Correlation between endoglucanase secretion and cell wall strength in oomycete hyphae: implications for growth and morphogenesis

Underlying Mechanisms of the Hedgehog-Like Panicle and Filamentous Leaf Tissue Symptoms Caused by Sclerospora graminicola in Foxtail Millet

Downy mildew caused by Sclerospora graminicola is a systemic infectious disease affecting foxtail millet production in Africa and Asia. S. graminicola-infected leaves could be decomposed to a state where only the veins remain, resulting in a filamentous leaf tissue symptom. The aim of the present study was to investigate how S. graminicola influences the formation of the filamentous leaf tissue symptoms in hosts at the morphological and molecular levels. We discovered that vegetative hyphae expanded rapidly, with high biomass accumulated at the early stages of S. graminicola infection. In addition, S. graminicola could affect spikelet morphological development at the panicle branch differentiation stage to the pistil and stamen differentiation stage by interfering with hormones and nutrient metabolism in the host, resulting in hedgehog-like panicle symptoms. S. graminicola could acquire high amounts of nutrients from host tissues through secretion of β-glucosidase, endoglucanase, and pectic enzyme, and destroyed host mesophyll cells by mechanical pressure caused by rapid expansion of hyphae. At the later stages, S. graminicola could rapidly complete sexual reproduction through tryptophan, fatty acid, starch, and sucrose metabolism and subsequently produce numerous oospores. Oospore proliferation and development further damage host leaves via mechanical pressure, resulting in a large number of degraded and extinct mesophyll cells and, subsequently, malformed leaves with only veins left, that is, "filamentous leaf tissue." Our study revealed the S. graminicola expansion characteristics from its asexual to sexual development stages, and the potential mechanisms via which the destructive effects of S. graminicola on hosts occur at different growth stages.

Modelling fungal hypha tip growth via viscous sheet approximation

In this paper we present a new model for single-celled, non-branching hypha tip growth. The growth mechanism of hypha cells consists of transport of cell wall building material to the cell wall and subsequent incorporation of this material in the wall as it arrives. To model the transport of cell wall building material to the cell wall we follow Bartnicki-Garcia and Gierz in assuming that the cell wall building material is transported in straight lines by an isotropic point source. To model the dynamics of the cell wall, including its growth by new material, we use the approach of Campàs and Mahadevan, which assumes that the cell wall is a thin viscous sheet sustained by a pressure difference. Furthermore, we include a novel equation which models the hardening of the cell wall as it ages. We validate the new model by comparing it to experimental data.

Elastic Properties of the Cell Wall of Aspergillus nidulans Studied with Atomic Force Microscopy

Currently, little is known about the mechanical properties of filamentous fungal hyphae. To study this topic, atomic force microscopy (AFM) was used to measure cell wall mechanical properties of the model fungus Aspergillus nidulans. Wild type and a mutant strain (deltacsmA), lacking one of the chitin synthase genes, were grown in shake flasks. Hyphae were immobilized on polylysine-coated coverslips and AFM force--displacement curves were collected. When grown in complete medium, wild-type hyphae had a cell wall spring constant of 0.29 +/- 0.02 N/m. When wild-type and mutant hyphae were grown in the same medium with added KCl (0.6 M), hyphae were significantly less rigid with spring constants of 0.17 +/- 0.01 and 0.18 +/- 0.02 N/m, respectively. Electron microscopy was used to measure the cell wall thickness and hyphal radius. By use of finite element analysis (FEMLAB v 3.0, Burlington, MA) to simulate AFM indentation, the elastic modulus of wild-type hyphae grown in complete medium was determined to be 110 +/- 10 MPa. This decreased to 64 +/- 4 MPa for hyphae grown in 0.6 M KCl, implying growth medium osmotic conditions have significant effects on cell wall elasticity. Mutant hyphae grown in KCl-supplemented medium were found to have an elastic modulus of 67 +/- 6 MPa. These values are comparable with other microbial systems (e.g., yeast and bacteria). It was also found that under these growth conditions axial variation in elastic modulus along fungal hyphae was small. To determine the relationship between composition and mechanical properties, cell wall composition was measured by anion-exchange liquid chromatography and pulsed electrochemical detection. Results show similar composition between wild-type and mutant strains. Together, these data imply differences in mechanical properties may be dependent on varying molecular structure of hyphal cell walls as opposed to wall composition.

An F-actin-depleted zone is present at the hyphal tip of invasive hyphae of Neurospora crassa

The distribution of filamentous actin (F-actin) in invasive and noninvasive hyphae of the ascomycete Neurospora crassa was investigated. Eighty six percent of noninvasive hyphae had F-actin in the tip region compared to only 9% of invasive hyphae. The remaining 91% of the invasive hyphae had no obvious tip high concentration of F-actin staining; instead they had an F-actin-depleted zone in this region, although some F-actin, possibly associated with the Spitzenkörper, remained at the tip. The size of the F-actin-depleted zone in invasive hyphae increased with an increase in agar concentration. The membrane stain FM 4-64 reveals a slightly larger accumulation of vesicles at the tips of invasive hyphae relative to noninvasive hyphae, although this difference is unlikely to be sufficient to account for the exclusion of F-actin from the depleted zone. Antibodies raised against the actin filament-severing protein cofilin from both yeast and human cells localize to the tips of invasive hyphae. The human cofilin antibody shows a more random distribution in noninvasive hyphae locating primarily at the hyphal periphery but with some diffuse cytoplasmic staining. This antibody also identifies a single band at 21 kDa in immunoblots of whole hyphal fractions. These data suggest that a protein with epitopic similarity to cofilin may function in F-actin dynamics that underlie invasive growth. The F-actin-depleted zone may play a role in the regulation of tip yielding to turgor pressure, thus increasing the protrusive force necessary for invasive growth.

Cell wall extensibility during branch formation in the xanthophycean alga Vaucheria terrestris

In the tip-growing filamentous cell of the xanthophycean alga Vaucheria terrestris sensu Götz, a new growing tip develops in the non-growing, cylindrical region of the cell that was exposed by local illumination. The present study examined changes in the strength and extensibility of the cell wall of the new growing tip and in the matrix components of the inner surface of the cell wall. The internal pressure required to rupture the cell walls decreased remarkably during the early to middle stages of growing tip development, but the cell wall hardly extended before rupture. In contrast, during the middle and late stages of development, cell walls were extended by internal pressure. Atomic force microscopy revealed that protease-resistant, fine granular matrix components were present only at the apical portion of a normal growing tip, and were absent in the non-growing cylindrical region. In the early and middle stages of new growing tip development, these matrix components appeared in the cell walls in patches. These results suggest that first cell wall strength decreases and then cell wall extensibility increases in the development of new growing tips, and that protease-resistant, fine granular matrix components may be involved in rendering a cell wall extensible.

Plant and fungal cytomechanics: quantifying and modeling cellular architectureThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Biomechanical studies aim at understanding the relationship between the mechanical properties of biological structures and their function. In cytomechanical investigations, this approach is brought down to the scale of cells and subcellular structures. In plant cells and the hyphae of fungi and water molds, interactions between turgor pressure, the cell wall, and the cytoskeleton are considered of primary importance. This review is an overview of how the mechanical properties of these individual features and their interactions have been measured and how the experimental data are used to produce theoretical mechanical models of cellular architecture and dynamics. Several models are discussed, and focusing on the example of tip-growing cells, various approaches to understanding the mechanical aspects of cellular morphogenesis are analyzed.

Surface ultrastructure and elasticity in growing tips and mature regions of Aspergillus hyphae describe wall maturation

This study reports the first direct, high-resolution physical and structural evidence of wall changes during hyphal tip growth, visualized by atomic force microscopy (AFM) inAspergillus nidulans. Images from AFM and cryo-scanning electron microscopy provided comparable information, but AFM was also able to image and physically probe living cells. AFM images showed changes in the surface ultrastructure ofA. nidulanshyphae, from newly deposited walls at hyphal tips to fully mature walls, as well as additional changes at young branches arising from mature walls. Surface architecture during wall maturation correlated with changes in the relative viscoelasticity (compliance per unit applied force) of walls measured by force spectroscopy (FS) in growingA. nidulanshyphae. Growing tips showed greater viscoelasticity than mature walls, despite equal support from turgor. Branch tips had comparable viscoelasticity to hyphal tips, unlike the mature wall from which they grew. FS also revealed differences in surface hydrophilicity between newly deposited and mature walls, with the tips being more hydrophilic. The hydrophilicity of young branch tips was similar to that of hyphal tips, and different from that of mature walls. Taken together, AFM images and FS data suggest that theA. nidulanswall matures following deposition at the hyphal tip.

Proteolytic release of membrane-bound endo-(1,4)-beta-glucanase activity associated with cell wall softening in Achlya ambisexualis

Branching and other cell wall softening events in fungi and oomycetes are thought to involve the activity of secreted enzymes, which are packaged in membrane vesicles and delivered to sites of cell expansion, there to work in a carefully regulated manner upon the structure of the wall. Here we demonstrate a latent endo-(1,4)-beta-glucanase activity in a mixed membrane fraction of the oomycete Achlya ambisexualis, which can be released by cysteine proteases with an increase of apparent activity. In addition, a similar endogenous process is strongly inhibited by the cysteine protease inhibitor iodoacetamide, while inhibitors of other types of proteases have a much smaller effect. Detergent treatment of membranes releases two glucanases detectable by electrophoretic activity staining, with apparent molecular masses of about 164 and 35 kDa. Proteolysis produces several activity bands, with major species having apparent molecular masses of about 149, 133, 48, 35, and 25 kDa. The ca. 35- and 25-kDa bands migrate in parallel with glucanases secreted during wall softening in vivo. We propose that the initiation of wall softening in Achlya involves the proteolytic processing and solubilization of at least some secreted endoglucanases. We also propose that the solubilization component of this process functions not just to provide the enzymes with access to wall matrix substrates but also may provide a mechanism for the eventual termination of their biological function.

Fine structure of the extracellular sheath and cell walls inOphiostoma novo-ulmigrowing on various substrates

The presence of microfilamentous-like structures of tubular appearance (MFS) in cell walls and extracellular sheath material (ES) in a number of isolates of Ophiostoma novo-ulmi Brasier grown on various substrates and following various treatments is reported. Standard fixation or high-pressure freezing methods were used, and cytochemical tests were carried out to detect fungal and host wall components and, in some cases, fungal DNA. In some cases, serial 0.2-μm-thick sections were examined at 120 kV and tilted to obtain stereoscopic images. Whether the fungal cell walls were thick and composed of an outer opaque and inner more electron-lucent layers, or thin and barely perceptible, MFS were observed to extend from the cell cytoplasm as parallel structures across the walls into the surrounding medium, including host cell components in infected elm tissues. MFS were associated (in samples from inoculated trees) with cleavage and desquamation of fungal walls. ES and MFS did not label for cellulose or chitin, but generally labelled slightly for β-(1-3)-glucan and mannose, and strongly for galactose. Only the lucent, inner fungal wall layer labelled for chitin and cellulose. DNA labelling was confined to nuclei and mitochondria in fungal cells from cultures on agar medium; in cells from cultures on millipore membranes, it was pronounced over imprecisely delimited cell regions. The possible ontogeny of MFS components and their importance are discussed. Key words: chitin, Dutch elm disease, fungal fimbriae, fungal walls, gold-complexed probes, microfilamentous structures (MFS).