane protein 4 and its subsequent endocytosis in tobacco pollen tube tips. We demonstrated that the pollen tube apex is the site for both vesicle polar exocytic fusion and endocytosis to take place. In addition, we disrupted either tip-focused exocytosis or endocytosis and found that their dynamic activities are closely correlated with one another basing on the spatial organization of actin fringe. Collectively, our findings attempt to propose a new exocytosis and endocytosis-coordinated yin-yang working model underlying the apical membrane organization and dynamics during pollen tube tip growth.

Going with the membrane flow: the impact of polarized secretion on bulk plasma membrane flows

Even the simplest cells show a remarkable degree of intracellular patterning. Like developing multicellular organisms, single cells break symmetry to establish polarity axes, pattern their cortex and interior, and undergo morphogenesis to acquire sometimes complex shapes. Symmetry-breaking and molecular patterns can be established through coupling of negative and positive feedback reactions in biochemical reaction-diffusion systems. Physical forces, perhaps best studied in the contraction of the metazoan acto-myosin cortex, which induces cortical and cytoplasmic flows, also serve to pattern-associated components. A less investigated physical perturbation is the in-plane flow of plasma membrane material caused by membrane trafficking. In this review, we discuss how bulk membrane flows can be generated at sites of active polarized secretion and growth, how they affect the distribution of membrane-associated proteins, and how they may be harnessed for patterning and directional movement in cells across the tree of life.

Sterols and Sphingolipids as New Players in Cell Wall Building and Apical Growth of Nicotiana tabacum L. Pollen Tubes

Pollen tubes are tip-growing cells that create safe routes to convey sperm cells to the embryo sac for double fertilization. Recent studies have purified and biochemically characterized detergent-insoluble membranes from tobacco pollen tubes. These microdomains, called lipid rafts, are rich in sterols and sphingolipids and are involved in cell polarization in organisms evolutionarily distant, such as fungi and mammals. The presence of actin in tobacco pollen tube detergent-insoluble membranes and the preferential distribution of these domains on the apical plasma membrane encouraged us to formulate the intriguing hypothesis that sterols and sphingolipids could be a "trait d'union" between actin dynamics and polarized secretion at the tip. To unravel the role of sterols and sphingolipids in tobacco pollen tube growth, we used squalestatin and myriocin, inhibitors of sterol and sphingolipid biosynthesis, respectively, to determine whether lipid modifications affect actin fringe morphology and dynamics, leading to changes in clear zone organization and cell wall deposition, thus suggesting a role played by these lipids in successful fertilization.

Advances in structural, spatial, and temporal mechanics of plant endocytosis

Endocytic trafficking underlies processes essential for plant growth and development, including the perception of and response to abiotic and extracellular stimuli, post-Golgi and exocytic trafficking, and cytokinesis. Protein adaptors and regulatory factors of clathrin-mediated endocytosis that contribute to the formation of endocytic clathrin-coated vesicles are evolutionarily conserved. Yet, work of the last ten years has identified differences between the endocytic mechanisms of plants and Opisthokonts involving the endocytic adaptor TPLATE complex, the requirement of actin during CME, and the function of clathrin-independent endocytosis in the uptake of plant-specific plasma membrane proteins. Here, we review clathrin-mediated and -independent pathways in plants and describe recent advances enabled by new proteomic and imaging methods, and conditional perturbation of endocytosis. In addition, we summarize the formation and trafficking of clathrin-coated vesicles based on temporal and structural data garnered from high-resolution quantitative imaging studies. Finally, new information about the cross-talk between endocytosis and other endomembrane trafficking pathways and organelles will also be discussed.

MATE-Type Proteins Are Responsible for Isoflavone Transportation and Accumulation in Soybean Seeds

Soybeans are nutritionally important as human food and animal feed. Apart from the macronutrients such as proteins and oils, soybeans are also high in health-beneficial secondary metabolites and are uniquely enriched in isoflavones among food crops. Isoflavone biosynthesis has been relatively well characterized, but the mechanism of their transportation in soybean cells is largely unknown. Using the yeast model, we showed that GmMATE1 and GmMATE2 promoted the accumulation of isoflavones, mainly in the aglycone forms. Using the tobacco BrightYellow-2 (BY-2) cell model, GmMATE1 and GmMATE2 were found to be localized in the vacuolar membrane. Such subcellular localization supports the notion that GmMATE1 and GmMATE2 function by compartmentalizing isoflavones in the vacuole. Expression analyses showed that GmMATE1 was mainly expressed in the developing soybean pod. Soybean mutants defective in GmMATE1 had significantly reduced total seed isoflavone contents, whereas the overexpression of GmMATE1 in transgenic soybean promoted the accumulation of seed isoflavones. Our results showed that GmMATE1, and possibly also GmMATE2, are bona fide isoflavone transporters that promote the accumulation of isoflavones in soybean seeds.

A potent endocytosis inhibitor Ikarugamycin up-regulates TNF production

Ikarugamycin (IK) is an antibiotic which has been reported to have a variety of functions, such as inhibition of clathrin-mediated endocytosis (CME), anti-tumor effects and regulation of the immune system. Whether IK influences cytokine production is poorly understood. We have investigated the relationship between IK and production of tumor necrosis factor-α (TNF). TNF plays a pivotal role in pathogenesis of many diseases. Although the dynamics of soluble TNF (sTNF) has been widely explored so far, the functions of the membrane form of TNF (mTNF) have not been fully elucidated. We demonstrated that IK increases the amount of mTNF and prolongs the duration of TNF expression. This effect is unrelated to the shedding activity of disintegrin and metalloproteinase domain-containing protein 17 (ADAM 17). Our results revealed that there is a mechanism to terminate inflammation at the cellular level which IK dysregulates. Furthermore, IK can be a tool to study TNF signaling due to its effect of increasing mTNF expression.

Fluid-phase and membrane markers reveal spatio-temporal dynamics of membrane traffic and repair in the green alga Chara australis

We investigated the mechanisms and the spatio-temporal dynamics of fluid-phase and membrane internalization in the green alga Chara australis using fluorescent hydrazides markers alone, or in conjunction with styryl dyes. Using live-cell imaging, immunofluorescence and inhibitor studies we revealed that both fluid-phase and membrane dyes were actively taken up into the cytoplasm by clathrin-mediated endocytosis and stained various classes of endosomes including brefeldin A- and wortmannin-sensitive organelles (trans-Golgi network and multivesicular bodies). Uptake of fluorescent hydrazides was poorly sensitive to cytochalasin D, suggesting that actin plays a minor role in constitutive endocytosis in Chara internodal cells. Sequential pulse-labelling experiments revealed novel aspects of the temporal progression of endosomes in Chara internodal cells. The internalized fluid-phase marker distributed to early compartments within 10 min from dye exposure and after about 30 min, it was found almost exclusively in late endocytic compartments. Notably, fluid cargo consecutively internalized at time intervals of more than 1h, was not targeted to the same vesicular structures, but was sorted into distinct late compartments. We further found that fluorescent hydrazide dyes distributed not only to rapidly recycling endosomes but also to long-lived compartments that participated in plasma membrane repair after local laser injury. Our approach highlights the benefits of combining different fluid-phase markers in conjunction with membrane dyes in simultaneous and sequential application modus for investigating vesicle traffic, especially in organisms, which are still refractory to genetic transformation like characean algae.

Global Identification of ANTH Genes Involved in Rice Pollen Germination and Functional Characterization of a Key Member, OsANTH3

Pollen in angiosperms plays a critical role in double fertilization by germinating and elongating pollen tubes rapidly in one direction to deliver sperm. In this process, the secretory vesicles deliver cell wall and plasma membrane materials, and excessive materials are sequestered via endocytosis. However, endocytosis in plants is poorly understood. AP180 N-terminal homology (ANTH) domain-containing proteins function as adaptive regulators for clathrin-mediated endocytosis in eukaryotic systems. Here, we identified 17 ANTH domain-containing proteins from rice based on a genome-wide investigation. Motif and phylogenomic analyses revealed seven asparagine-proline-phenylalanine (NPF)-rich and 10 NPF-less subgroups of these proteins, as well as various clathrin-mediated endocytosis-related motifs in their C-terminals. To investigate their roles in pollen germination, we performed meta-expression analysis of all genes encoding ANTH domain-containing proteins in Oryza sativa (OsANTH genes) in anatomical samples, including pollen, and identified five mature pollen-preferred OsANTH genes. The subcellular localization of four OsANTH proteins that were preferentially expressed in mature pollen can be consistent with their role in endocytosis in the plasma membrane. Of them, OsANTH3 represented the highest expression in mature pollen. Functional characterization of OsANTH3 using T-DNA insertional knockout and gene-edited mutants revealed that a mutation in OsANTH3 decreased seed fertility by reducing the pollen germination percentage in rice. Thus, our study suggests OsANTH3-mediated endocytosis is important for rice pollen germination.

Protein Analysis of Pollen Tubes after the Treatments of Membrane Trafficking Inhibitors Gains Insights on Molecular Mechanism Underlying Pollen Tube Polar Growth

Pollen tube elongation is characterized by a highly-polarized tip growth process dependent on an efficient vesicular transport system and largely mobilized by actin cytoskeleton. Pollen tubes are an ideal model system to study exocytosis, endocytosis, membrane recycling, and signaling network coordinating cellular processes, structural organization and vesicular trafficking activities required for tip growth. Proteomic analysis was applied to identify Nicotiana tabacum Differentially Abundant Proteins (DAPs) after in vitro pollen tube treatment with membrane trafficking inhibitors Brefeldin A, Ikarugamycin and Wortmannin. Among roughly 360 proteins separated in two-dimensional gel electrophoresis, a total of 40 spots visibly changing between treated and control samples were identified by MALDI-TOF MS and LC-ESI-MS/MS analysis. The identified proteins were classified according to biological processes, and most proteins were related to pollen tube energy metabolism, including ammino acid synthesis and lipid metabolism, structural features of pollen tube growth as well modification and actin cytoskeleton organization, stress response, and protein degradation. In-depth analysis of proteins corresponding to energy-related pathways revealed the male gametophyte to be a reliable model of energy reservoir and dynamics.

A Complex Journey: Cell Wall Remodeling, Interactions, and Integrity During Pollen Tube Growth

In flowering plants, pollen tubes undergo a journey that starts in the stigma and ends in the ovule with the delivery of the sperm cells to achieve double fertilization. The pollen cell wall plays an essential role to accomplish all the steps required for the successful delivery of the male gametes. This extended path involves female tissue recognition, rapid hydration and germination, polar growth, and a tight regulation of cell wall synthesis and modification, as its properties change not only along the pollen tube but also in response to guidance cues inside the pistil. In this review, we focus on the most recent advances in elucidating the molecular mechanisms involved in the regulation of cell wall synthesis and modification during pollen germination, pollen tube growth, and rupture.

Experimental toolbox for quantitative evaluation of clathrin-mediated endocytosis in the plant model Arabidopsis

Clathrin-mediated endocytosis (CME) is a crucial cellular process implicated in many aspects of plant growth, development, intra- and intercellular signaling, nutrient uptake and pathogen defense. Despite these significant roles, little is known about the precise molecular details of how CME functions in planta To facilitate the direct quantitative study of plant CME, we review current routinely used methods and present refined, standardized quantitative imaging protocols that allow the detailed characterization of CME at multiple scales in plant tissues. These protocols include: (1) an efficient electron microscopy protocol for the imaging of Arabidopsis CME vesicles in situ, thus providing a method for the detailed characterization of the ultrastructure of clathrin-coated vesicles; (2) a detailed protocol and analysis for quantitative live-cell fluorescence microscopy to precisely examine the temporal interplay of endocytosis components during single CME events; (3) a semi-automated analysis to allow the quantitative characterization of global internalization of cargos in whole plant tissues; and (4) an overview and validation of useful genetic and pharmacological tools to interrogate the molecular mechanisms and function of CME in intact plant samples.This article has an associated First Person interview with the first author of the paper.

Exocytosis and endocytosis: coordinating and fine-tuning the polar tip growth domain in pollen tubes

Pollen tubes rapidly elongate, penetrate, and navigate through multiple female tissues to reach ovules for sperm delivery by utilizing a specialized form of polar growth known as tip growth. This process requires a battery of cellular activities differentially occurring at the apical growing region of the plasma membrane (PM), such as the differential cellular signaling involving calcium (Ca2+), phospholipids, and ROP-type Rho GTPases, fluctuation of ions and pH, exocytosis and endocytosis, and cell wall construction and remodeling. There is an emerging understanding of how at least some of these activities are coordinated and/or interconnected. The apical active ROP modulates exocytosis to the cell apex for PM and cell wall expansion differentially occurring at the tip. The differentiation of the cell wall involves at least the preferential distribution of deformable pectin polymers to the apex and non-deformable pectin polymers to the shank of pollen tubes, facilitating the apical cell expansion driven by high internal turgor pressure. Recent studies have generated inroads into how the ROP GTPase-based intracellular signaling is coordinated spatiotemporally with the external wall mechanics to maintain the tubular cell shape and how the apical cell wall mechanics are regulated to allow rapid tip growth while maintaining the cell wall integrity under the turgor pressure. Evidence suggests that exocytosis and endocytosis play crucial but distinct roles in this spatiotemporal coordination. In this review, we summarize recent advances in the regulation and coordination of the differential pectin distribution and the apical domain of active ROP by exocytosis and endocytosis in pollen tubes.

The Biological and Chemical Diversity of Tetramic Acid Compounds from Marine-Derived Microorganisms

Tetramic acid (pyrrolidine-2,4-dione) compounds, isolated from a variety of marine and terrestrial organisms, have attracted considerable attention for their diverse, challenging structural complexity and promising bioactivities. In the past decade, marine-derived microorganisms have become great repositories of novel tetramic acids. Here, we discuss the biological activities of 277 tetramic acids of eight classifications (simple 3-acyl tetramic acids, 3-oligoenoyltetramic acids, 3-decalinoyltetramic acid, 3-spirotetramic acids, macrocyclic tetramic acids, N-acylated tetramic acids, α-cyclopiazonic acid-type tetramic acids, and other tetramic acids) from marine-derived microbes, including fungi, actinobacteria, bacteria, and cyanobacteria, as reported in 195 research studies up to 2019.

Exocytosis and Endocytosis: Yin-Yang Crosstalk for Sculpting a Dynamic Growing Pollen Tube Tip

The growing pollen tube has become one of the most fascinating model cell systems for investigations into cell polarity and polar cell growth in plants. Rapidly growing pollen tubes achieve tip-focused cell expansion by vigorous anterograde exocytosis, through which various newly synthesized macromolecules are directionally transported and deposited at the cell apex. Meanwhile, active retrograde endocytosis counter balances the exocytosis at the tip which is believed to recycle the excessive exocytic components for multiple rounds of secretion. Therefore, apical exocytosis and endocytosis are the frontline cellular processes which drive the polar growth of pollen tubes, although they represent opposite vesicular trafficking events with distinct underpinning mechanisms. Nevertheless, the molecular basis governing the spatiotemporal crosstalk and counterbalance of exocytosis and endocytosis during pollen tube polarization and growth remains elusive. Here we discuss recent insight into exocytosis and endocytosis in sculpturing high rates of polarized pollen tube growth. In addition, we especially introduce the novel integration of mathematical modeling in uncovering the mysteries of cell polarity and polar cell growth.

A Peptoid Delivers CoQ-derivative to Plant Mitochondria via Endocytosis

Controlled delivery of molecules interfering specifically with target activities in a cell of interest can be a powerful tool for experimental manipulation, because it can be administered at a defined time point and does not require genetic transformation, which in some systems is difficult and time consuming. Peptides as versatile tools that can be tailored for binding numerous binding partners, are of special interest. However, their passage through membranes, their intracellular targeting, and their sensitivity to proteases is limiting. The use of peptoids, where cationic amino-acid side chains are linked to nitrogen (rather than to carbon) of the peptide bond, can circumvent these limitations, because they are not cleavable by proteases. In the current work, we provide a proof-of-concept that such Trojan Peptoids, the plant PeptoQ, can be used to target a functional cargo (i.e. a rhodamine-labelled peptoid and a coenzyme Q10 derivative) into mitochondria of tobacco BY-2 cells as experimental model. We show that the uptake is specific for mitochondria, rapid, dose-dependent, and requires clathrin-mediated endocytosis, as well as actin filaments, while microtubules seem to be dispensable. Viability of the treated cells is not affected, and they show better survival under salt stress, a condition that perturbs oxidative homeostasis in mitochondria. In congruence with improved homeostasis, we observe that the salt induced accumulation of superoxide is mitigated and even inverted by pretreatment with PeptoQ. Using double labelling with appropriate fluorescent markers, we show that targeting of this Trojan Peptoid to the mitochondria is not based on a passage through the plasma membrane (as thought hitherto), but on import via endocytotic vesicles and subsequent accumulation in the mitochondrial intermembrane space, from where it can enter the matrix, e.g. when the permeability of the inner membrane is increased under salt stress.

Mycorrhizal Fungi Respond to Resource Inequality by Moving Phosphorus from Rich to Poor Patches across Networks

The world's ecosystems are characterized by an unequal distribution of resources [1]. Trade partnerships between organisms of different species-mutualisms-can help individuals cope with such resource inequality [2-4]. Trade allows individuals to exchange commodities they can provide at low cost for resources that are otherwise impossible or more difficult to access [5, 6]. However, as resources become increasingly patchy in time or space, it is unknown how organisms alter their trading strategies [7, 8]. Here, we show how a symbiotic fungus mediates trade with a host root in response to different levels of resource inequality across its network. We developed a quantum-dot-tracking technique to quantify phosphorus-trading strategies of arbuscular mycorrhizal fungi simultaneously exposed to rich and poor resource patches. By following fluorescent nanoparticles of different colors across fungal networks, we determined where phosphorus was hoarded, relocated, and transferred to plant hosts. We found that increasing exposure to inequality stimulated trade. Fungi responded to high resource variation by (1) increasing the total amount of phosphorus distributed to host roots, (2) decreasing allocation to storage, and (3) differentially moving resources within the network from rich to poor patches. Using single-particle tracking and high-resolution video, we show how dynamic resource movement may help the fungus capitalize on value differences across the trade network, physically moving resources to areas of high demand to gain better returns. Such translocation strategies can help symbiotic organisms cope with exposure to resource inequality.

Plant AP180 N-Terminal Homolog Proteins Are Involved in Clathrin-Dependent Endocytosis during Pollen Tube Growth in Arabidopsis thaliana

Polarized cell growth in plants is maintained under the strict control and exquisitely choreographed balance of exocytic and endocytic membrane trafficking. The pollen tube has become a model system for rapid polar growth in which delivery of cell wall material and membrane recycling are controlled by membrane trafficking. Endocytosis plays an important role that is poorly understood. The plant AP180 N-Terminal Homolog (ANTH) proteins are putative homologs of Epsin 1 that recruits clathrin to phosphatidylinositol 4, 5-bisphosphate (PIP2) containing membranes to facilitate vesicle budding during endocytosis. Two Arabidopsis ANTH encoded by the genes AtAP180 and AtECA2 are highly expressed in pollen tubes. Pollen tubes from T-DNA inserted knockout mutant lines display significant morphological defects and unique pectin deposition. Fluorescent tagging reveals organization into dynamic foci located at the lateral flanks of the pollen tube. This precisely defined subapical domain coincides which clathrin-mediated endocytosis (CME) and PIP2 localization. Using a liposome-protein binding test, we showed that AtECA2 protein and ANTH domain recombinant proteins have strong affinity to PIP2 and phosphatidic acid containing liposomes in vitro. Taken together these data suggest that Arabidopsis ANTH proteins may play an important role in CME, proper cell wall assembly and morphogenesis.

Endocytosis acts as transport pathway in wood

In trees, dead and living cells of secondary xylem (wood) function collectively, rendering cell-to-cell communication challenging. Water and solutes are transported over long distances from the roots to the above-ground organs via vessels, the main component of wood, and then radially over short distances to the neighboring cells. This enables proper functioning of trees and integrates whole-plant activity. In this study, tracer loading, immunolocalization experiments and inhibitor assays were used to decipher the mechanisms enabling transport in wood of Acer pseudoplatanus (maple), Fraxinus excelsior (ash) and Populus tremula × tremuloides (poplar) trees. We show that tracer uptake from dead water-conducting vessels, elements of the apoplasm, to living vessel-associated cells (VACs) of the xylem parenchyma of the symplasm system proceeds via the endocytic pathway, including clathrin-mediated and clathrin-independent processes. These findings enhance our understanding of the transport pathways in complex wood tissue, providing experimental evidence of the involvement of VACs and endocytosis in radial uptake from vessels.

Drought stress stimulates endocytosis and modifies membrane lipid order of rhizodermal cells of Medicago truncatula in a genotype-dependent manner

BACKGROUND

Drought stress negatively affects plant growth and productivity. Plants sense soil drought at the root level but the underlying mechanisms remain unclear. At the cell level, we aim to reveal the short-term root perception of drought stress through membrane dynamics.

RESULTS

In our study, 15 Medicago truncatula accessions were exposed to a polyethylene glycol (PEG)-induced drought stress, leading to contrasted ecophysiological responses, in particular related to root architecture plasticity. In the reference accession Jemalong A17, identified as drought susceptible, we analyzed lateral roots by imaging of membrane-localized fluorescent probes using confocal microscopy. We found that PEG stimulated endocytosis especially in cells belonging to the growth differentiation zone (GDZ). The mapping of membrane lipid order in cells along the root apex showed that membranes of root cap cells were more ordered than those of more differentiated cells. Moreover, PEG triggered a significant increase in membrane lipid order of rhizodermal cells from the GDZ. We initiated the membrane analysis in the drought resistant accession HM298, which did not reveal such membrane modifications in response to PEG.

CONCLUSIONS

Our data demonstrated that the plasma membranes of root cells from a susceptible genotype perceived drought stress by modulating their physical state both via a stimulation of endocytosis and a modification of the degree of lipid order, which could be proposed as mechanisms required for signal transduction.

Microtubules play a role in trafficking prevacuolar compartments to vacuoles in tobacco pollen tubes

Fine regulation of exocytosis and endocytosis plays a basic role in pollen tube growth. Excess plasma membrane secreted during pollen tube elongation is known to be retrieved by endocytosis and partially reused in secretory pathways through the Golgi apparatus. Dissection of endocytosis has enabled distinct degradation pathways to be identified in tobacco pollen tubes and has shown that microtubules influence the transport of plasma membrane internalized in the tip region to vacuoles. Here, we used different drugs affecting the polymerization state of microtubules together with SYP21, a marker of prevacuolar compartments, to characterize trafficking of prevacuolar compartments in Nicotiana tabacum pollen tubes. Ultrastructural and biochemical analysis showed that microtubules bind SYP21-positive microsomes. Transient transformation of pollen tubes with LAT52-YFP-SYP21 revealed that microtubules play a key role in the delivery of prevacuolar compartments to tubular vacuoles.

Nanostructures: between natural environment and medical practice

Nanoparticles (NPs) are small structures under 100 nm in dimension. Interrelationships among the morphological parameters and toxicity of NPs have been the focus of several investigations that assessed potential health risk in environmentally-exposed subjects and the realistic uses of NPs in medical practice. In the current review, we provide a summary of the cellular mechanisms of membrane-mediated transport, including old and novel molecules that transport nanostructures across cellular membranes. The effects of geochemical exposure to natural NPs are evaluated through epidemiological data and cancerous pathways activated by Fe2+ NPs. Specifically, we discuss screening for papillary thyroid carcinomas in the inhabitants of the Sicilian volcanic area surrounding Mount Etna to compare the incidence of thyroid carcinoma in this population. Lastly, considering the increased production of carbon nanotubes (CNTs), we examine the toxicity and potential use of these engineered NPs in drug delivery of an extensive amount of therapeutic and imaging molecules (theranosis) that can be conjugated to CNTs.

The REN4 rheostat dynamically coordinates the apical and lateral domains of Arabidopsis pollen tubes

The dynamic maintenance of polar domains in the plasma membrane (PM) is critical for many fundamental processes, e.g., polar cell growth and growth guidance but remains poorly characterized. Rapid tip growth of Arabidopsis pollen tubes requires dynamic distribution of active ROP1 GTPase to the apical domain. Here, we show that clathrin-mediated endocytosis (CME) coordinates lateral REN4 with apical ROP1 signaling. REN4 interacted with but antagonized active ROP1. REN4 also interacts and co-localizes with CME components, but exhibits an opposite role to CME, which removes both REN4 and active ROP1 from the PM. Mathematical modeling shows that REN4 restrains the spatial distribution of active ROP1 and is important for the robustness of polarity control. Hence our results indicate that REN4 acts as a spatiotemporal rheostat by interacting with ROP1 to initiate their removal from the PM by CME, thereby coordinating a dynamic demarcation between apical and lateral domains during rapid tip growth.

Endocytic pathways involved in PLGA nanoparticle uptake by grapevine cells and role of cell wall and membrane in size selection

PLGA NPs' cell uptake involves different endocytic pathways. Clathrin-independent endocytosis is the main internalization route. The cell wall plays a more prominent role than the plasma membrane in NPs' size selection. In the last years, many studies on absorption and cell uptake of nanoparticles by plants have been conducted, but the understanding of the internalization mechanisms is still largely unknown. In this study, polydispersed and monodispersed poly(lactic-co-glycolic) acid nanoparticles (PLGA NPs) were synthesized, and a strategy combining the use of transmission electron microscopy (TEM), confocal analysis, fluorescently labeled PLGA NPs, a probe for endocytic vesicles (FM4-64), and endocytosis inhibitors (i.e., wortmannin, ikarugamycin, and salicylic acid) was employed to shed light on PLGA NP cell uptake in grapevine cultured cells and to assess the role of the cell wall and plasma membrane in size selection of PLGA NPs. The ability of PLGA NPs to cross the cell wall and membrane was confirmed by TEM and fluorescence microscopy. A strong adhesion of PLGA NPs to the outer side of the cell wall was observed, presumably due to electrostatic interactions. Confocal microscopy and treatment with endocytosis inhibitors suggested the involvement of both clathrin-dependent and clathrin-independent endocytosis in cell uptake of PLGA NPs and the latter appeared to be the main internalization pathway. Experiments on grapevine protoplasts revealed that the cell wall plays a more prominent role than the plasma membrane in size selection of PLGA NPs. While the cell wall prevents the uptake of PLGA NPs with diameters over 50 nm, the plasma membrane can be crossed by PLGA NPs with a diameter of 500-600 nm.

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.

Clathrin in Chara australis: Molecular Analysis and Involvement in Charasome Degradation and Constitutive Endocytosis

Charasomes are convoluted plasma membrane domains in characean green algae. They are known to form in response to light via secretion of trans-Golgi network (TGN) vesicles and local inhibition of endocytosis. Charasomes are involved in the acidification of their aqueous environment, thereby facilitating photosynthesis-dependent carbon uptake. Charasome formation is reversible to allow cells to adapt to different light conditions. Here, we show that darkness-induced degradation of charasomes involves the formation of coated pits and coated vesicles. The darkness-induced degradation of charasomes can be inhibited by 1-2 μM ikarugamycin (IKA), which is considered to be a specific inhibitor of clathrin-dependent endocytosis. At a much higher concentration (100 μM), IKA also significantly reduces the internalization of styryl dyes, indicating uptake via clathrin-coated vesicles (CV). We are the first to present evidence, based on fine structure investigation, that IKA does not interfere with the formation of clathrin coat, but inhibits the detachment and/or further processing of coated vesicles. Both charasome degradation and constitutive endocytosis are also significantly inhibited by sterol complexing agents (methyl-ß-cyclodextrin and filipin). The absence of an additive effect, when applied together with IKA, suggests that charasome degradation and constitutive endocytosis (measured via styryl dye uptake) is not inhibited due to membrane retrieval via lipid rafts, but due to clathrin coat formation requirement of a specific set of sterols. Analysis of Chara australis clathrin proteins revealed two heavy chains and several light chains with sequence peculiarities, suggesting functional and/or species specific differences. The data obtained indicate that clathrin plays a central role not only in constitutive endocytosis but also in the degradation of charasomes, thereby representing a valuable system for studying targeted exo- and endocytosis.

Practical review on the use of synchrotron based micro- and nano- X-ray fluorescence mapping and X-ray absorption spectroscopy to investigate the interactions between plants and engineered nanomaterials

The increased use of engineered nanomaterials (ENMs) in commercial products and the continuous development of novel applications, is leading to increased intentional and unintentional release of ENMs into the environment with potential negative impacts. Particularly, the partition of nanoparticles (NPs) to waste water treatment plant (WWTP) sludge represents a potential threat to agricultural ecosystems where these biosolids are being applied as fertilizers. Moreover, several applications of ENMs in agriculture and soil remediation are suggested. Therefore, detailed risk assessment should be done to evaluate possible secondary negative impacts. The impact of ENMS on plants as central component of ecosystems and worldwide food supply is of primary relevance. Understanding the fate and physical and chemical modifications of NPs in plants and their possible transfer into food chains requires specialized analytical techniques. Due to the importance of both chemical and physical factors to consider for a better understanding of ENMs behavior in complex matrices, these materials can be considered a new type of analyte. An ideal technique should require minimal sample preparation, be non-destructive, and offer the best balance between sensitivity, chemical specificity, and spatial resolution. Synchrotron radiation (SR) techniques are particularly adapted to investigate localization and speciation of ENMs in plants. SR X-ray fluorescence mapping (SR-XFM) offers multi-elemental detection with lateral resolution down to the tens of nm, in combination with spatially resolved X-ray absorption spectroscopy (XAS) speciation. This review will focus on important methodological aspects regarding sample preparation, data acquisition and data analysis of SR-XFM/XAS to investigate interactions between plants and ENMs.

Effects of surface ligands on the uptake and transport of gold nanoparticles in rice and tomato

Nanotechnology is advancing rapidly and substantial amounts of nanomaterials are released into the environment. Plants are an essential base component of the ecological environment and play a critical role in the fate and transport of nanomaterials in the environment through plant uptake and bioaccumulation. In this study, plant uptake of gold nanoparticles (GNPs) functionalized with three types of short ligands [cysteamine (CA), cysteine (CYS) and thioglycolic acid (TGA)] and of nearly identical hydrodynamic size (8-12nm) was investigated in the major crops rice (Oryza sativa L.) and tomato (Solanum lycopersicum). Uptake and translocation of GNPs not only depended on particle surface charge, but were also related to the species of ligand on the GNPs. The negatively charged GNPs capped with the CYS ligand (GNP-CYS) were more efficiently absorbed in roots and transferred to shoots (including stems and leaves) than that of GNPs capped with CA and TGA. The absorption process of GNPs involved a combination of both clathrin-dependent and -independent mechanisms. The endocytosis of GNPs was strongly inhibited by wortmannin, suggesting that clathrin-independent endocytosis was an important pathway of nanoparticle internalization in plants. Competition experiments with a free ligand (CYS) showed that the CYS ligand probably facilitated the endocytosis process of GNPs and increased the internalization of GNP-CYS in plants. The results will aid understanding of the mechanisms of nanoparticle uptake and translocation in plants.

Ikarugamycin: A Natural Product Inhibitor of Clathrin-Mediated Endocytosis

Ikarugamycin (IKA) is a previously discovered antibiotic, which has been shown to inhibit the uptake of oxidized low-density lipoproteins in macrophages. Furthermore, several groups have previously used IKA to inhibit clathrin-mediated endocytosis (CME) in plant cell lines. However, detailed characterization of IKA has yet to be performed. Consequently, we performed biochemistry and microscopy experiments to further characterize the effects of IKA on CME. We show that IKA has an IC50 of 2.7 μm in H1299 cells and acutely inhibits CME, but not other endocytic pathways, in a panel of cell lines. Although long-term incubation with IKA has cytotoxic effects, the short-term inhibitory effects on CME are reversible. Thus, IKA can be a useful tool for probing routes of endocytic trafficking.

Arabidopsis COG Complex Subunits COG3 and COG8 Modulate Golgi Morphology, Vesicle Trafficking Homeostasis and Are Essential for Pollen Tube Growth

Spatially and temporally regulated membrane trafficking events incorporate membrane and cell wall materials into the pollen tube apex and are believed to underlie the rapid pollen tube growth. In plants, the molecular mechanisms and physiological functions of intra-Golgi transport and Golgi integrity maintenance remain largely unclear. The conserved oligomeric Golgi (COG) complex has been implicated in tethering of retrograde intra-Golgi vesicles in yeast and mammalian cells. Using genetic and cytologic approaches, we demonstrate that T-DNA insertions in Arabidopsis COG complex subunits, COG3 and COG8, cause an absolute, male-specific transmission defect that can be complemented by expression of COG3 and COG8 from the LAT52 pollen promoter, respectively. No obvious abnormalities in the microgametogenesis of the two mutants are observed, but in vitro and in vivo pollen tube growth are defective. COG3 or COG8 proteins fused to green fluorescent protein (GFP) label the Golgi apparatus. In pollen of both mutants, Golgi bodies exhibit altered morphology. Moreover, γ-COP and EMP12 proteins lose their tight association with the Golgi. These defects lead to the incorrect deposition of cell wall components and proteins during pollen tube growth. COG3 and COG8 interact directly with each other, and a structural model of the Arabidopsis COG complex is proposed. We believe that the COG complex helps to modulate Golgi morphology and vesicle trafficking homeostasis during pollen tube tip growth.

Male functions and malfunctions: the impact of phosphoinositides on pollen development and pollen tube growth

Phosphoinositides in pollen. In angiosperms, sexual reproduction is a series of complex biological events that facilitate the distribution of male generative cells for double fertilization. Angiosperms have no motile gametes, and the distribution units of generative cells are pollen grains, passively mobile desiccated structures, capable of delivering genetic material to compatible flowers over long distances and in an adverse environment. The development of pollen (male gametogenesis) and the formation of a pollen tube after a pollen grain has reached a compatible flower (pollen tube growth) are important aspects of plant developmental biology. In recent years, a wealth of information has been gathered about the molecular control of cell polarity, membrane trafficking and cytoskeletal dynamics underlying these developmental processes. In particular, it has been found that regulatory membrane phospholipids, such as phosphoinositides (PIs), are critical regulatory players, controlling key steps of trafficking and polarization. Characteristic features of PIs are the inositol phosphate headgroups of the lipids, which protrude from the cytosolic surfaces of membranes, enabling specific binding and recruitment of numerous protein partners containing specific PI-binding domains. Such recruitment is globally an early event in polarization processes of eukaryotic cells and also of key importance to pollen development and tube growth. Additionally, PIs serve as precursors of other signaling factors with importance to male gametogenesis. This review highlights the recent advances about the roles of PIs in pollen development and pollen function.

The cytoskeleton in the pollen tube

The cytoskeleton in pollen tubes has been intensively studied, because of its abundance and prominent roles and because the pollen tube is an excellent experimental system for cell biological studies. Pollen actin microfilaments (MFs) exist as multiple distinct populations, each participating in a specific cellular trafficking or organization process. Consequently, MFs are essential for pollen tube growth and are tightly regulated in response to various signals. Pollen microtubules (MTs) are non-essential and less characterized, but recent studies have implicated MTs in vesicle trafficking and cell wall construction in pollen tubes. This review summarizes recent advances in understanding the organization and regulation of both MFs and MTs and discusses their roles in cellular trafficking and the modulation of pollen-tube tip growth.

Vesicular trafficking and salinity responses in plants

Research spanning three decades has demonstrated that vesicles pinch off from the plasma membrane and traffic through the cytoplasm of plant cells, much as previously reported in animal cells. Although the well-conserved clathrin-mediated mechanism of endocytosis has been well characterized, relatively little is known about clathrin-independent pathways in plants. Modulation of endocytosis by both physical stimuli and chemical ligands has been reported in plants. Here, we review the effect of salinity-one of the most deleterious environmental assaults-on endocytosis and intracellular trafficking.

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.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Insight into S-RNase-based self-incompatibility in Petunia: recent findings and future directions

S-RNase-based self-incompatibility in Petunia is a self/non-self recognition system that allows the pistil to reject self-pollen to prevent inbreeding and to accept non-self pollen for outcrossing. Cloning of S-RNase in 1986 marked the beginning of nearly three decades of intensive research into the mechanism of this complex system. S-RNase was shown to be the sole female determinant in 1994, and the first male determinant, S-locus F-box protein1 (SLF1), was identified in 2004. It was discovered in 2010 that additional SLF proteins are involved in pollen specificity, and recently two S-haplotypes of Petunia inflata were found to possess 17 SLF genes based on pollen transcriptome analysis, further increasing the complexity of the system. Here, we first summarize the current understanding of how the interplay between SLF proteins and S-RNase in the pollen tube allows cross-compatible pollination, but results in self-incompatible pollination. We then discuss some of the aspects that are not yet elucidated, including uptake of S-RNase into the pollen tube, nature, and assembly of SLF-containing complexes, the biochemical basis for differential interactions between SLF proteins and S-RNase, and fate of non-self S-RNases in the pollen tube.

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.

Ion and lipid signaling in apical growth-a dynamic machinery responding to extracellular cues

Apical cell growth seems to have independently evolved throughout the major lineages of life. To a certain extent, so does our body of knowledge on the mechanisms regulating this morphogenetic process. Studies on pollen tubes, root hairs, rhizoids, fungal hyphae, even nerve cells, have highlighted tissue and cell specificities but also common regulatory characteristics (e.g., ions, proteins, phospholipids) that our focused research sometimes failed to grasp. The working hypothesis to test how apical cell growth is established and maintained have thus been shaped by the model organism under study and the type of methods used to study them. The current picture is one of a dynamic and adaptative process, based on a spatial segregation of components that network to achieve growth and respond to environmental (extracellular) cues. Here, we explore some examples of our live imaging research, namely on cyclic nucleotide gated ion channels, lipid kinases and syntaxins involved in exocytosis. We discuss how their spatial distribution, activity and concentration suggest that the players regulating apical cell growth may display more mobility than previously thought. Furthermore, we speculate on the implications of such perspective in our understanding of the mechanisms regulating apical cell growth and their responses to extracellular cues.

In vitro inhibition of incompatible pollen tubes in Nicotiana alata involves the uncoupling of the F-actin cytoskeleton and the endomembrane trafficking system

In the S-RNase-based self-incompatibility system, subcellular events occurring in the apical region of incompatible pollen tubes during the pollen rejection process are poorly understood. F-actin dynamics and endomembrane trafficking are crucial for polar growth, which is temporally and spatially controlled in the tip region of pollen tubes. Thus, we developed a simple in vitro assay to study the changes in the F-actin cytoskeleton and the endomembrane system at the apical region of incompatible pollen tubes in Nicotiana alata. Growth but not germination of pollen tubes of S c₁₀-, S₇₀-, and S₇₅-haplotypes was selectively inhibited by style extracts carrying the same haplotypes. Pollen F-actin cytoskeleton and endomembrane system, visualized by fluorescent markers, were normal during the initial 60 min of pollen culture in the presence of compatible and incompatible style extracts. Additional culture resulted in complete growth arrest and critical alterations in the integrity of the F-actin cytoskeleton and the endomembrane system of incompatible pollen tubes. The F-actin ring and the V-shaped zone disappeared from the apical region, while distorted F-actin cables and progressive formation of membrane aggregates evolved in the subapical region and the shank. The vacuolar network of incompatible pollen tubes invaded the tip region, but vacuolar membrane integrity remained mostly unaffected. The polar growth machinery of incompatible pollen tubes was uncoupled, as evidenced by the severe disruption of colocalization between the F-actin cytoskeleton and the endomembrane compartments. A model of pollen rejection integrating the main subcellular events occurring in incompatible pollen is discussed.

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.

RISAP is a TGN-associated RAC5 effector regulating membrane traffic during polar cell growth in tobacco

RAC/ROP GTPases coordinate actin dynamics and membrane traffic during polar plant cell expansion. In tobacco (Nicotiana tabacum), pollen tube tip growth is controlled by the RAC/ROP GTPase RAC5, which specifically accumulates at the apical plasma membrane. Here, we describe the functional characterization of RISAP, a RAC5 effector identified by yeast (Saccharomyces cerevisiae) two-hybrid screening. RISAP belongs to a family of putative myosin receptors containing a domain of unknown function 593 (DUF593) and binds via its DUF593 to the globular tail domain of a tobacco pollen tube myosin XI. It also interacts with F-actin and is associated with a subapical trans-Golgi network (TGN) compartment, whose cytoplasmic position at the pollen tube tip is maintained by the actin cytoskeleton. In this TGN compartment, apical secretion and endocytic membrane recycling pathways required for tip growth appear to converge. RISAP overexpression interferes with apical membrane traffic and blocks tip growth. RAC5 constitutively binds to the N terminus of RISAP and interacts in an activation-dependent manner with the C-terminal half of this protein. In pollen tubes, interaction between RAC5 and RISAP is detectable at the subapical TGN compartment. We present a model of RISAP regulation and function that integrates all these findings.

Dynamic, high precision targeting of growth modulating agents is able to trigger pollen tube growth reorientation

The pollen tube is the most rapidly growing cell in the plant kingdom and has the function to deliver the sperm cells for fertilization. The growing tip region of the cell behaves in a chemotropic manner to respond to the guidance cues emitted by the pistil and the female gametophyte, but how it perceives and responds to these directional triggers is virtually unknown. Quantitative assessment of chemotropic behavior can greatly be enhanced by the administration of pharmacological or other biologically active agents at subcellular precision, which is a technical challenge when the target area moves as it grows. We developed a laminar flow based microfluidic device that allows for continuous administration of two different solutions with a movable interface that permits the dynamic targeting of the growing pollen tube apex over prolonged periods of time. Asymmetric administration of calcium revealed that rather than following the highest calcium concentration as would be expected with simple chemotropic behavior, the pollen tube of Camellia targets an optimal concentration suggesting the presence of two superimposed mechanisms. Subcellular application of pectin methyl esterase (PME), an enzyme that modifies the growth behavior by rigidifying the pollen tube cell wall, caused the tube to turn away from the agent - providing important evidence for a previously proposed conceptual model of the growth mechanism.