Homogalacturonan Pectins Tuned as an Effect of Susceptible rbohD, Col-0-Reactions, and Resistance rbohF-, rbohD/F-Reactions to TuMV

The plant cell wall is an actively reorganized network during plant growth and triggered immunity in response to biotic stress. While the molecular mechanisms managing perception, recognition, and signal transduction in response to pathogens are well studied in the context of damaging intruders, the current understanding of plant cell wall rebuilding and active defense strategies in response to plant virus infections remains poorly characterized. Pectins can act as major elements of the primary cell wall and are dynamic compounds in response to pathogens. Homogalacturonans (HGs), a main component of pectins, have been postulated as defensive molecules in plant-pathogen interactions and linked to resistance responses. This research focused on examining the regulation of selected pectin metabolism components in susceptible (rbohD-, Col-0-TuMV) and resistance (rbohF-, rbohD/F-TuMV) reactions. Regardless of the interaction type, ultrastructural results indicated dynamic cell wall rebuilding. In the susceptible reaction promoted by RbohF, there was upregulation of AtPME3 (pectin methylesterase) but not AtPME17, confirmed by induction of PME3 protein deposition. Moreover, the highest PME activity along with a decrease in cell wall methylesters compared to resistance interactions in rbohD-TuMV were noticed. Consequently, the susceptible reaction of rbohD and Col-0 to TuMV was characterized by a significant domination of low/non-methylesterificated HGs. In contrast, cell wall changes during the resistance response of rbohF and rbohD/F to TuMV were associated with dynamic induction of AtPMEI2, AtPMEI3, AtGAUT1, and AtGAUT7 genes, confirmed by significant induction of PMEI2, PMEI3, and GAUT1 protein deposition. In both resistance reactions, a dynamic decrease in PME activity was documented, which was most intense in rbohD/F-TuMV. This decrease was accompanied by an increase in cell wall methylesters, indicating that the domination of highly methylesterificated HGs was associated with cell wall rebuilding in rbohF and rbohD/F defense responses to TuMV. These findings suggest that selected PME with PMEI enzymes have a diverse impact on the demethylesterification of HGs and metabolism as a result of rboh-TuMV interactions, and are important factors in regulating cell wall changes depending on the type of interaction, especially in resistance responses. Therefore, PMEI2 and PMEI3 could potentially be important signaling resistance factors in the rboh-TuMV pathosystem.

Pollen tube invasive growth is promoted by callose

Callose, a β-1,3-glucan, lines the pollen tube cell wall except for the apical growing region, and it constitutes the main polysaccharide in pollen tube plugs. These regularly deposited plugs separate the active portion of the pollen tube cytoplasm from the degenerating cell segments. They have been hypothesized to reduce the total amount of cell volume requiring turgor regulation, thus aiding the invasive growth mechanism. To test this, we characterized the growth pattern of Arabidopsis callose synthase mutants with altered callose deposition patterns. Mutant pollen tubes without callose wall lining or plugs had a wider diameter but grew slower compared to their respective wildtype. To probe the pollen tube's ability to perform durotropism in the absence of callose, we performed mechanical assays such as growth in stiffened media and assessed turgor through incipient plasmolysis. We found that mutants lacking plugs had lower invading capacity and higher turgor pressure when faced with a mechanically challenging substrate. To explain this unexpected elevation in turgor pressure in the callose synthase mutants we suspected that it is enabled by feedback-driven increased levels of de-esterified pectin and/or cellulose in the tube cell wall. Through immunolabeling we tested this hypothesis and found that the content and spatial distribution of these cell wall polysaccharides was altered in callose-deficient mutant pollen tubes. Combined, the results reveal how callose contributes to the pollen tube's invasive capacity and thus plays an important role in fertilization. In order to understand, how the pollen tube deposits callose, we examined the involvement of the actin cytoskeleton in the spatial targeting of callose synthases to the cell surface. The spatial proximity of actin with locations of callose deposition and the dramatic effect of pharmacological interference with actin polymerization suggest a potential role for the cytoskeleton in the spatial control of the characteristic wall assembly process in pollen tubes.

An osmotic model of the growing pollen tube

Pollen tube growth is central to the sexual reproduction of plants and is a longstanding model for cellular tip growth. For rapid tip growth, cell wall deposition and hardening must balance the rate of osmotic water uptake, and this involves the control of turgor pressure. Pressure contributes directly to both the driving force for water entry and tip expansion causing thinning of wall material. Understanding tip growth requires an analysis of the coordination of these processes and their regulation. Here we develop a quantitative physiological model which includes water entry by osmosis, the incorporation of cell wall material and the spreading of that material as a film at the tip. Parameters of the model have been determined from the literature and from measurements, by light, confocal and electron microscopy, together with results from experiments made on dye entry and plasmolysis in Lilium longiflorum. The model yields values of variables such as osmotic and turgor pressure, growth rates and wall thickness. The model and its predictive capacity were tested by comparing programmed simulations with experimental observations following perturbations of the growth medium. The model explains the role of turgor pressure and its observed constancy during oscillations; the stability of wall thickness under different conditions, without which the cell would burst; and some surprising properties such as the need for restricting osmotic permeability to a constant area near the tip, which was experimentally confirmed. To achieve both constancy of pressure and wall thickness under the range of conditions observed in steady-state growth the model reveals the need for a sensor that detects the driving potential for water entry and controls the deposition rate of wall material at the tip.

Wall architecture with high porosity is established at the tip and maintained in growing pollen tubes of Nicotiana tabacum

A major question in pollen tube growth in planta remains: do the pollen tube walls form a barrier to interaction with the environment? Using cryo-FESEM, we directly assessed the 3D construction and porosity of tobacco pollen tube walls. Fractured mature primary walls showed a 40-50 nm spaced lattice of continuous fibers interconnected by short rods in the primary wall. These observations agree with TEM observations of sectioned walls. In the secondary callose wall, for which no structure is visible using TEM, cryo-FESEM also revealed a 50 nm lattice consisting of longer fibers, approximately 10-15 nm wide, with rod-like, thinner interconnections at angles of approximately 90° with the longer fibers. Such architecture may reflect functional needs with respect to porosity and mechanical strength. The wall does not form a mechanical barrier to interaction with the environment and is gained at low cost. Cryo-FESEM additionally revealed another special feature of the wall: the tubes were tiled with scales or rings that were highly conspicuous after pectin extraction with EDTA. These rings cause the typical banding patterns of pectin that are commonly seen in pollen tubes during oscillatory growth, as confirmed by staining with toluidine blue as well as by DIC microscopy. Growth analysis by VEC-LM showed that the ring- or scale-like structures of the primary wall consist of material deposited prior to the growth pulses. The alternating band pattern seen in the callose wall is probably imposed by constrictions resulting from the rings of the primary wall.

Under pressure, cell walls set the pace

Significant controversy still swirls around the regulation of extension by tip-growing cells, particularly during stable, oscillatory growth of pollen tubes. One explanation proposes that turgor pressure is both the controlling and driving force. We refute this hypothesis on theoretical and evidentiary grounds. Direct measurement of intracellular pressure reveals constant turgor even as growth rates change. Measured ion fluxes, notably potassium, are insufficient to account for the requisite osmotic changes. Water movement, and hence pressure gradients, occur throughout the cell, unrestricted to local domains. Increases in hydrostatic pressure alone would force water out of the cell rather than cause increased growth. We have recently demonstrated concomitant changes in the apical cell wall that account fully for observed changes in growth rate.

Pectin and the role of the physical properties of the cell wall in pollen tube growth of Solanum chacoense

The cell wall is one of the structural key players regulating pollen tube growth, since plant cell expansion depends on an interplay between intracellular driving forces and the controlled yielding of the cell wall. Pectin is the main cell wall component at the growing pollen tube apex. We therefore assessed its role in pollen tube growth and cytomechanics using the enzymes pectinase and pectin methyl esterase (PME). Pectinase activity was able to stimulate pollen germination and tube growth at moderate concentrations whereas higher concentrations caused apical swelling or bursting in Solanum chacoense Bitt. pollen tubes. This is consistent with a modification of the physical properties of the cell wall affecting its extensibility and thus the growth rate, as well as its capacity to withstand turgor. To prove that the enzyme-induced effects were due to the altered cell wall mechanics, we subjected pollen tubes to micro-indentation experiments. We observed that cellular stiffness was reduced and visco-elasticity increased in the presence of pectinase. These are the first mechanical data that confirm the influence of the amount of pectins in the pollen tube cell wall on the physical parameters characterizing overall cellular architecture. Cytomechanical data were also obtained to analyze the role of the degree of pectin methyl-esterification, which is known to exhibit a gradient along the pollen tube axis. This feature has frequently been suggested to result in a gradient of the physical properties characterizing the cell wall and our data provide, for the first time, mechanical support for this concept. The gradient in cell wall composition from apical esterified to distal de-esterified pectins seems to be correlated with an increase in the degree of cell wall rigidity and a decrease of visco-elasticity. Our mechanical approach provides new insights concerning the mechanics of pollen tube growth and the architecture of living plant cells.

More than a leak sealant. The mechanical properties of callose in pollen tubes

While callose is a well-known permeability barrier and leak sealant in plant cells, it is largely unknown whether this cell wall polymer can also serve as a load-bearing structure. Since callose occurs in exceptionally large amounts in pollen, we assessed its role for resisting tension and compression stress in this cell. The effect of callose digestion in Solanum chacoense and Lilium orientalis pollen grains demonstrated that, depending on the species, this cell wall polymer represents a major stress-bearing structure at the aperture area of germinating grains. In the pollen tube, it is involved in cell wall resistance to circumferential tension stress, and despite its absence at the growing apex, callose is indirectly involved in the establishment of tension stress resistance in this area. To investigate whether or not callose is able to provide mechanical resistance against compression stress, we subjected pollen tubes to local deformation by microindentation. The data revealed that lowering the amount of callose resulted in reduced cellular stiffness and increased viscoelasticity, thus indicating clearly that callose is able to resist compression stress. Whether this function is relevant for pollen tube mechanics, however, is unclear, as stiffened growth medium caused a decrease in callose deposition. Together, our data provide clear evidence for the capacity of cell wall callose to resist tension and compression stress, thus demonstrating that this amorphous cell wall substance can have a mechanical role in growing plant cells.

Identification and characterization of stretch-activated ion channels in pollen protoplasts

Pollen tube growth requires a Ca2+ gradient, with elevated levels of cytosolic Ca2+ at the growing tip. This gradient's magnitude oscillates with growth oscillation but is always maintained. Ca2+ influx into the growing tip is necessary, and its magnitude also oscillates with growth. It has been widely assumed that stretch-activated Ca2+ channels underlie this influx, but such channels have never been reported in either pollen grains or pollen tubes. We have identified and characterized stretch-activated Ca2+ channels from Lilium longiflorum pollen grain and tube tip protoplasts. The channels were localized to a small region of the grain protoplasts associated with the site of tube germination. In addition, we find a stretch-activated K+ channel as well as a spontaneous K+ channel distributed over the entire grain surface, but neither was present at the germination site or at the tip. Neither stretch-activated channel was detected in the grain protoplasts unless the grains were left in germination medium for at least 1 h before protoplast preparation. The stretch-activated channels were inhibited by a spider venom that is known to block stretch-activated channels in animal cells, but the spontaneous channel was unaffected by the venom. The venom also stopped pollen tube germination and elongation and blocked Ca2+ entry into the growing tip, suggesting that channel function is necessary for growth.

Control of pollen tube growth: role of ion gradients and fluxes

Pollen tube growth attracts our attention as a model system for studying cell elongation in plants. The process is fast, it is confined to the tip of the tube, and it is crucial for sexual reproduction in plants. In the enclosed review we focus on the control of pollen tube growth, giving special attention to the role of ions, especially calcium and protons. During the last decade technical advances have made it possible to detect localized intracellular gradients, and extracellular fluxes of calcium and protons in the apical domain. Other ions, notably potassium and chloride, are also receiving attention. An important development has been the realization that pollen tube growth oscillates in rate; in addition, the ion gradients and fluxes oscillate in magnitude. Although all the ionic oscillations show the same period as that of the growth rate, with the exception of extracellular chloride efflux, they are not in phase with growth. Considerable effort is devoted to the elucidation of these different phase relationships, with the view that a hierarchical order may provide clues about those events that are primary vs. secondary in growth control. Attention is also given to the targets for the ions, for example, the secretory system, the cytoskeleton, the cell wall, in an attempt to provide a global understanding of pollen tube growth. Contents Summary 539 I. Introduction 540 II. Ion gradients and flux patterns 541 III. Oscillations 544 IV. The need for a Ca²⁺ store 547 V. Intracellular targets for Ion activity 549 VI. Extracellular targets for ions: the cell wall 552 VII. Ions in navigation 554 VIII. Role of ions in self-incompatibility 555 IX. The plasma membrane; site of global coordination and control 556 X. A model for pollen tube growth 557 IX. Conclusions 558 Acknowledgements 559 References 559.

First patch, then catch: measuring the activity and the mRNA transcripts of a proton pump in individual Lilium pollen protoplasts

Combining the patch-clamp method with single-cell reverse transcription polymerase chain reaction (scRT-PCR) a fusicoccin-induced current reflecting the activity of the plasma membrane H(+) ATPase of lily pollen protoplasts was measured and subsequently, the ATPase-encoding mRNAs were collected and amplified. Southern blot signals were observed in all 'patch-catch' experiments and could be detected even in 2560-fold dilutions of the pollen contents. H(+) ATPase mRNAs were detectable only in the vegetative but not in the generative cell of pollen as confirmed by immunolocalisation. In 15% of the scRT-PCR experiments, a random non-reproducibility of the PCR was observed, probably caused by varying amounts of ATPase mRNAs in the protoplasts.

Pollen tube development and competitive ability are impaired by disruption of a Shaker K(+) channel in Arabidopsis

Sexual reproduction in plants requires elongation of the pollen tube through the transmitting tissues toward the ovary. Tube growth rate is a major determinant of pollen competitive ability. We report that a K(+) channel of the Shaker family in Arabidopsis, SPIK, plays an important role in pollen tube development. SPIK was found to be specifically expressed in pollen. When SPIK was heterologously expressed in COS cells, its product formed hyperpolarization-activated K(+) channels. Disruption (T-DNA insertion) of the SPIK coding sequence strongly affected inwardly rectifying K(+)-channel activity in the pollen-grain plasma membrane. Measurements of membrane potential in growing pollen tubes yielded data compatible with a contribution of SPIK to K(+) influx. In vitro pollen germination assays were performed, revealing that the disruption results in impaired pollen tube growth. Analysis of the transmission rate of the disrupted allele in the progeny of heterozygous plants revealed a decrease in pollen competitive ability, the probability of fertilization by mutant pollen being approximately 1.6 times lower than that by wild-type pollen. The whole set of data supports the hypothesis that functional expression of SPIK plays a role in K(+) uptake in the growing pollen tube, and thereby in tube development and pollen competitive ability.

Cellular oscillations and the regulation of growth: the pollen tube paradigm

The occurrence of oscillatory behaviours in living cells can be viewed as a visible consequence of stable, regulatory homeostatic cycles. Therefore, they may be used as experimental windows on the underlying physiological mechanisms. Recent studies show that growing pollen tubes are an excellent biological model for these purposes. They unite experimental simplicity with clear oscillatory patterns of both structural and temporal features, most being measurable during real-time in live cells. There is evidence that these cellular oscillators involve an integrated input of plasma membrane ion fluxes, and a cytosolic choreography of protons, calcium and, most likely, potassium and chloride. In turn, these can create positive feedback regulation loops that are able to generate and self-sustain a number of spatial and temporal patterns. Other features, including cell wall assembly and rheology, turgor, and the cytoskeleton, play important roles and are targets or modulators of ion dynamics. Many of these features have similarities with other cell types, notably with apical-growing cells. Pollen tubes may thus serve as a powerful model for exploring the basis of cell growth and morphogenesis. BioEssays 23:86-94, 2001.

Cell biology and genetics of root hair formation in Arabidopsis thaliana

In this review we integrate the information available on the cell biology of root hair formation with recent findings from the analysis of root hair mutants of Arabidopsis thaliana. The mature Arabidopsis root epidermis consists of root-hair-producing cells and non-root-hair-producing cells. Root hair growth begins with a swelling of the outer epidermal wall. It has been postulated that this is due to a pH-mediated localised cell wall loosening. From the bulge a single root hair emerges which grows by tip growth. The root hair tip consists of a vesicle-rich zone and an organelle-rich subapical zone. The vesicles supply new plasma membrane and cell wall material for elongation. The cytoskeleton and its associated regulatory proteins such as profilin and spectrin are proposed to be involved in the targeting of vesicles. Ca2+ influxes and gradients are present in hair tips, but their function is still unclear. Mutants have been isolated with lesions in various parts of the root hair developmental pathway from bulge identity and initiation to control of tip diameter and extent and polarity of elongation.

Periodic increases in elongation rate precede increases in cytosolic Ca2+ during pollen tube growth

Pollen tubes grown in vitro require an intracellular tip-high gradient of Ca2+ in order to elongate. Moreover, after about 2 h in vitro both the tip Ca2+ and the elongation rate of lily tubes begin to oscillate regularly with large amplitudes. This raises the question of the phase relation between these two oscillations. Previous studies lacked the temporal resolution to accurately establish this relationship. We have studied these oscillations with a newly developed, high temporal resolution system and the complementary use of both luminescent and fluorescent calcium reporters. We hereby show that the periodic increases in elongation rate during oscillatory growth of Lilium longiflorum pollen tubes clearly precede those in subtip calcium and do so by 4.1 +/- 0.2 s out of average periods of 38.7 +/- 1.8 s. Also, by collecting images of the light output of aequorin, we find that the magnitude of the [Ca2+] at the tip oscillates between 3 and 10 microM, which is considerably greater than that reported by fluorescent indicators. We propose an explanatory model that features cyclic growth and secretion in which growth oscillations give rise to secretion that is essential for the subsequent growth oscillation. We also critically compile data on L. longiflorum stylar growth rates, which show little variation from in vitro rates of pollen tubes grown in optimal medium.

The first caldesmon-like protein in higher plants

Using anti-caldesmon polyclonal and monoclonal (raised against the N-terminal fragment of chicken gizzard caldesmon) antibodies, a plant caldesmon-like protein, 107 kDa as determined by SDS-gel electrophoresis, has been identified based on Western blotting of total extracts of Ornithogalum virens pollen tubes. Biochemical investigations showed common properties of this protein with animal caldesmon--it binds to actin and, in a Ca(2+)-dependent manner, to calmodulin. In contrast to animal caldesmon, this plant cell counterpart is relatively resistant to proteolysis by endogenous proteases and sensitive to heat treatment. Our results show the presence of a caldesmon-like protein in higher plants for the first time.

Endocytosis and membrane turnover in the germ tube of uromyces fabae

We have used the fluorescent dye FM4-64 as a tracer to demonstrate bulk membrane internalization (endocytosis) and redistribution of the dye within the cytoplasm of the germ tube of the rust fungus Uromyces fabae. Staining of the hyphal membrane was detected 4 s after application of FM4-64 and reached a maximum after 1 min. The highest fluorescence intensity occurred in the apex. Subsequently, staining of the plasma membrane decreased and a subapical region of the fungal protoplast (5-20 &mgr;m from the tip) displayed increasing fluorescence with a maximum after 5 min. Fluorescence in the subapical region was redistributed to an area in the hyphal tip, which corresponds to the accumulation of apical vesicles, after 10-15 min and subsequently to a cytoplasmic region in front of the two nuclei (35-45 &mgr;m from the tip). We conclude from our measurements of membrane fluorescence that the turnover time from endocytosis to secretion of the dye amounts to 15 min. The uptake of the dye into the cytoplasm, but not membrane loading, could be inhibited completely with 5 mM NaN3 or by a temperature shift to 4 degreesC. This is the first evidence for endocytosis in a fungal germ tube. Copyright 1998 Academic Press.

Plant cell morphogenesis: plasma membrane interactions with the cytoskeleton and cell wall

Because plants are composed of immobile cells, plant morphogenesis requires mechanisms allowing precise control of cell expansion and cell division patterns. Cortical domains, localized in response to directional cues, are of central importance in establishing cell polarity, orienting cell division, and determining daughter cell fates in a wide variety of prokaryotic and eukaryotic organisms. Such domains consist of localized macromolecular complexes that, in plant cells, provide spatial control of cell expansion and cell division functions. The role of the cytoskeleton, plasma membrane, and targeted secretion to the cell wall in the spatial regulation of cell morphogenesis in plants is discussed in light of recent results from model organisms, including brown algal zygotes (e.g. Fucus). A general model, emphasizing the importance of cortical sites and targeted secretion, is proposed for morphogenesis in higher plant cells based on current knowledge and principles derived from analysis of the establishment of a stable cortical asymmetry in Fucus. The model illustrates mechanisms to direct the orientation of an asymmetric division resulting in daughter cells with different fates.