Two distinct distributions of F-actin are present in the hyphal apex of the oomycete Achlya bisexualis

A fitness landscape instability governs the morphological diversity of tip-growing cells

Cellular morphology affects many aspects of cellular and organismal physiology. This makes it challenging to dissect the evolutionary basis for specific morphologies since various cellular functions may exert competing selective pressures on this trait, and the influence of these pressures will depend on the specific mechanisms of morphogenesis. In this light, we combined experiment and theory to investigate the complex basis for morphological diversity among tip-growing cells from across the tree of life. We discovered that an instability in the widespread mechanism of "inflationary" tip growth leads directly to a bifurcation in the common fitness landscape of tip-growing cells, which imposes a strict global constraint on their morphologies. This result rationalizes the morphology of an enormous diversity of important fungal, plant, protistan, and bacterial systems. More broadly, our study elucidates the principle that strong evolutionary constraints on complex traits, like biological form, may emerge from emergent instabilities within developmental systems.

Effects of latrunculin B on the actin cytoskeleton and hyphal growth in Phytophthora infestans

The actin cytoskeleton is conserved in all eukaryotes, but its functions vary among different organisms. In oomycetes, the function of the actin cytoskeleton has received relatively little attention. We have performed a bioinformatics study and show that oomycete actin genes fall within a distinct clade that is divergent from plant, fungal and vertebrate actin genes. To obtain a better understanding of the functions of the actin cytoskeleton in hyphal growth of oomycetes, we studied the actin organization in Phytophthora infestans hyphae and the consequences of treatment with the actin depolymerising drug latrunculin B (latB). This revealed that latB treatment causes a concentration dependent inhibition of colony expansion and aberrant hyphal growth. The most obvious aberrations observed upon treatment with 0.1 μM latB were increased hyphal branching and irregular tube diameters whereas at higher concentrations latB (0.5 and 1 μM) tips of expanding hyphae changed into balloon-like shapes. This aberrant growth correlated with changes in the organization of the actin cytoskeleton. In untreated hyphae, staining with fluorescently tagged phalloidin revealed two populations of actin filaments: long, axially oriented actin filament cables and cortical actin filament plaques. Two hyphal subtypes were recognized, one containing only plaques and the other containing both cables and plaques. In the latter, some hyphae had an apical zone without actin filament plaques. Upon latB treatment, the proportion of hyphae without actin filament cables increased and there were more hyphae with a short apical zone without actin filament plaques. In general, actin filament plaques were more resilient against actin depolymerisation than actin filament cables. Besides disturbing hyphal growth and actin organization, actin depolymerisation also affected the positioning of nuclei. In the presence of latB, the distance between nuclei and the hyphal tip decreased, suggesting that the actin cytoskeleton plays a role in preventing the movement of nuclei towards the hyphal tip.

An investigation into plasmolysis in the oomycete Achlya bisexualis reveals that membrane–wall attachment points are sensitive to peptides containing the sequence RGD and that cell wall deposition can occur despite retraction of the protoplast

The structure and function of membrane–wall attachment sites in walled cells, and how these relate to animal focal adhesions, is an area that is poorly understood. In view of this, we investigated how membrane–wall attachments that form upon plasmolysis, respond to peptides that disrupt animal focal adhesions. The degree of cytoplasmic disruption during plasmolysis was also investigated. Upon hyperosmotic challenge, the protoplast in hyphae of the oomycete Achlya bisexualis typically retracted incompletely due to membrane–wall attachments. The inclusion, in the plasmolysing solution, of peptides containing the sequence RGD disrupted these attachments in a dose-dependent manner. In some hyphae, protoplast retraction stopped temporarily at attachment points — upon resumption of retraction, material was left that traced the outline of the static protoplast. Staining of this material with fluorescence brightener indicated the presence of cellulose, which suggests that wall deposition was able to occur despite plasmolysis. The F-actin cytoskeleton was disrupted during plasmolysis; peripheral F-actin staining was observed, but there was no distinct F-actin cap; staining was more diffuse; and there were fewer plaques compared with nonplasmolysed hyphae. Our data indicate that membrane–wall attachment points are sensitive to RGD-containing peptides and that wall deposition continues despite protoplast retraction and F-actin disruption.

An investigation of the effects of Ca²+ channel inhibitors on branching and chemotropism in the oomycete Achlya bisexualis: Support for a role for Ca²+ in apical dominance

In an attempt to better understand branching and chemotropism, we describe the effects of Ca²+ channel inhibitors on these processes in Achlya bisexualis, using a branch induction technique and whole plate assays. Branching appears to be a two step process with the initial formation of a bump from which a branch emerges. Verapamil increased numbers of branches in whole plate assays and decreased the distance from the first branch to the tip. In induction assays verapamil increased the number of bumps formed, although in some hyphae it inhibited the transition from an initial bump to a branch. When a branch formed it did not affect the time taken to branch. It had no effect on chemotropism. Lanthanum (La³+) and gadolinium (Gd³+) also increased branching in whole plate assays but their effect was much less marked and they had no effect on bump/branch number in induction assays. Gd³+ decreased the time taken to branch. Both La³+ and Gd³+ increased chemotropism. These data suggest firstly that the respective inhibitors may affect different parts of the branching process and secondly that Ca²+ influx through channels may not be a requirement for branching, indeed such movements may suppress branching. This would fit with elevated Ca²+ at the tip playing a role in apical dominance.

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.

Invasive hyphal growth: An F-actin depleted zone is associated with invasive hyphae of the oomycetes Achlya bisexualis and Phytophthora cinnamomi

We have compared F-actin patterns in invasive and non-invasive oomycete hyphae. In Achlya bisexualis an F-actin depleted zone is present in 70% of invasive but only 9% of non-invasive hyphae. In Phytophthora cinnamomi these figures are 74 and 20%, respectively. Thus, the F-actin depleted zone appears to be associated with invasive growth. TEM images indicate that it is unlikely to represent areas of vesicle accumulation. Measurements of turgor indicate no significant increase under invasive conditions (0.65 MPa (invasive) and 0.63 MPa (non-invasive)). Similarly we found no difference in burst pressures (1.04 MPa (invasive) and 1.06 MPa (non-invasive)), although surrounding agarose may lead to overestimates of invasive tip strength. An F-actin depleted zone has the potential, along with wall softening, to increase protrusive force in the absence of turgor increases. Staining of F-actin in hyphae under hyperosmotic conditions suggests that decreases in F-actin at growing tips may also enable non-invasive growth at very low turgor.

A beta4 integrin-like protein co-localises with a phosphotyrosine containing protein in the oomycete Achlya bisexualis: inhibition of tyrosine phosphorylation slows tip growth

We present immunocytochemical data that indicate the presence of, and a close association between beta4 integrin-like proteins and proteins containing phosphorylated tyrosine residues in the oomycete Achlya bisexualis. When hyphae were plasmolysed, these proteins were present in wall-membrane attachment sites where there was also F-actin. A combination of immunoblots, ELISA, and a coupled enzyme assay suggest that phosphorylation may occur by both autophosphorylation and through the action of a tyrosine kinase. Tyrphostins, which are inhibitors of tyrosine kinases, abolished the anti-phosphotyrosine staining, inhibited the kinase activity, slowed tip growth and affected the organisation of the actin cytoskeleton, in a dose-dependent manner. By analogy with the integrins and associated kinases of the metazoa we suggest that these proteins could contribute to the process of tip growth by providing a means of bidirectional signaling between the cell wall and the cytoplasm.

Enhanced fixation reveals the apical cortical fringe of actin filaments as a consistent feature of the pollen tube

The actin cytoskeleton plays a crucial role in the growth and polarity of the pollen tube. Due to inconsistencies in the conventional preservation methods, we lack a unified view of the organization of actin microfilaments, especially in the apical domain, where tip growth occurs. In an attempt to improve fixation methods, we have developed a rapid freeze-whole mount procedure, in which growing pollen tubes (primarily lily) are frozen in liquid propane at -180 degrees C, substituted at -80 degrees C in acetone containing glutaraldehyde, rehydrated, quenched with sodium borohydride, and probed with antibodies. Confocal microscopy reveals a distinct organization of actin in the apical domain that consists of a dense cortical fringe or collar of microfilaments starting about 1-5 microm behind the extreme apex and extending basally for an additional 5-10 microm. In the shank of the pollen tube, basal to the fringe, actin forms abundant longitudinal filaments that are evenly dispersed throughout the cytoplasm. We have also developed an improved ambient-temperature chemical fixation procedure, modified from a protocol based on simultaneous fixation and phalloidin staining. We removed EGTA, elevated the pH to 9, and augmented the fixative with ethylene glycol bis[sulfosuccinimidylsuccinate] (sulfo-EGS). Notably, this protocol preserves the actin cytoskeleton in a pattern similar to that produced by cryofixation. These procedures provide a reproducible way to preserve the actin cytoskeleton; employing them, we find that a cortical fringe in the apex and finely dispersed longitudinal filaments in the shank are consistent features of the actin cytoskeleton.