Cllmodulin in tip-growing plant cells, visualized by fluorescing calmodulin-binding phenothiazines

Calmodulin concentrates at the apex of growing hyphae and localizes to the Spitzenkörper in Aspergillus nidulans

The calmodulin (CaM) localization pattern in the growing hyphal tip of Aspergillus nidulans was studied with the functional GFP::CaM fusion protein. A faint tip-high gradient of CaM was found in the growing hyphal tip, with CaM highly localized in the region corresponding to the Spitzenkörper forming a bright granule. The position of highly concentrated CaM in the extreme apex seemed to determine the orientation of the hypha. The normal pattern of CaM localization was also shown to be dependent on the integrated actin cytoskeleton. When the growth of the hyphal tip ceased, CaM failed to localize in the bright granule and was evenly distributed in the hyphal tip. These findings suggest that CaM may play an important role in establishing and maintaining apical organization, morphogenesis, and growth in Aspergillus nidulans.

Calmodulin activity and cAMP signalling modulate growth and apical secretion in pollen tubes

Our present understanding implicates both calmodulin (CaM) and 3',5'-cyclicAMP (cAMP) in the regulation of pollen tube growth. However, downstream molecules of these signalling pathways and the cellular processes they modulate remain largely unknown. In order to elucidate the role of CaM, we mapped its activity in growing pollen tubes. 2-chloro-(epsilon-amino-Lys(75))-[6-4-(N,N'-diethylaminophenyl)-1,3,5-triazin-4-yl]-calmodulin (TA-CaM) and fluorescein-calmodulin (FL-CaM), fluorescent analogues of CaM, were loaded into pollen tubes and CaM activity was mapped by fluorescence ratio imaging. It was found that CaM activity exhibits a tip-focused gradient, similar to the distribution of cytosolic-free calcium ([Ca(2+)](c)). In long pollen tubes, apical CaM activity was also found to oscillate with a period similar to [Ca(2+)](c) (40-80 sec). This oscillatory behaviour was not observed in small pollen tubes or in tubes that had stopped growing. Changes in CaM activity within the dome of the pollen tube apex resulting from the photolysis of caged photolysis of RS-20 (a peptide antagonist of CaM) induced re-orientation of the growth axis, suggesting that CaM is also involved in the guidance mechanism. CaM activity was strongly modulated by intracellular changes in cAMP (induced by activators and antagonists of adenylyl cyclase). These results indicate that the action of this protein is dependent not solely on [Ca(2+)](c) but also on a cross-talk with other signalling pathways. A putative target of this cross-talk is the secretory machinery as observed in pollen tubes loaded with the FM (N-(3-triethylammoniumpropyl)-4-(4-dibutylamino)styryl)pyridinium dibromide 1-43 dye and exposed to different antagonists and activators of these molecules. Our data thus suggest that pollen tube growth and orientation depend on an intricate cross-talk between multiple signalling pathways in which CaM is a key element.

A pollen-specific novel calmodulin-binding protein with tetratricopeptide repeats

Calcium is essential for pollen germination and pollen tube growth. A large body of information has established a link between elevation of cytosolic Ca(2+) at the pollen tube tip and its growth. Since the action of Ca(2+) is primarily mediated by Ca(2+)-binding proteins such as calmodulin (CaM), identification of CaM-binding proteins in pollen should provide insights into the mechanisms by which Ca(2+) regulates pollen germination and tube growth. In this study, a CaM-binding protein from maize pollen (maize pollen calmodulin-binding protein, MPCBP) was isolated in a protein-protein interaction-based screening using (35)S-labeled CaM as a probe. MPCBP has a molecular mass of about 72 kDa and contains three tetratricopeptide repeats (TPR) suggesting that it is a member of the TPR family of proteins. MPCBP protein shares a high sequence identity with two hypothetical TPR-containing proteins from Arabidopsis. Using gel overlay assays and CaM-Sepharose binding, we show that the bacterially expressed MPCBP binds to bovine CaM and three CaM isoforms from Arabidopsis in a Ca(2+)-dependent manner. To map the CaM-binding domain several truncated versions of the MPCBP were expressed in bacteria and tested for their ability to bind CaM. Based on these studies, the CaM-binding domain was mapped to an 18-amino acid stretch between the first and second TPR regions. Gel and fluorescence shift assays performed with CaM and a CaM-binding synthetic peptide further confirmed MPCBP binding to CaM. Western, Northern, and reverse transcriptase-polymerase chain reaction analysis have shown that MPCBP expression is specific to pollen. MPCBP was detected in both soluble and microsomal proteins. Immunoblots showed the presence of MPCBP in mature and germinating pollen. Pollen-specific expression of MPCBP, its CaM-binding properties, and the presence of TPR motifs suggest a role for this protein in Ca(2+)-regulated events during pollen germination and growth.

Calcium-calmodulin suppresses the filamentous actin-binding activity of a 135-kilodalton actin-bundling protein isolated from lily pollen tubes

We have isolated a 135-kD actin-bundling protein (P-135-ABP) from lily (Lilium longiflorum) pollen tubes and have shown that this protein is responsible for bundling actin filaments in lily pollen tubes (E. Yokota, K. Takahara, T. Shimmen [1998] Plant Physiol 116: 1421-1429). However, only a few thin actin-filament bundles are present in random orientation in the tip region of pollen tubes, where high concentrations of Ca(2+) have also been found. To elucidate the molecular mechanism for the temporal and spatial regulation of actin-filament organization in the tip region of pollen tubes, we explored the possible presence of factors modulating the filamentous actin (F-actin)-binding activity of P-135-ABP. The F-actin-binding activity of P-135-ABP in vitro was appreciably reduced by Ca(2+) and calmodulin (CaM), although neither Ca(2+) alone nor CaM in the presence of low concentrations of Ca(2+) affects the activity of P-135-ABP. A micromolar order of Ca(2+) and CaM were needed to induce the inhibition of the binding activity of P-135-ABP to F-actin. An antagonist for CaM, W-7, cancelled this inhibition. W-5 also alleviated the inhibition effect of Ca(2+)-CaM, however, more weakly than W-7. These results suggest the specific interaction of P-135-ABP with Ca(2+)-CaM. In the presence of both Ca(2+) and CaM, P-135-ABP organized F-actin into thin bundles, instead of the thick bundles observed in the absence of CaM. These results suggest that the inhibition of the P-135-ABP activity by Ca(2+)-CaM is an important regulatory mechanism for organizing actin filaments in the tip region of lily pollen tubes.

Whorl morphogenesis in the dasycladalean algae: the pattern formation viewpoint

The dasycladalean algae produce diverse whorled structures, among which the best known are the vegetative and reproductive whorls of Acetabularia acetabulum. In this paper, we review the literature pertaining to the origin of these structures. The question is addressed in terms of the necessary pattern-forming events and the possible mechanisms involved, an outlook we call the pattern formation viewpoint. The pattern-forming events involved in the morphogenesis of the vegetative and reproductive whorls of Acetabularia have been used to define five and six morphogenetic stages, respectively. We discuss three published mechanisms which account, at least in part, for the pattern-forming events. The mechanisms are mechanical buckling of the cell wall, reaction-diffusion of morphogen molecules along the cell membrane, and mechanochemical interactions between Ca2+ ions and the cytoskeleton in the cytosol. The numerous differences between these mechanisms provide experimental grounds to test their validity. To date, the results of these experiments point towards reaction diffusion as the most likely patterning mechanism. Finally, we consider the evolutionary origin of the vegetative and reproductive whorls and provide mechanistic explanations for some of the major evolutionary advances.

Characterization and immunolocalization of a cytosolic calcium-binding protein from Brassica napus and Arabidopsis pollen

Two low-molecular-weight proteins have been purified from Brassica napus pollen and a gene corresponding to one of them has been isolated. The gene encodes an 8.6-kD protein with two EF-hand calcium-binding motifs and is a member of a small gene family in B. napus. The protein is part of a family of pollen allergens recently identified in several evolutionarily distant dicot and monocot plants. Homologs have been detected in Arabidopsis, from which one gene has been cloned in this study, and in snapdragon (Antirrhinum majus), but not in tobacco (Nicotiana tabacum). Expression of the gene in B. napus was limited to male tissues and occurred during the pollen-maturation phase of anther development. Both the B. napus and Arabidopsis proteins interact with calcium, and the potential for a calcium-dependent conformational change was demonstrated. Given this affinity for calcium, the cloned genes were termed BPC1 and APC1 (B. napus and Arabidopsis pollen calcium-binding protein 1, respectively). Immunolocalization studies demonstrated that BPC1 is found in the cytosol of mature pollen. However, upon pollen hydration and germination, there is some apparent leakage of the protein to the pollen wall. BPC1 is also concentrated on or near the surface of the elongating pollen tube. The essential nature of calcium in pollen physiology, combined with the properties of BPC1 and its high evolutionary conservation suggests that this protein plays an important role in pollination by functioning as a calcium-sensitive signal molecule.

Calcium and related channels in fertilization and early development ofFucus

Unfertilized eggs ofFucus serratusare primed to respond rapidly to the fertilizing sperm. The unfertilized egg plasma membrane is excitable due to the presence of voltage-regulated Ca2+and K+channels. Sperm-egg interaction elicits a fertilization potential as the first observable fertilization event. It is speculated that sperm-gated Na+channels are responsible for the initial depolarization phase, leading to opening of Ca2+channels, allowing Ca2+influx and further depolarizing the membrane to the threshold for outward K+channels. K+efflux repolarizes the membrane and the zygote plasmalemma quickly becomes dominated by a large K+conductance. The involvement of Ca2+in axis formation and fixation is not clear. Ca2+carries a proportion of the inward current at the future rhizoid pole and asymmetric45Ca influx has been detected in polarizing zygotes. However, there is no requirement for external Ca2+in axis fixation. In contrast, Ca2+influx is required for expression of polarity and rhizoid growth. New developments in patch clamping can now enable localized areas of the plasma membrane in polarized cells to be studied. So far, both inward and outward single channel currents have been observed in the growing rhizoid tip, most probably carrying Cl-and K+respectively. These channels can be related to the currents identified by previous studies using the extracellular vibrating probe.

Characterisation of isolated egg cells, in vitro fusion products and zygotes of Zea mays L. using the technique of image analysis and confocal laser scanning microscopy

Changes in membrane Ca2+, calcium receptor protein calmodulin, endoplasmic reticulum (ER), mitochondria and cellulose in unfixed, living, isolated egg cells and fusion products of pairs of one egg and one sperm cell of Zea mays L. have been investigated using chlorotetracycline, fluphenazine, immunocytochemical techniques, 3,3'-dihexyloxa-carbocyanine iodide (DiOC6(3)) and calcofluor white in conjunction with computer-controlled video image analysis. In addition, confocal laser scanning microscopy has been used in conjunction with ethidium bromide to detect the nature and location of the sperm cell nuclear chromatin before and after karyogamy. Digitised video images of chlorotetracycline (CTC) fluorescence reveal that egg cells contain high levels of membrane Ca2+ in organelles present around the nucleus while the cytosolic signal is relatively low. Intense CTC fluorescence is invariably present just below the plasma membrane of egg cells and a certain degree of regionalised distribution of Ca2+ in cytoplasm is also discernible. Similarly, the fluphenazine (FPZ)-detectable calmodulin (CaM) and that localised immunocytochemically using monoclonal anti-CaM antibodies reveal high levels of CaM in the vicinity of the nucleus in egg cells. Only a few ER profiles and mitochondria could be visualised in the egg cell and no calcofluor fluorescence could be detected. Following in vitro fertilisation of single isolated eggs substantial changes in the Ca2+ levels occur which include an increase in the membrane Ca2+ of the fusion product, particularly in the cytosol and around the nucleus. Unlike in the eggs the fine CTC fluorescence signal below the plasma membrane is not detectable in the fusion products.(ABSTRACT TRUNCATED AT 250 WORDS)

Confocal image analysis of spatial variations in immunocytochemically identified calmodulin during pollen hydration, germination and pollen tube tip growth in Nicotiana tabacum L

Using monoclonal anti-calmodulin antibodies in conjunction with confocal scanning laser microscopy we have analysed the spatial variations in the distribution pattern of calmodulin (CaM) during the sequential events of pollen hydration, germination and tube growth in Nicotiana tabacum. These immunocytochemical observations have been complemented by immunochemical studies wherein the anti-calmodulin antibody raised against pea CaM recognises a polypeptide of c. 18 kDa in the pollen extracts. Digitisation of confocally acquired optical sections of immunofluorescence images reveals that in hydrated pollen a high level of CaM is consistently present in the region of the germinal apertures. Subsequently, with the onset of germination a high CaM concentration was found associated with the plasma membrane of the germination bubble and in the cytoplasm in its vicinity, while in the vegetative cytoplasm a weak diffuse and intense punctate signal was registered. CaM immunostain was also detected in association with the plasma membrane of the tube tips in both short and long pollen tubes. Furthermore, the cytosol of the tubes invariably manifested an apically focused CaM gradient. We were, however, unable to detect any vacuolar association of CaM in the older regions of the pollen tubes. Although punctate immunostain was obvious across the pollen tube numerous punctate structures were invariably present in the extreme tip. The possible implications of these findings in development of cell polarity, polarised growth, maintenance of calcium homeostasis and CaM interactions with other mechanochemical motor proteins in effecting propulsion of organelles during pollen hydration, germination and pollen tube growth are discussed.

Cytological indication of the involvement of calcium and calcium-related proteins in the early responses of Bryonia dioica to mechanical stimulus

The distribution of membrane-bound calcium, activated calmodulin, and callose synthesis was visualized in Bryonia dioica internodes before and after mechanical stimulus, using fluorescent probes, respectively, chlorotetracycline, fluphenazine, and aniline blue. Bright chlorotetracycline fluorescence remains localized in the plasma membrane of control cells, 30 s after stimulation calcium left the plasmalemma. A delocalization of activated calmodulin was observed after wounding and deposition of callose, which could not be detected before, appeared in the same times in most cells. The callose formation and the decrease in membrane-associated calcium suggest a rapid influx of calcium in the cytosol and an intervention of this ion in the cascade of the early events underlying Bryonia dioica thigmomorphogenesis.

Pollen tube tip growth

This review considers pollen tube growth with regard to current information on pollen tube cytoplasm, wall structure and calcium ion interactions with pollen tubes. Pollen tubes have a marked cytoplasmic Polarity with a number of distinct zones along the tube, each with a characteristic complement of cytoplasmic and nuclear structures. The cytoplasmic structures are characteristic of secretory cells with extensive endoplasmic reticulum cisternae and numerous dictyosomes. The dictyosomes produce secretory vesicles that are mainly directed to the extending tip of the tube, where they provide new plasma membrane and wall components. The rates of secretory vesicle production and delivery have been estimated, allowing quantitative assessments of the rate of delivery of materials to the tip. Pollen tubes contain cytoskeletal components, with microtubules and microfilament strands lying axially in the main tube and diffuse microfilament strands at the tip. The tube wall consists of an outer fibrous layer containing pectins and an inner, more homogeneous layer containing callose and cellulose-like microfibrils, possessing both β-1,4 and β-1,3 linkages. Protein is also present in the wall. The tube tip lacks the inner callosic wall. This type of structure is considered to be different from that of elongating sporophyte tissue cells which are enclosed by a wall containing layers of cellulose microfibrils. Calcium ions are required for pollen tube growth and, in at least some species, act as a chemotropic agent. High concentrations of calcium ions in the external medium inhibit growth. Pollen tubes contain some calcium ions bound to the cell wall and larger amounts located intracellularly, which enter the tube at the tip. This intracellular calcium is present as ions that exist freely within the cytoplasmic Matrix and as ions bound to membrane systems. The highest concentrations in both of these pools are found at the tip and in both they decline towards the base. The structure of the tip and the activity involved in providing components for plasma membrane and Wall assembly provide a basis for considering possible mechanisms of tip growth. Two hypotheses to account for the regulation of tip extension are considered, cell wall control and cytoskeletal control. In the cell wall hypothesis, control depends on an interaction between internal turgor pressure and a plastic cell wall. The mechanical properties of the wall are assumed to be partly dependent on the availability of external calcium ions to crosslink acidic pectin chains. According to this hypothesis, high external calcium ion concentrations cause cessation of tip growth due to increased mechanical resistance of the tip wall. Various observations on plant cell-wall interactions with calcium ions and on experimentally-treated pollen tubes provide evidence that does not support this hypothesis. The cytoskeletal control hypothesis of tip growth depends on the internal tip cytoskeleton to contain the tube tip cytoplasm against the internal turgor pressure during cell wall assembly. The activities and mechanical properties of the cytoskeleton are assumed to depend on the availability of external calcium ions. High external concentrations are believed to cause a state of rigor in the cytoskeleton and hence a cessation of tip growth. Some experimental evidence is presented which suggests that the effects of excess calcium ions are on intracellular processes, and not extracellular ones. The mitochondrial zone behind the tip is believed to maintain the tip calcium ion concentration at an optimal level for growth. Some comparisons are made between tip growth in pollen tubes and that in other tip growing cells. CONTENTS Summary 323 I. Introduction 324 II. Cytoplasm 326 III. Wall structure 332 IV. Calcium 335 V. Tip growth 339 VI. Conclusions 350 References 351.

Electron spectroscopic imaging (ESI) applied to the detection of potential Ca(2+)-binding sites in plant cells

The applicability of the electron spectroscopic imaging technique for detection of the intracellular distribution of calcium in plant cells was tested with calyptra cells ofZea mays and with pollen tubes ofLilium longiflorum. After fixation in enhanced Ca(2+) levels and embedding in resin, ultrathin sections were analyzed for the elemental distribution. Calcium and phosphorus were enriched in cell wall, plasma membrane, endoplasmic reticulum, mitochondria, and Golgi vesicles, mainly in granular or globular deposits appearing electron dense in transmission electron microscopy. The results demonstrated that the ESI-technique allows exact localization of calcium enrichment relative to specific cell organelles.

Development of membrane- and calcium-gradients during pollen germination of Lilium longiflorum

Chlorotetracyclin (10(-4)M) has been used to observe the distribution of membrane-associated calcium during pollen germination of Lilium longiflorum. For comparison, the general membrane distribution has been determined with 4·10(-5) M fluorescamine. The pollen grains show a calcium gradient with either weak or strong chlorotetracycline-fluorescence intensity, but always increasing toward the germination colpus. This gradient intensifies during germination, reaching a maximum before the pollen tube emerges. The typical tip-to-base calcium gradient of the tube does not change during growth. Independent of the developmental stage, the pollen grains show a flat fluorescamine-fluorescence gradient with the highest intensity in one half of the grain. Pollen tubes reveal a tip-to-base membrane gradient, independent of their length. As an additional marker for membrane distribution, the distribution of phosphorus, measured by proton-induced X-ray emission in chemically fixed tubes, has been used. A tip-to-base phosphorus gradient, distinct from the calcium gradient measured with the same method, was detected.