Nicholas P. Money

Osmotic pressure of fungal compatible osmolytes.

Filamentous fungi and yeasts control cytoplasmic osmotic pressure through ion accumulation and synthesis of compatible osmolytes including polyhydric alcohols (polyols), proline, and trehalose. Authoritative data on the osmotic effects of these compounds were obtained using vapour pressure deficit osmometry. All osmolytes tested were characterised by nonlinear relationships between concentration and osmotic pressure. At high concentrations larger polyols generated higher osmotic pressures than smaller ones, though differences between the osmotic effects of polyols with three, four, five and six carbon atoms were not pronounced at lower (physiological) concentrations. Proline shared a similar relationship between concentration and osmotic pressure with polyols with five carbon atoms, while at concentrations above 0.5 M trehalose generated higher osmotic pressures than any of the polyols tested. Mixtures of trehalose and glycerol boosted osmotic pressure in a synergistic rather than additive fashion. These data provide new clues to the adaptive significance of glycerol accumulation, and also suggest that complex patterns of osmolyte synthesis are not due to differences between the osmotic effects of these compounds.

The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi

Background A variety of spore discharge processes have evolved among the fungi. Those with the longest ranges are powered by hydrostatic pressure and include “squirt guns” that are most common in the Ascomycota and Zygomycota. In these fungi, fluid-filled stalks that support single spores or spore-filled sporangia, or cells called asci that contain multiple spores, are pressurized by osmosis. Because spores are discharged at such high speeds, most of the information on launch processes from previous studies has been inferred from mathematical models and is subject to a number of errors. Methodology/Principal Findings In this study, we have used ultra-high-speed video cameras running at maximum frame rates of 250,000 fps to analyze the entire launch process in four species of fungi that grow on the dung of herbivores. For the first time we have direct measurements of launch speeds and empirical estimates of acceleration in these fungi. Launch speeds ranged from 2 to 25 m s−1 and corresponding accelerations of 20,000 to 180,000 g propelled spores over distances of up to 2.5 meters. In addition, quantitative spectroscopic methods were used to identify the organic and inorganic osmolytes responsible for generating the turgor pressures that drive spore discharge. Conclusions/Significance The new video data allowed us to test different models for the effect of viscous drag and identify errors in the previous approaches to modeling spore motion. The spectroscopic data show that high speed spore discharge mechanisms in fungi are powered by the same levels of turgor pressure that are characteristic of fungal hyphae and do not require any special mechanisms of osmolyte accumulation.

Biomechanics of Invasive Hyphal Growth

Filamentous fungi penetrate diverse solid substrates, including plant and animal tissues, by a process called invasive hyphal growth. Extending hyphae overcome the resistance of their food sources by the secretion of lytic enzymes and the exertion of mechanical force. The forces utilized for invasive growth are derived from turgor pressure and are regulated through loosening of the apical cell wall of the hypha. This chapter explains how hyphae are pressurized and how they apply this pressure during invasive growth. Recent experimental work is discussed, including the use of miniature strain gauges and laser tweezers to measure the forces exerted by hyphae, and information on hyphal mechanics obtained by atomic force microscopy. Other topics in this chapter include current thinking on the role of secreted enzymes and the cytoskeleton in the invasive process, and the remarkable mechanism of leaf penetration by melanized appressoria.

More g's than the Space Shuttle: ballistospore discharge

Ballistospores of basidiomycete fungi form at the tips of spear-shaped projections called sterig- mata that extend from basidia. At maturity, a spher- ical drop of fluid appears at the base of each spore, and a few seconds later the spore is propelled into the surrounding air. The development of the fluid drop was first reported in 1889, but a century of in- novative research was necessary to solve the mecha- nistic link between the drop and spore discharge. Through an extraordinary series of experiments the composition of the drop has now been established, its development is explained, and an effective solu- tion to the relationship between drop appearance and spore discharge has been proposed. Drop for- mation is initiated when a femtomole quantity of mannitol and hexoses is excreted from a specific site at the base of the spore, forming a hygroscopic nu- cleus upon which water condenses from the sur- rounding air. Discharge of the spore occurs when the drop fuses with a film of liquid that curves over the adjacent spore surface. This rapid coalescence results in a decrease in surface free energy within the liquid and displaces the center of mass of the spore. The change in weight distribution exerts a force that is opposed by the pressurized sterigma, and the spore is shot away from the basidium into the surrounding air. The mechanism is described as a surface-tension catapult. During discharge, ballistospores are subject- ed to an acceleration of 25 000 g, which is about ten thousand times the acceleration experienced by as- tronauts during the launch of the Space Shuttle! Even more impressive is the fact that while the Shut- tle consumes 50% of its weight in fuel in the first 2 min of flight, ballistospore discharge is fueled by the mannitol and hexoses that cause water to condense on the spore surface, and these solutes represent only 1% of the mass of the spore.

Adaptation of the Spore Discharge Mechanism in the Basidiomycota

Background Spore discharge in the majority of the 30,000 described species of Basidiomycota is powered by the rapid motion of a fluid droplet, called Bullers drop, over the spore surface. In basidiomycete yeasts, and phytopathogenic rusts and smuts, spores are discharged directly into the airflow around the fungal colony. Maximum discharge distances of 1–2 mm have been reported for these fungi. In mushroom-forming species, however, spores are propelled over much shorter ranges. In gilled mushrooms, for example, discharge distances of <0.1 mm ensure that spores do not collide with opposing gill surfaces. The way in which the range of the mechanism is controlled has not been studied previously. Methodology/Principal Findings In this study, we report high-speed video analysis of spore discharge in selected basidiomycetes ranging from yeasts to wood-decay fungi with poroid fruiting bodies. Analysis of these video data and mathematical modeling show that discharge distance is determined by both spore size and the size of the Bullers drop. Furthermore, because the size of Bullers drop is controlled by spore shape, these experiments suggest that seemingly minor changes in spore morphology exert major effects upon discharge distance. Conclusions/Significance This biomechanical analysis of spore discharge mechanisms in mushroom-forming fungi and their relatives is the first of its kind and provides a novel view of the incredible variety of spore morphology that has been catalogued by traditional taxonomists for more than 200 years. Rather than representing non-selected variations in micromorphology, the new experiments show that changes in spore architecture have adaptive significance because they control the distance that the spores are shot through air. For this reason, evolutionary modifications to fruiting body architecture, including changes in gill separation and tube diameter in mushrooms, must be tightly linked to alterations in spore morphology.

The fungal dining habit: a biomechanical perspective

Invasive hyphal growth allows filamentous fungi to insinuate themselves in the solid materials that serve as their food sources. Hyphae overcome the mechanical resistance of plant and animal tissues, and other substances through the secretion of digestive enzymes and the exertion of force. This force is derived from the osmotically-generated turgor pressure within the hypha and is governed by wall loosening at the growing apex. This article offers a concise description of the biomechanics of this process.

How far and how fast can mushroom spores fly? Physical limits on ballistospore size and discharge distance in the Basidiomycota

Active discharge of basidiospores in most species of Basidiomycota is powered by the rapid movement of a droplet of fluid, called Bullers drop, over the spore surface. This paper is concerned with the operation of the launch mechanism in species with the largest and smallest ballistospores. Aleurodiscus gigasporus (Russulales) produces the largest basidiospores on record. The maximum dimensions of the spores, 34 × 28 µm, correspond to a volume of 14 pL and to an estimated mass of 17 ng. The smallest recorded basidiospores are produced by Hyphodontia latitans (Hymenochaetales). Minimum spore dimensions in this species, 3.5 × 0.5 µm, correspond to a volume of 0.5 fL and mass of 0.6 pg. Neither species has been studied using high-speed video microscopy, but this technique was used to examine ballistospore discharge in species with spores of similar sizes (slightly smaller than A. gigasporus and slightly larger than those of H. latitans). Extrapolation of velocity measurements from these fungi provided estimates of discharge distances ranging from a maximum of almost 2 mm in A. gigasporus to a minimum of 4 µm in H. latitans. These are, respectively, the longest and shortest predicted discharge distances for ballistospores. Limitations to the distances traveled by basidiospores are discussed in relation to the mechanics of the discharge process and the types of fruit-bodies from which the spores are released.

Pulses in turgor pressure and water potential: resolving the mechanics of hyphal growth

Hyphae do not grow at perfectly constant rates, but exhibit rapid oscillations in speed that are thought to reflect waves of vesicular fusion with the apical plasma membrane and dynamic changes in the mechanical properties of the cell wall. Theoretical considerations suggest that hyphal turgor pressure falls in response to wall loosening, and that the resulting differential between the water potential of the cell and its surroundings causes water influx. Measurements of micronewton forces exerted by single hyphal apices using a miniature strain gauge reveal the predicted fluctuations in turgor, and these show similar frequency to oscillations in the growth rate. This paper offers a conceptual framework for understanding the biomechanical processes that operate during hyphal growth.

Biophysics: fungus punches its way in.

When certain fungi infect grasses and cereals, they develop a specialized structure called the appressorium. This microscopic structure inflates on the surface of the grass leaf, then generates enough force to push through its cuticle and cell wall to tap the juices within. The development of a technique to measure the forces generated by the appressorium should help in understanding how this infection platform works.

Mechanics of Invasive Fungal Growth and the Significance of Turgor in Plant Infection

In many plant diseases, fungi execute direct cuticular penetration and then breach the epidermal cell wall. A mycelium is established by further invasive growth through host tissues. The role of specific exoenzymes in the disease process is still questionable, but mechanical aspects of invasion are well defined. Invasive growth by tip-growing hyphae is largely driven by the force available from turgor pressure, though cytoskeletal expansion may play a subsidiary role. The delivery of this force to the substrate in contact with the hyphal tip depends upon the interplay between turgor, the cytoskeleton, and processes that control wall-yielding; this is formalized in a novel mathematical model for plant infection. Empirical support for the model comes from studies on hyphal growth, and from experiments on appressorial development in the rice blast fungus Magnaporthe grisea. There is substantial evidence that turgor-driven invasive growth provides the link between melanin synthesis and pathogenicity in M. grisea.