Pollen tube growth oscillations and intracellular calcium levels are reversibly modulated by actin polymerization

Addition of Phenylboronic Acid to Malus domestica Pollen Tubes Alters Calcium Dynamics, Disrupts Actin Filaments and Affects Cell Wall Architecture

A key role of boron in plants is to cross-link the cell wall pectic polysaccharide rhamnogalacturonan-II (RG-II) through borate diester linkages. Phenylboronic acid (PBA) can form the same reversible ester bonds but cannot cross-link two molecules, so can be used as an antagonist to study the function of boron. This study aimed to evaluate the effect of PBA on apple (Malus domestica) pollen tube growth and the underlying regulatory mechanism. We observed that PBA caused an inhibition of pollen germination, tube growth and led to pollen tube morphological abnormalities. Fluorescent labeling, coupled with a scanning ion-selective electrode technique, revealed that PBA induced an increase in extracellular Ca²⁺ influx, thereby elevating the cytosolic Ca²⁺ concentration [Ca²⁺]c and disrupting the [Ca²⁺]c gradient, which is critical for pollen tube growth. Moreover the organization of actin filaments was severely perturbed by the PBA treatment. Immunolocalization studies and fluorescent labeling, together with Fourier-transform infrared analysis (FTIR) suggested that PBA caused an increase in the abundance of callose, de-esterified pectins and arabinogalactan proteins (AGPs) at the tip. However, it had no effect on the deposition of the wall polymers cellulose. These effects are similar to those of boron deficiency in roots and other organs, indicating that PBA can induce boron deficiency symptoms. The results provide new insights into the roles of boron in pollen tube development, which likely include regulating [Ca²⁺]c and the formation of the actin cytoskeleton, in addition to the synthesis and assembly of cell wall components.

Actin3 promoter reveals undulating F-actin bundles at shanks and dynamic F-actin meshworks at tips of tip-growing pollen tubes

The dynamic actin cytoskeleton of pollen tubes is both the driver of the tip growth and the organizer of cell polarity. In order to understand this fast re-arranging cytoskeletal system, we need reliable constructs expressed under relevant promoters. Here we are reporting that the Lifeact reporter, expressed under the pollen-specific Actin3 promoter, visualizes very dynamic F-actin elements both in germinating pollen grains and tip-growing pollen tubes. Importantly, we have documented very active actin polymerization at the cell periphery, especially in the bulging area during pollen germination and in the apical clear zone. Expression of the Lifeact reporter under control of the pollen-specific Actin3 promoter revealed 2 new aspects: (i) long F-actin bundles in pollen tube shanks are dynamic, showing undulating movements, (ii) subapical 'actin collars' or 'fringes' are absent.

The Cytoskeleton and Its Regulation by Calcium and Protons

Calcium and protons exert control over the formation and activity of the cytoskeleton, usually by modulating an associated motor protein or one that affects the structural organization of the polymer.

The ROP2-RIC7 pathway negatively regulates light-induced stomatal opening by inhibiting exocyst subunit Exo70B1 in Arabidopsis

Stomata are the tiny valves on the plant surface that mediate gas exchange between the plant and its environment. Stomatal opening needs to be tightly regulated to facilitate CO2 uptake and prevent excess water loss. Plant Rho-type (ROP) GTPase 2 (ROP2) is a molecular component of the system that negatively regulates light-induced stomatal opening. Previously, ROP-interactive Cdc42- and Rac-interactive binding motif-containing protein 7 (RIC7) was suggested to function downstream of ROP2. However, the underlying molecular mechanism remains unknown. To understand the mechanism by which RIC7 regulates light-induced stomatal opening, we analyzed the stomatal responses of ric7 mutant Arabidopsis plants and identified the target protein of RIC7 using a yeast two-hybrid screen. Light-induced stomatal opening was promoted by ric7 knockout, whereas it was inhibited by RIC7 overexpression, indicating that RIC7 negatively regulates stomatal opening in Arabidopsis. RIC7 interacted with exocyst subunit Exo70 family protein B1 (Exo70B1), a component of the vesicle trafficking machinery. RIC7 and Exo70B1 localized to the plasma membrane region under light or constitutively active ROP2 conditions. The knockout mutant of Exo70B1 and ric7/exo70b1 exhibited retarded light-induced stomatal opening. Our results suggest that ROP2 and RIC7 suppress excess stomatal opening by inhibiting Exo70B1, which most likely participates in the vesicle trafficking required for light-induced stomatal opening.

NADPH Oxidase-Dependent Superoxide Production in Plant Reproductive Tissues

In the life cycle of a flowering plant, the male gametophyte (pollen grain) produced in the anther reaches the stigmatic surface and initiates the pollen-pistil interaction, an important step in plant reproduction, which ultimately leads to the delivery of two sperm cells to the female gametophyte (embryo sac) inside the ovule. The pollen tube undergoes a strictly apical expansion characterized by a high growth rate, whose targeting should be tightly regulated. A continuous exchange of signals therefore takes place between the haploid pollen and diploid tissue of the pistil until fertilization. In compatible interactions, theses processes result in double fertilization to form a zygote (2n) and the triploid endosperm. Among the large number of signaling mechanisms involved, the redox network appears to be particularly important. Respiratory burst oxidase homologs (Rbohs) are superoxide-producing enzymes involved in a broad range of processes in plant physiology. In this study, we review the latest findings on understanding Rboh activity in sexual plant reproduction, with a particular focus on the male gametophyte from the anther development stages to the crowning point of fertilization. Rboh isoforms have been identified in both the male and female gametophyte and have proven to be tightly regulated. Their role at crucial points such as proper growth of pollen tube, self-incompatibility response and eventual fertilization is discussed.

The pH sensibility of actin-bundling LIM proteins is governed by the acidic properties of their C-terminal domain

Actin-bundling Arabidopsis LIM proteins are subdivided into two subfamilies differing in their pH sensitivity. Widely-expressed WLIMs are active under low and high physiologically-relevant pH conditions, whereas pollen-enriched PLIMs are inactivated by pH values above 6.8. By a domain swapping approach we identified the C-terminal (Ct) domain of PLIMs as the domain responsible for pH responsiveness. Remarkably, this domain conferred pH sensitivity to LIM proteins, when provided "in trans" (i.e., as a single, independent, peptide), indicating that it operates through the interaction with another domain. An acidic 6xc-Myc peptide functionally mimicked the Ct domain of PLIMs and efficiently inhibited LIM actin bundling activity under high pH conditions. Together, our data suggest a model where PLIMs are regulated by an intermolecular interaction between their acidic Ct domain and another, yet unidentified, domain.

Live imaging of inorganic phosphate in plants with cellular and subcellular resolution

Despite variable and often scarce supplies of inorganic phosphate (Pi) from soils, plants must distribute appropriate amounts of Pi to each cell and subcellular compartment to sustain essential metabolic activities. The ability to monitor Pi dynamics with subcellular resolution in live plants is, therefore, critical for understanding how this essential nutrient is acquired, mobilized, recycled, and stored. Fluorescence indicator protein for inorganic phosphate (FLIPPi) sensors are genetically encoded fluorescence resonance energy transfer-based sensors that have been used to monitor Pi dynamics in cultured animal cells. Here, we present a series of Pi sensors optimized for use in plants. Substitution of the enhanced yellow fluorescent protein component of a FLIPPi sensor with a circularly permuted version of Venus enhanced sensor dynamic range nearly 2.5-fold. The resulting circularly permuted FLIPPi sensor was subjected to a high-efficiency mutagenesis strategy that relied on statistical coupling analysis to identify regions of the protein likely to influence Pi affinity. A series of affinity mutants was selected with dissociation constant values of 0.08 to 11 mm, which span the range for most plant cell compartments. The sensors were expressed in Arabidopsis (Arabidopsis thaliana), and ratiometric imaging was used to monitor cytosolic Pi dynamics in root cells in response to Pi deprivation and resupply. Moreover, plastid-targeted versions of the sensors expressed in the wild type and a mutant lacking the PHOSPHATE TRANSPORT4;2 plastidic Pi transporter confirmed a physiological role for this transporter in Pi export from root plastids. These circularly permuted FLIPPi sensors, therefore, enable detailed analysis of Pi dynamics with subcellular resolution in live plants.

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.

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.

Arabidopsis thaliana calmodulin-like protein CML24 regulates pollen tube growth by modulating the actin cytoskeleton and controlling the cytosolic Ca(2+) concentration

Cytosolic free calcium ([Ca(2+)]cyt), which is essential during pollen germination and pollen tube growth, can be sensed by calmodulin-like proteins (CMLs). The Arabidopsis thaliana genome encodes over 50 CMLs, the physiological role(s) of most of which are unknown. Here we show that the gene AtCML24 acts as a regulator of pollen germination and pollen tube extension, since the pollen produced by loss-of-function mutants germinated less rapidly than that of wild-type (WT) plants, the rate of pollen tube extension was slower, and the final length of the pollen tube was shorter. The [Ca(2+)]cyt within germinated pollen and extending pollen tubes produced by the cml24 mutant were higher than their equivalents in WT plants, and pollen tube extension was less sensitive to changes in external [K(+)] and [Ca(2+)]. The pollen and pollen tubes produced by cml24 mutants were characterized by a disorganized actin cytoskeleton and lowered sensitivity to the action of latrunculin B. The observations support an interaction between CML24 and [Ca(2+)]cyt and an involvement of CML24 in actin organization, thereby affecting pollen germination and pollen tube elongation.

Trans-Golgi network localized small GTPase RabA1d is involved in cell plate formation and oscillatory root hair growth

BACKGROUND

Small Rab GTPases are important regulators of vesicular trafficking in plants. AtRabA1d, a member of the RabA1 subfamily of small GTPases, was previously found in the vesicle-rich apical dome of growing root hairs suggesting a role during tip growth; however, its specific intracellular localization and role in plants has not been well described.

RESULTS

The transient expression of 35S::GFP:RabA1d construct in Allium porrum and Nicotiana benthamiana revealed vesicular structures, which were further corroborated in stable transformed Arabidopsis thaliana plants. GFP-RabA1d colocalized with the trans-Golgi network marker mCherry-VTI12 and with early FM4-64-labeled endosomal compartments. Late endosomes and endoplasmic reticulum labeled with FYVE-DsRed and ER-DsRed, respectively, were devoid of GFP-RabA1d. The accumulation of GFP-RabA1d in the core of brefeldin A (BFA)-induced-compartments and the quantitative upregulation of RabA1d protein levels after BFA treatment confirmed the association of RabA1d with early endosomes/TGN and its role in vesicle trafficking. Light-sheet microscopy revealed involvement of RabA1d in root development. In root cells, GFP-RabA1d followed cell plate expansion consistently with cytokinesis-related vesicular trafficking and membrane recycling. GFP-RabA1d accumulated in disc-like structures of nascent cell plates, which progressively evolved to marginal ring-like structures of the growing cell plates. During root hair growth and development, GFP-RabA1d was enriched at root hair bulges and at the apical dome of vigorously elongating root hairs. Importantly, GFP-RabA1d signal intensity exhibited an oscillatory behavior in-phase with tip growth. Progressively, this tip localization dissapeared in mature root hairs suggesting a link between tip localization of RabA1d and root hair elongation. Our results support a RabA1d role in events that require vigorous membrane trafficking.

CONCLUSIONS

RabA1d is located in early endosomes/TGN and is involved in vesicle trafficking. RabA1d participates in both cell plate formation and root hair oscillatory tip growth. The specific GFP-RabA1d subcellular localization confirms a correlation between its specific spatio-temporal accumulation and local vesicle trafficking requirements during cell plate and root hair formation.

Sucrose concentration in the growth medium affects the cell wall composition of tobacco pollen tubes

The cell wall of pollen tubes is organized in both spatial and temporal order to allow the pollen tube to grow according to external conditions. The deposition of methyl-esterified and acid pectins in addition to callose/cellulose occurs according to a series of temporally succeeding events. In this work, we attempted to determine how the composition of the external growth medium (in terms of osmolarity) could affect the deposition of cell wall components. Pollen tubes of tobacco were grown in a hypotonic medium and then analyzed for the distribution of pectins and callose/cellulose [as well as for the distribution of the enzyme callose synthase (CALS)]. The data indicate that pollen tubes grown in a hypotonic medium show changes of the initial growth rate followed by modification of the deposition of acid pectins and, to a lesser extent, of CALS. These observations indicate that, under the osmolarity determined by the growth medium, pollen tubes adapt their cell wall to the changing conditions of growth.

Annexin5 plays a vital role in Arabidopsis pollen development via Ca2+-dependent membrane trafficking

The regulation of pollen development and pollen tube growth is a complicated biological process that is crucial for sexual reproduction in flowering plants. Annexins are widely distributed from protists to higher eukaryotes and play multiple roles in numerous cellular events by acting as a putative "linker" between Ca2+ signaling, the actin cytoskeleton and the membrane, which are required for pollen development and pollen tube growth. Our recent report suggested that downregulation of the function of Arabidopsis annexin 5 (Ann5) in transgenic Ann5-RNAi lines caused severely sterile pollen grains. However, little is known about the underlying mechanisms of the function of Ann5 in pollen. This study demonstrated that Ann5 associates with phospholipid membrane and this association is stimulated by Ca2+ in vitro. Brefeldin A (BFA) interferes with endomembrane trafficking and inhibits pollen germination and pollen tube growth. Both pollen germination and pollen tube growth of Ann5-overexpressing plants showed increased resistance to BFA treatment, and this effect was regulated by calcium. Overexpression of Ann5 promoted Ca2+-dependent cytoplasmic streaming in pollen tubes in vivo in response to BFA. Lactrunculin (LatB) significantly prohibited pollen germination and tube growth by binding with high affinity to monomeric actin and preferentially targeting dynamic actin filament arrays and preventing actin polymerization. Overexpression of Ann5 did not affect pollen germination or pollen tube growth in response to LatB compared with wild-type, although Ann5 interacts with actin filaments in a manner similar to some animal annexins. In addition, the sterile pollen phenotype could be only partially rescued by Ann5 mutants at Ca2+-binding sites when compared to the complete recovery by wild-type Ann5. These data demonstrated that Ann5 is involved in pollen development, germination and pollen tube growth through the promotion of endomembrane trafficking modulated by calcium. Our results provide reliable molecular mechanisms that underlie the function of Ann5 in pollen.

Divergent roles for maize PAN1 and PAN2 receptor-like proteins in cytokinesis and cell morphogenesis

Pangloss1 (PAN1) and PAN2 are leucine-rich repeat receptor-like proteins that function cooperatively to polarize the divisions of subsidiary mother cells (SMCs) during stomatal development in maize (Zea mays). PANs colocalize in SMCs, and both PAN1 and PAN2 promote polarization of the actin cytoskeleton and nuclei in these cells. Here, we show that PAN1 and PAN2 have additional functions that are unequal or divergent. PAN1, but not PAN2, is localized to cell plates in all classes of dividing cells examined. pan1 mutants exhibited no defects in cell plate formation or in the recruitment or removal of a variety of cell plate components; thus, they did not demonstrate a function for PAN1 in cytokinesis. PAN2, in turn, plays a greater role than PAN1 in directing patterns of postmitotic cell expansion that determine the shapes of mature stomatal subsidiary cells and interstomatal cells. Localization studies indicate that PAN2 impacts subsidiary cell shape indirectly by stimulating localized cortical actin accumulation and polarized growth in interstomatal cells. Localization of PAN1, Rho of Plants2, and PIN1a suggests that PAN2-dependent cell shape changes do not involve any of these proteins, indicating that PAN2 function is linked to actin polymerization by a different mechanism in interstomatal cells compared with SMCs. Together, these results demonstrate that PAN1 and PAN2 are not dedicated to SMC polarization but instead play broader roles in plant development. We speculate that PANs may function in all contexts to regulate polarized membrane trafficking either directly or indirectly via their influence on actin polymerization.

Ca2+-activated reactive oxygen species production by Arabidopsis RbohH and RbohJ is essential for proper pollen tube tip growth

In flowering plants, pollen germinates on the stigma and pollen tubes grow through the style to fertilize the ovules. Enzymatic production of reactive oxygen species (ROS) has been suggested to be involved in pollen tube tip growth. Here, we characterized the function and regulation of the NADPH oxidases RbohH and RbohJ (Respiratory burst oxidase homolog H and J) in pollen tubes in Arabidopsis thaliana. In the rbohH and rbohJ single mutants, pollen tube tip growth was comparable to that of the wild type; however, tip growth was severely impaired in the double mutant. In vivo imaging showed that ROS accumulation in the pollen tube was impaired in the double mutant. Both RbohH and RbohJ, which contain Ca(2+) binding EF-hand motifs, possessed Ca(2+)-induced ROS-producing activity and localized at the plasma membrane of the pollen tube tip. Point mutations in the EF-hand motifs impaired Ca(2+)-induced ROS production and complementation of the double mutant phenotype. We also showed that a protein phosphatase inhibitor enhanced the Ca(2+)-induced ROS-producing activity of RbohH and RbohJ, suggesting their synergistic activation by protein phosphorylation and Ca(2+). Our results suggest that ROS production by RbohH and RbohJ is essential for proper pollen tube tip growth, and furthermore, that Ca(2+)-induced ROS positive feedback regulation is conserved in the polarized cell growth to shape the long tubular cell.

Apoplastic calmodulin promotes self-incompatibility pollen tube growth by enhancing calcium influx and reactive oxygen species concentration in Pyrus pyrifolia

Calmodulin (CaM) has been associated with various physiological and developmental processes in plants, including pollen tube growth. In this study, we showed that CaM regulated the pear pollen tube growth in a concentration-dependent bi-phasic response. Using a whole-cell patch-clamp configuration, we showed that apoplastic CaM induced a hyperpolarization-activated calcium ion (Ca²⁺) current, and anti-CaM largely inhibited this type of Ca²⁺ current. Moreover, upon anti-CaM treatment, the reactive oxygen species (ROS) concentration decreased and actin filaments depolymerized in the pollen tube. Interestingly, CaM could partially rescue the inhibition of self-incompatible pear pollen tube growth. This phenotype could be mediated by CaM-enhanced pollen plasma membrane Ca²⁺ current, tip-localized ROS concentration and stabilized actin filaments. These data indicated that Ca²⁺, ROS and actin filaments were involved with CaM in regulating pollen tube growth and provide a potential way for overcoming pear self-incompatibility.

Dynamic secretion changes in the salt glands of the mangrove tree species Avicennia officinalis in response to a changing saline environment

The specialized salt glands on the epidermis of halophytic plants secrete excess salts from tissues by a mechanism that is poorly understood. We examined the salt glands as putative salt and water bi-regulatory units that can respond swiftly to altering environmental cues. The tropical mangrove tree species (Avicennia officinalis) is able to grow under fluctuating salinities (0.7-50.0 dS m(-1)) at intertidal zones, and its salt glands offer an excellent platform to investigate their dynamic responses under rapidly changing salinities. Utilizing a novel epidermal peel system, secretion profiles of hundreds of individual salt glands examined revealed that these glands could secrete when exposed to varying salinities. Notably, rhythmic fluctuations observed in secretion rates were reversibly inhibited by water channel (aquaporin) blocker, and two aquaporin genes (PIP and TIP) preferentially expressed in the salt gland cells were rapidly induced in response to increasing salt concentration. We propose that aquaporins are involved and contribute to the re-absorption of water during salt removal in Avicennia officinalis salt glands. This constitutes an adaptive feature that contributes to salt balance of trees growing in saline environments where freshwater availability is limited.

Control of cell wall extensibility during pollen tube growth

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

LlSR28 is involved in pollen germination by affecting filamentous actin dynamics

Alternative splicing plays important roles in gene regulation and contributes to protein complexity. Previous studies suggest that alternative splicing exists in members of the villin/gelsolin/fragmin superfamily. In this study, a serine/argine-rich (SR) protein cDNA with 28 kDa protein (LlSR28) was isolated from a lily (Lilium longiflorum) expression library. Protein domain analysis showed that LlSR28 had similar structures to Arabidopsis SR45 (AtSR45), and LlSR28 could complement the phenotype of loss of AtSR45 function. Therefore, overexpression of LlSR28 and AtSR45 mutant (atsr45-1) were used in the following experiments. Overexpression of LlSR28 in Arabidopsis completely inhibited pollen germination. In contrast, the pollen germination of atsr45-1 was earlier than that of wild-type. In addition, pollen of atsr45-1 contained less F-actin at the corresponding hydration stage during pollen germination compared to that of wild-type. Alternative splicing analysis showed that Arabidopsis villin1 (AtVLN1) transcript encoding the full-length protein was increased, and that encoding the truncated protein was decreased in atst45-1. Moreover, the mRNA expression level of other actin-binding proteins (ABPs) abundant in Arabidopsis pollen was also changed in atsr45-1. In conclusion, we hypothesize that LlSR28 alters F-actin dynamics probably through its alternative splicing activities to affect directly or indirectly the alternative splicing of AtVLN1 and the expression of different ABPs, which then affects the pollen germination.

Involvement of the sieve element cytoskeleton in electrical responses to cold shocks

This study dealt with the visualization of the sieve element (SE) cytoskeleton and its involvement in electrical responses to local cold shocks, exemplifying the role of the cytoskeleton in Ca(2+)-triggered signal cascades in SEs. High-affinity fluorescent phalloidin as well as immunocytochemistry using anti-actin antibodies demonstrated a fully developed parietal actin meshwork in SEs. The involvement of the cytoskeleton in electrical responses and forisome conformation changes as indicators of Ca(2+) influx was investigated by the application of cold shocks in the presence of diverse actin disruptors (latrunculin A and cytochalasin D). Under control conditions, cold shocks elicited a graded initial voltage transient, ΔV1, reduced by external La(3+) in keeping with the involvement of Ca(2+) channels, and a second voltage transient, ΔV2. Cytochalasin D had no effect on ΔV1, while ΔV1 was significantly reduced with 500 nm latrunculin A. Forisome dispersion was triggered by cold shocks of 4°C or greater, which was indicative of an all-or-none behavior. Forisome dispersion was suppressed by incubation with latrunculin A. In conclusion, the cytoskeleton controls cold shock-induced Ca(2+) influx into SEs, leading to forisome dispersion and sieve plate occlusion in fava bean (Vicia faba).

Arabidopsis villins promote actin turnover at pollen tube tips and facilitate the construction of actin collars

Apical actin filaments are crucial for pollen tube tip growth. However, the specific dynamic changes and regulatory mechanisms associated with actin filaments in the apical region remain largely unknown. Here, we have investigated the quantitative dynamic parameters that underlie actin filament growth and disappearance in the apical regions of pollen tubes and identified villin as the major player that drives rapid turnover of actin filaments in this region. Downregulation of Arabidopsis thaliana VILLIN2 (VLN2) and VLN5 led to accumulation of actin filaments at the pollen tube apex. Careful analysis of single filament dynamics showed that the severing frequency significantly decreased, and the lifetime significantly increased in vln2 vln5 pollen tubes. These results indicate that villin-mediated severing is critical for turnover and departure of actin filaments originating in the apical region. Consequently, the construction of actin collars was affected in vln2 vln5 pollen tubes. In addition to the decrease in severing frequency, actin filaments also became wavy and buckled in the apical cytoplasm of vln2 vln5 pollen tubes. These results suggest that villin confers rigidity upon actin filaments. Furthermore, an observed decrease in skewness of actin filaments in the subapical region of vln2 vln5 pollen tubes suggests that villin-mediated bundling activity may also play a role in the construction of actin collars. Thus, our data suggest that villins promote actin turnover at pollen tube tips and facilitate the construction of actin collars.

Endocytic Pathways and Recycling in Growing Pollen Tubes

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

Distribution of transglutaminase in pear pollen tubes in relation to cytoskeleton and membrane dynamics

Transglutaminases (TGases) are ubiquitous enzymes that take part in a variety of cellular functions. In the pollen tube, cytoplasmic TGases are likely to be involved in the incorporation of primary amines at selected peptide-bound glutamine residues of cytosolic proteins (including actin and tubulin), while cell wall-associated TGases are believed to regulate pollen tube growth. Using immunological probes, we identified TGases associated with different subcellular compartments (cytosol, membranes, and cell walls). Binding of cytosolic TGase to actin filaments was shown to be Ca(2+) dependent. The membrane TGase is likely associated with both Golgi-derived structures and the plasma membrane, suggesting a Golgi-based exocytotic delivery of TGase. Association of TGase with the plasma membrane was also confirmed by immunogold transmission electron microscopy. Immunolocalization of TGase indicated that the enzyme was present in the growing region of pollen tubes and that the enzyme colocalizes with cell wall markers. Bidimensional electrophoresis indicated that different TGase isoforms were present in distinct subcellular compartments, suggesting either different roles or different regulatory mechanisms of enzyme activity. The application of specific inhibitors showed that the distribution of TGase in different subcellular compartments was regulated by both membrane dynamics and cytoskeleton integrity, suggesting that delivery of TGase to the cell wall requires the transport of membranes along cytoskeleton filaments. Taken together, these data indicate that a cytoplasmic TGase interacts with the cytoskeleton, while a different TGase isoform, probably delivered via a membrane/cytoskeleton-based transport system, is secreted in the cell wall of pear (Pyrus communis) pollen tubes, where it might play a role in the regulation of apical growth.

Regulation of Pollen Tube Growth by Transglutaminase

In pollen tubes, cytoskeleton proteins are involved in many aspects of pollen germination and growth, from the transport of sperm cells to the asymmetrical distribution of organelles to the deposition of cell wall material. These activities are based on the dynamics of the cytoskeleton. Changes to both actin filaments and microtubules are triggered by specific proteins, resulting in different organization levels suitable for the different functions of the cytoskeleton. Transglutaminases are enzymes ubiquitous in all plant organs and cell compartments. They catalyze the post-translational conjugation of polyamines to different protein targets, such as the cytoskeleton. Transglutaminases are suggested to have a general role in the interaction between pollen tubes and the extracellular matrix during fertilization and a specific role during the self-incompatibility response. In such processes, the activity of transglutaminases is enhanced, leading to the formation of cross-linked products (including aggregates of tubulin and actin). Consequently, transglutaminases are suggested to act as regulators of cytoskeleton dynamics. The distribution of transglutaminases in pollen tubes is affected by both membrane dynamics and the cytoskeleton. Transglutaminases are also secreted in the extracellular matrix, where they may take part in the assembly and/or strengthening of the pollen tube cell wall.

MAP18 regulates the direction of pollen tube growth in Arabidopsis by modulating F-actin organization

For fertilization to occur in plants, the pollen tube must be guided to enter the ovule via the micropyle. Previous reports have implicated actin filaments, actin binding proteins, and the tip-focused calcium gradient as key contributors to polar growth of pollen tubes; however, the regulation of directional pollen tube growth is largely unknown. We reported previously that Arabidopsis thaliana MICROTUBULE-ASSOCIATED PROTEIN18 (MAP18) contributes to directional cell growth and cortical microtubule organization. The preferential expression of MAP18 in pollen and in pollen tubes suggests that MAP18 also may function in pollen tube growth. In this study, we demonstrate that MAP18 functions in pollen tubes by influencing actin organization, rather than microtubule assembly. In vitro biochemical results indicate that MAP18 exhibits Ca(2+)-dependent filamentous (F)-actin-severing activity. Abnormal expression of MAP18 in map18 and MAP18 OX plants was associated with disorganization of the actin cytoskeleton in the tube apex, resulting in aberrant pollen tube growth patterns and morphologies, inaccurate micropyle targeting, and fewer fertilization events. Experiments with MAP18 mutants created by site-directed mutagenesis suggest that F-actin-severing activity is essential to the effects of MAP18 on pollen tube growth direction. Our study demonstrates that in Arabidopsis, MAP18 guides the direction of pollen tube growth by modulating actin filaments.

Cyclic nucleotide gated channels 7 and 8 are essential for male reproductive fertility

The Arabidopsis thaliana genome contains 20 CNGCs, which are proposed to encode cyclic nucleotide gated, non-selective, Ca²⁺-permeable ion channels. CNGC7 and CNGC8 are the two most similar with 74% protein sequence identity, and both genes are preferentially expressed in pollen. Two independent loss-of-function T-DNA insertions were identified for both genes and used to generate plant lines in which only one of the two alleles was segregating (e.g., cngc7-1+/-/cngc8-2-/- and cngc7-3-/-/cngc8-1+/-). While normal pollen transmission was observed for single gene mutations, pollen harboring mutations in both cngc7 and 8 were found to be male sterile (transmission efficiency reduced by more than 3000-fold). Pollen grains harboring T-DNA disruptions of both cngc7 and 8 displayed a high frequency of bursting when germinated in vitro. The male sterile defect could be rescued through pollen expression of a CNGC7 or 8 transgene including a CNGC7 with an N-terminal GFP-tag. However, rescue efficiencies were reduced ∼10-fold when the CNGC7 or 8 included an F to W substitution (F589W and F624W, respectively) at the junction between the putative cyclic nucleotide binding-site and the calmodulin binding-site, identifying this junction as important for proper functioning of a plant CNGC. Using confocal microscopy, GFP-CNGC7 was found to preferentially localize to the plasma membrane at the flanks of the growing tip. Together these results indicate that CNGC7 and 8 are at least partially redundant and provide an essential function at the initiation of pollen tube tip growth.

Apical myosin XI anticipates F-actin during polarized growth of Physcomitrella patens cells

Tip growth is essential for land colonization by bryophytes, plant sexual reproduction and water and nutrient uptake. Because this specialized form of polarized cell growth requires both a dynamic actin cytoskeleton and active secretion, it has been proposed that the F-actin-associated motor myosin XI is essential for this process. Nevertheless, a spatial and temporal relationship between myosin XI and F-actin during tip growth is not known in any plant cell. Here, we use the highly polarized cells of the moss Physcomitrella patens to show that myosin XI and F-actin localize, in vivo, at the same apical domain and that both signals fluctuate. Surprisingly, phase analysis shows that increase in myosin XI anticipates that of F-actin; in contrast, myosin XI levels at the tip fluctuate in identical phase with a vesicle marker. Pharmacological analysis using a low concentration of the actin polymerization inhibitor latrunculin B showed that the F-actin at the tip can be significantly diminished while myosin XI remains elevated in this region, suggesting that a mechanism exists to cluster myosin XI-associated structures at the cell's apex. In addition, this approach uncovered a mechanism for actin polymerization-dependent motility in the moss cytoplasm, where myosin XI-associated structures seem to anticipate and organize the actin polymerization machinery. From our results, we inferred a model where the interaction between myosin XI-associated vesicular structures and F-actin polymerization-driven motility function at the cell's apex to maintain polarized cell growth. We hypothesize this is a general mechanism for the participation of myosin XI and F-actin in tip growing cells.

FIMBRIN1 is involved in lily pollen tube growth by stabilizing the actin fringe

An actin fringe structure in the subapex plays an important role in pollen tube tip growth. However, the precise mechanism by which the actin fringe is generated and maintained remains largely unknown. Here, we cloned a 2606-bp full-length cDNA encoding a deduced 77-kD fimbrin-like protein from lily (Lilium longiflorum), named FIMBRIN1 (FIM1). Ll-FIM1 was preferentially expressed in pollen and concentrated at actin fringe in the subapical region, as well as in longitudinal actin-filament bundles in the shank of pollen tubes. Microinjection of Ll-FIM1 antibody into lily pollen tubes inhibited tip growth and disrupted the actin fringe. Furthermore, we verified the function of Ll-FIM1 in the fim5 mutant of its closest relative, Arabidopsis thaliana. Pollen tubes of fim5 mutants grew with a larger diameter in early stages but could recover into normal forms in later stages, despite significantly slower growth rates. The actin fringe of the fim5 mutants, however, was impaired during both early and late stages. Impressively, stable expression of fim5pro:GFP:Ll-FIM1 rescued the actin fringe and the growth rate of Arabidopsis fim5 pollen tubes. In vitro biochemical analysis showed that Ll-FIM1 could bundle actin filaments. Thus, our study has identified a fimbrin that may stabilize the actin fringe by cross-linking actin filaments into bundles, which is important for proper tip growth of lily pollen tubes.

Calcium - a central regulator of pollen germination and tube growth

Pollen tubes grow rapidly by very fast rates and reach extended lengths to bring about fertilization during plant reproduction. The pollen tube grows exclusively at its tip. Fundamental for such local, tip-focused growth are the presence of internal gradients and transmembrane fluxes of ions. Consequently, vegetative pollen tube cells are an excellent single cell model system to investigate cell biological processes of vesicle transport, cytoskeleton reorganization and regulation of ion transport. The second messenger Ca(2+) has emerged as a central and crucial modulator that not only regulates but also integrates the coordination each of these processes. In this review we reflect on recent advances in our understanding of the mechanisms of Ca(2+) function in pollen tube growth, focusing on its role in basic cellular processes such as control of cell growth, vesicular transport and intracellular signaling by localized gradients of second messengers. In particular we discuss new insights into the identity and role of Ca(2+) conductive ion channels and present experimental addressable hypotheses about their regulation. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Inhibition of actin polymerisation by low concentration Latrunculin B affects endocytosis and alters exocytosis in shank and tip of tobacco pollen tubes

Pollen tube growth depends on the integrity of the actin cytoskeleton that regulates cytoplasmic streaming and secretion. To clarify whether actin also plays a role in pollen tube endocytosis, Latrunculin B (LatB) was employed in internalisation experiments with tobacco pollen tubes, using the lipophilic dye FM4-64 and charged nanogold. Time-lapse analysis and dissection of endocytosis allowed us to identify internalisation pathways with different sensitivity to LatB. Co-localisation experiments and ultrastructural observations using positively charged nanogold revealed that LatB significantly inhibited endocytosis in the pollen tube shank, affecting internalisation of the plasma membrane (PM) recycled for secretion, as well as that conveyed to vacuoles. In contrast, endocytosis of negatively charged nanogold in the tip, which is also conveyed to vacuoles, was not influenced. Experiments of fluorescence recovery after photobleaching (FRAP) of the apical and subapical PM revealed domains with different rates of fluorescence recovery and showed that these differences depend on the actin cytoskeleton integrity. These results show the presence of distinct degradation pathways by demonstrating that actin-dependent and actin-indepedent endocytosis both operate in pollen tubes, internalising tracts of PM to be recycled and broken down. Intriguingly, although most studies concentrate on exocytosis and distension in the apex, the present paper shows that uncharacterised, actin-dependent secretory activity occurs in the shank of pollen tubes.

The quest for four-dimensional imaging in plant cell biology: it's just a matter of time

BACKGROUND

Analysis of plant cell dynamics over time, or four-dimensional imaging (4-DI), represents a major goal of plant science. The ability to resolve structures in the third dimension within the cell or tissue during developmental events or in response to environmental or experimental stresses (i.e. 4-DI) is critical to our understanding of gene expression, post-expression modulations of macromolecules and sub-cellular system interactions.

SCOPE

Microscopy-based technologies have been profoundly integral to this type of investigation, and new and refined microscopy technologies now allow for the visualization of cell dynamics with unprecedented resolution, contrast and experimental versatility. However, certain realities of light and electron microscopy, choice of specimen and specimen preparation techniques limit the scope of readily attaining 4-DI. Today, the plant microscopist must use a combinatorial strategy whereby multiple microscopy-based investigations are used. Modern fluorescence, confocal laser scanning, transmission electron and scanning electron microscopy provide effective conduits for synthesizing data detailing live cell dynamics and highly resolved snapshots of specific cell structures that will ultimately lead to 4-DI. This review provides a synopsis of such technologies available.

Expression of the Cameleon calcium biosensor in fungi reveals distinct Ca(2+) signatures associated with polarized growth, development, and pathogenesis

Calcium is a universal messenger that translates diverse environmental stimuli and developmental cues into specific cellular and developmental responses. While individual fungal species have evolved complex and often unique biochemical and structural mechanisms to exploit specific ecological niches and to adjust growth and development in response to external stimuli, one universal feature to all is that Ca(2+)-mediated signaling is involved. The lack of a robust method for imaging spatial and temporal dynamics of subcellular Ca(2+) (i.e., "Ca(2+) signature"), readily available in the plant and animal systems, has severely limited studies on how this signaling pathway controls fungal growth, development, and pathogenesis. Here, we report the first successful expression of a FRET (Förster Resonance Energy Transfer)-based Ca(2+) biosensor in fungi. Time-lapse imaging of Magnaporthe oryzae, Fusarium oxysporum, and Fusarium graminearum expressing this sensor showed that instead of a continuous gradient, the cytoplasmic Ca(2+) ([Ca(2+)](c)) change occurred in a pulsatile manner with no discernable gradient between pulses, and each species exhibited a distinct Ca(2+) signature. Furthermore, occurrence of pulsatile Ca(2+) signatures was age and development dependent, and major [Ca(2+)](c) transients were observed during hyphal branching, septum formation, differentiation into specialized plant infection structures, cell-cell contact and in planta growth. In combination with the sequenced genomes and ease of targeted gene manipulation of these and many other fungal species, the data, materials and methods developed here will help understand the mechanism underpinning Ca(2+)-mediated control of cellular and developmental changes, its role in polarized growth forms and the evolution of Ca(2+) signaling across eukaryotic kingdoms.

AGD1, a class 1 ARF-GAP, acts in common signaling pathways with phosphoinositide metabolism and the actin cytoskeleton in controlling Arabidopsis root hair polarity

The Arabidopsis thaliana AGD1 gene encodes a class 1 adenosine diphosphate ribosylation factor-gtpase-activating protein (ARF-GAP). Previously, we found that agd1 mutants have root hairs that exhibit wavy growth and have two tips that originate from a single initiation point. To gain new insights into how AGD1 modulates root hair polarity we analyzed double mutants of agd1 and other loci involved in root hair development, and evaluated dynamics of various components of root hair tip growth in agd1 by live cell microscopy. Because AGD1 contains a phosphoinositide (PI) binding pleckstrin homology (PH) domain, we focused on genetic interactions between agd1 and root hair mutants altered in PI metabolism. Rhd4, which is knocked-out in a gene encoding a phosphatidylinositol-4-phosphate (PI-4P) phosphatase, was epistatic to agd1. In contrast, mutations to PIP5K3 and COW1, which encode a type B phosphatidylinositol-4-phosphate 5-kinase 3 and a phosphatidylinositol transfer protein, respectively, enhanced the root hair defects of agd1. Enhanced root hair defects were also observed in double mutants to AGD1 and ACT2, a root hair-expressed vegetative actin isoform. Consistent with our double-mutant studies, targeting of tip growth components involved in PI signaling (PI-4P), secretion (RABA4b) and actin regulation (ROP2), were altered in agd1 root hairs. Furthermore, tip cytosolic calcium ([Ca²⁺](cyt) ) oscillations were disrupted in root hairs of agd1. Taken together, our results indicate that AGD1 links PI signaling to cytoskeletal-, [Ca²⁺](cyt-) , ROP2-, and RABA4b-mediated root hair development.

Plant organellar calcium signalling: an emerging field

This review provides a comprehensive overview of the established and emerging roles that organelles play in calcium signalling. The function of calcium as a secondary messenger in signal transduction networks is well documented in all eukaryotic organisms, but so far existing reviews have hardly addressed the role of organelles in calcium signalling, except for the nucleus. Therefore, a brief overview on the main calcium stores in plants-the vacuole, the endoplasmic reticulum, and the apoplast-is provided and knowledge on the regulation of calcium concentrations in different cellular compartments is summarized. The main focus of the review will be the calcium handling properties of chloroplasts, mitochondria, and peroxisomes. Recently, it became clear that these organelles not only undergo calcium regulation themselves, but are able to influence the Ca(2+) signalling pathways of the cytoplasm and the entire cell. Furthermore, the relevance of recent discoveries in the animal field for the regulation of organellar calcium signals will be discussed and conclusions will be drawn regarding potential homologous mechanisms in plant cells. Finally, a short overview on bacterial calcium signalling is included to provide some ideas on the question where this typically eukaryotic signalling mechanism could have originated from during evolution.

Calcium regulation of tip growth: new genes for old mechanisms

We review the recent advances on Ca(2+) in tip-growing cells, with a special focus on pollen tubes. New genes for Ca(2+) pumps, channels and sensing proteins have been recently described, with special emphasis on cyclic nucleotide gated channels (CNGCs) and glutamate receptor-like channels (GLRs). We also review the current state of knowledge in what concerns Ca(2+) sensor and relay proteins, where the knowledge of the cell models is less advanced. While these newly described genes offer promise to a better understanding of the spatial and temporal patterns of Ca(2+) signalling that may be relevant for the formation of the phenotype, we discuss the necessity to investigate further links in the network downstream of the Ca(2+) signature, with a special need for mechanisms of feed-back that might render functional feed-back loops approachable by modelling and genetics. Given the available literature, we conclude on the need to investigate more on the role of two specific classes of proteins, the calcium binding protein kinases (CPKs) and the Calcineurin B-like proteins (CBLs) and their regulatory relationships to ion channels (summarized in Figure 3b).

Propidium iodide competes with Ca(2+) to label pectin in pollen tubes and Arabidopsis root hairs

We have used propidium iodide (PI) to investigate the dynamic properties of the primary cell wall at the apex of Arabidopsis (Arabidopsis thaliana) root hairs and pollen tubes and in lily (Lilium formosanum) pollen tubes. Our results show that in root hairs, as in pollen tubes, oscillatory peaks in PI fluorescence precede growth rate oscillations. Pectin forms the primary component of the cell wall at the tip of both root hairs and pollen tubes. Given the electronic structure of PI, we investigated whether PI binds to pectins in a manner analogous to Ca(2+) binding. We first show that Ca(2+) is able to abrogate PI growth inhibition in a dose-dependent manner. PI fluorescence itself also relies directly on the amount of Ca(2+) in the growth solution. Exogenous pectin methyl esterase treatment of pollen tubes, which demethoxylates pectins, freeing more Ca(2+)-binding sites, leads to a dramatic increase in PI fluorescence. Treatment with pectinase leads to a corresponding decrease in fluorescence. These results are consistent with the hypothesis that PI binds to demethoxylated pectins. Unlike other pectin stains, PI at low yet useful concentration is vital and specifically does not alter the tip-focused Ca(2+) gradient or growth oscillations. These data suggest that pectin secretion at the apex of tip-growing plant cells plays a critical role in regulating growth, and PI represents an excellent tool for examining the role of pectin and of Ca(2+) in tip growth.

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

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

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

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

Cytoplasmic connection of sperm cells to the pollen vegetative cell nucleus: potential roles of the male germ unit revisited

The male germ cells of angiosperm plants are neither free-living nor flagellated and therefore are dependent on the unique structure of the pollen grain for fertilization. During angiosperm male gametogenesis, an asymmetric mitotic division produces the generative cell, which is completely enclosed within the cytoplasm of the larger pollen grain vegetative cell. Mitotic division of the generative cell generates two sperm cells that remain connected by a common extracellular matrix with potential intercellular connections. In addition, one sperm cell has a cytoplasmic projection in contact with the vegetative cell nucleus. The shared extracellular matrix of the two sperm cells and the physical association of one sperm cell to the vegetative cell nucleus forms a linkage of all the genetic material in the pollen grain, termed the male germ unit. Found in species representing both the monocot and eudicot lineages, the cytoplasmic projection is formed by vesicle formation and microtubule elongation shortly after the formation of the generative cell and tethers the male germ unit until just prior to fertilization. The cytoplasmic projection plays a structural role in linking the male germ unit, but potentially plays other important roles. Recently, it has been speculated that the cytoplasmic projection and the male germ unit may facilitate communication between the somatic vegetative cell nucleus and the germinal sperm cells, via RNA and/or protein transport. This review focuses on the nature of the sperm cell cytoplasmic projection and the potential communicative function of the male germ unit.

Phosphatidylinositol-4,5-bisphosphate influences Nt-Rac5-mediated cell expansion in pollen tubes of Nicotiana tabacum

The regulation of pollen tube growth by the phospholipid phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2) ) is not well understood. The Arabidopsis genome encodes two type A phosphatidylinositol-4-phosphate (PI4P) 5-kinases, PIP5K10 and PIP5K11, which are exclusively expressed in pollen and produce PtdIns(4,5)P(2) in vitro. Fluorescence-tagged PIP5K10 and PIP5K11 localized to lateral subapical plasma membrane microdomains in tobacco pollen tubes in a pattern closely resembling the distribution of PtdIns(4,5)P(2,) with the exception of notably weaker association at the extreme apex. Overexpression of PIP5K10 or PIP5K11 in tobacco pollen tubes resulted in severe tip swelling and altered actin fine structure similar to that reported for overexpression of tobacco Nt-Rac5, a monomeric GTPase known to regulate the actin cytoskeleton. Increased sensitivity of Arabidopsis pip5k10 pip5k11 double mutant pollen tubes to Latrunculin B (LatB) further supports a role for type A PI4P 5-kinases in controlling the actin cytoskeleton. Despite the disruption of both its type A PI4P 5-kinases, the pip5k10 pip5k11 double mutant was fertile, indicating that one of the remaining type B PI4P 5-kinase isoforms might be functionally redundant with PIP5K10 and PIP5K11. Antagonistic effects of PIP5K11 and the Nt-Rac5-specific guanine nucleotide dissociation inhibitor, Nt-RhoGDI2, on tip swelling observed in coexpression-titration experiments indicate a link between PtdIns(4,5)P(2) and Rac-signaling in pollen tubes. The data suggest that type A PI4P 5-kinases influence the actin cytoskeleton in pollen tubes in part by counteracting Nt-RhoGDI2, possibly contributing to the control of the pool of plasma membrane-associated Nt-Rac5.

Pollen tube energetics: respiration, fermentation and the race to the ovule

BACKGROUND

Pollen tubes grow by transferring chemical energy from stored cellular starch and newly assimilated sugars into ATP. This drives myriad processes essential for cell elongation, directly or through the creation of ion gradients. Respiration plays a central role in generating and regulating this energy flow and thus in the success of plant reproduction. Pollen tubes are easily grown in vitro and have become an excellent model for investigating the contributions of respiration to plant cellular growth and morphogenesis at the molecular, biochemical and physiological levels.

SCOPE

In recent decades, pollen tube research has become increasingly focused on the molecular mechanisms involved in cellular processes. Yet, effective growth and development requires an intact, integrated set of cellular processes, all supplied with a constant flow of energy. Here we bring together information from the current and historical literature concerning respiration, fermentation and mitochondrial physiology in pollen tubes, and assess the significance of more recent molecular and genetic investigations in a physiological context.

CONCLUSIONS

The rapid growth of the pollen tube down the style has led to the evolution of high rates of pollen tube respiration. Respiration rates in lily predict a total energy turnover of 40-50 fmol ATP s(-1) per pollen grain. Within this context we examine the energetic requirements of cell wall synthesis, osmoregulation, actin dynamics and cyclosis. At present, we can only estimate the amount of energy required, because data from growing pollen tubes are not available. In addition to respiration, we discuss fermentation and mitochondrial localization. We argue that the molecular pathways need to be examined within the physiological context to understand better the mechanisms that control tip growth in pollen tubes.

Mutations in two putative phosphorylation motifs in the tomato pollen receptor kinase LePRK2 show antagonistic effects on pollen tube length

The tip-growing pollen tube is a useful model for studying polarized cell growth in plants. We previously characterized LePRK2, a pollen-specific receptor-like kinase from tomato (1). Here, we showed that LePRK2 is present as multiple phosphorylated isoforms in mature pollen membranes. Using comparative sequence analysis and phosphorylation site prediction programs, we identified two putative phosphorylation motifs in the cytoplasmic juxtamembrane (JM) domain. Site-directed mutagenesis in these motifs, followed by transient overexpression in tobacco pollen, showed that both motifs have opposite effects in regulating pollen tube length. Relative to LePRK2-eGFP pollen tubes, alanine substitutions in residues of motif I, Ser(277)/Ser(279)/Ser(282), resulted in longer pollen tubes, but alanine substitutions in motif II, Ser(304)/Ser(307)/Thr(308), resulted in shorter tubes. In contrast, phosphomimicking aspartic substitutions at these residues gave reciprocal results, that is, shorter tubes with mutations in motif I and longer tubes with mutations in motif II. We conclude that the length of pollen tubes can be negatively and positively regulated by phosphorylation of residues in motif I and II respectively. We also showed that LePRK2-eGFP significantly decreased pollen tube length and increased pollen tube tip width, relative to eGFP tubes. The kinase activity of LePRK2 was relevant for this phenotype because tubes that expressed a mutation in a lysine essential for kinase activity showed the same length and width as the eGFP control. Taken together, these results suggest that LePRK2 may have a central role in pollen tube growth through regulation of its own phosphorylation status.

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

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