MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 13. FUNGI

Fungi differ from Algae, mosses, and vascular plants just as profoundly as they do from animals. We describe them here largely out of tradition. However, fungi share one fundamental feature with plants that is absent in all other eukaryotic groups: a filamentous or elongated form of multicellular growth (with only a few fungi, such as Yeasts, being unicellular). Fungi never form motile Cells at any stage of their life cycle, and they have no direct evolutionary link to plants. These two groups arose independently from two different forms of Unicellular Eukaryotes, which justifies classifying fungi as a distinct kingdom.

Along with heterotrophic Bacteria and certain other groups of heterotrophs, fungi function as decomposers in the biosphere (Fig. 13-1). Their activity is just as essential to the Maintenance of the modern world as The activity of producers. The decomposition process releases carbon dioxide into the atmosphere, while nitrogen and Other Compounds are returned to the soil, where they can be utilized once again by plants and animals. This cycle maintains the Circulation of matter in nature. It has been estimated that the top 20 cm of fertile soil contains an average of 5 tons of fungi and bacteria per hectare.

Class="center">Fig. 13-1. Mycelium of a basidiomycete on a fallen tree trunk. The activity of fungi and bacteria drives the cycling of organic matter within the ecosystem.

As decomposers, fungi often inflict direct damage on human endeavors. They make no distinction between rotting fallen timber and wooden fences, actively attacking both. Armed with a powerful arsenal of organic-degrading Enzymes, fungi can be a constant nuisance and occasionally cause major destruction (Fig. 13-2). This is especially evident in the tropics, where warmth and high humidity promote rampant growth. It has been estimated that during World War II, less than 50% of military supplies shipped to such regions arrived in usable condition. Fungi spoil fabrics, paint, cardboard, leather, wax, jet fuel, cable and wire insulation, photographic film, and even optical lenses—virtually every known substance. Even in temperate zones, they are the bane of food production and storage, growing on bread, fruit, vegetables, meat, etc., thereby reducing their nutritional value and palatability. They also produce toxins, some of which (aflatoxins) are potent carcinogens effective even at concentrations in the parts-per-ten-million range.

Fig. 13-2. A. Common Aspergillus mold on strawberries; B. Aspergillus, acting alongside other fungi and bacteria, will decompose this stump until it is level with the soil.

The economic Damage caused by fungi is further compounded by their ability to thrive across a remarkably wide range of environmental conditions. For instance, certain strains of Cladosporium herbarum, which spoil meat in cold storage, can grow at -6°C. Conversely, one species of Chaetomium has an optimal growth Temperature of 50°C and can sometimes even grow at 60°C.

At the same time, certain properties of fungi are highly beneficial to humans. Many species, particularly yeasts, are capable of synthesizing alcohol and carbon dioxide, a process fundamental to baking. The Industrial Applications of fungi are steadily expanding (Fig. 13-3). Some species serve as a valuable source of Antibiotics, including such widely known medications as penicillin.

Fig. 13-3. This massive fermenter, nearly 30 m high, is used in England by Imperial Chemical Industries to produce Yeast, which is sold as a protein-rich feed substitute for soybean extract.

A striking example of the potential of fungi as sources of novel compounds is cyclosporine, a remarkable new substance isolated from a soil fungus. It was first produced on an industrial scale in 1979 by the Swiss company Sandoz. This cyclic molecule consists of 13 different Amino Acids, one of which was discovered here for the first time. Cyclosporine suppresses the immune responses triggered by organ transplantation without the undesirable side Effects of Other drugs used for this purpose (which destroy Bone Marrow—the source of all Blood Cells—thereby risking leukemia). The discovery of this remarkable drug made Heart and other organ transplantations feasible in the early 1980s.

The Nature of the interactions between fungi and other organisms is exceptionally diverse. For example, about 80% of all vascular plants are associated with fungi through their ROOT systems. This association, known as mycorrhiza, plays a pivotal role in PLANT Nutrition AND distribution (see p. 206). Another example is Lichens (see p. 194), a Symbiosis between fungi and algae or cyanobacteria that allows them to colonize the most inhospitable habitats. Many fungi prefer to attack living rather than dead organisms, sometimes doing so in extraordinary ways (see "Predatory Fungi," p. 211). They are the most prevalent phytopathogens; approximately 5,000 species attack economically important crops as well as many wild plants. Some fungi damage living trees, inflicting severe losses on forests worldwide, while others cause serious diseases in humans and domestic animals.

Currently, about 100,000 distinct species of fungi have been described, and it is estimated that another 200,000 await description. This total may eventually correspond to the number of plant species, although far fewer fungi have been given scientific names.

Conventionally, heterotrophic protists discussed in Chapter 14 were also classified as fungi. However, evidence for a direct relationship between them is scarce, and we discuss these groups separately. Virtually none of the known protist groups can be considered the ancestor of fungi, even though they must have evolved from some ancestral stock.

Biology of Fungi

Fungi are predominantly terrestrial organisms. While some unicellular forms exist, the majority possess a filamentous body, and structures such as mushrooms consist of densely packed filaments (Fig. 13-4). Fungal filaments are called hyphae, and their collective mass is termed the mycelium (see Fig. 13-1). Hyphae grow exclusively at their tips, but Protein Synthesis occurs throughout the entire length of the mycelium, after which Proteins are transported to the growth zone by cytoplasmic streaming—a process particularly well developed in fungi (Fig. 13-4). An individual fungus can produce a mycelium over a kilometer long in just 24 hours. The terms "mycelium" and "mycology" (The Study of fungi) are derived from the Greek word myketos, meaning "mushroom."

Fig. 13-4. A mushroom (photographed in a redwood forest in central California) is composed of densely packed hyphae, a mass collectively known as the mycelium. Such fungi reproduce via spores formed on specialized structures lining the gills on the underside of the cap.

The Cell walls of plants and many protists feature a structural framework of Cellulose microfibrils embedded in a matrix of hemicellulose and pectic substances. In contrast, the fungal Cell wall is composed primarily of another polysaccharide, Chitin, which is also a major component of the exoskeleton in Arthropods such as insects, arachnids, and crustaceans (Fig. 13-5). This substance is significantly more resistant to microbial decomposition than cellulose.

Fig. 13-5. The Structure of chitin, composed of N-acetylglucosamine units linked by a β-1,4 bond. Because a similar bond is found in cellulose and bacterial cell walls, it is believed to confer exceptional structural strength to the polysaccharide. Chitin is characteristic of the cell walls of many fungi and also forms the exoskeleton of arthropods.

The rapid growth and filamentous STRUCTURE OF FUNGI determine a unique type of interaction with their environment that is unlike any other group of organisms. The surface-to-volume ratio in fungi is exceptionally high, meaning they contact the outside world almost as intimately as bacteria do. With few exceptions, all PARTS OF THE mycelium lie no more than a few micrometers away from the external environment, separated from it only by a thin cell wall and a Plasma Membrane. Through its extensive mycelium, a fungus can profoundly influence its environment, for instance, by binding soil particles together. Frequently, hyphae—even those originating from different spores—fuse, forming an increasingly complex network.

This intimate contact between fungi and their environment is due to the fact that all their parts are metabolically active, lacking layers of dead cells such as those found in plant wood. Enzymes and other substances secreted by fungi instantly affect their surroundings, which is vital for the fungus's own survival.

All fungi are heterotrophs and function either as saprobionts (i.e., living on dead organic matter) or parasites (i.e., feeding on other organisms). In either case, food is absorbed after being partially digested by extracellular enzymes secreted by the fungal cells. Some fungi, notably yeasts, derive energy through Fermentation, such as converting glucose into ethanol. The primary reserve polysaccharide in some fungi is Glycogen (as in animals and bacteria), whereas in others, the main storage substances are Lipids.

Saprobic fungi sometimes attach to the substrate by means of specialized hyphae called rhizoids. Parasitic forms frequently develop specialized hyphae known as haustoria, which absorb nutrients directly from the cells of other organisms (Fig. 13-6).

Fig. 13-6. Haustorium of the flax rust fungus Melampsora lini within a leaf cell of flax Linum usitatissimum

All fungi possess cell walls, are capable of producing various types of spores, are incapable of locomotion throughout their life cycle, and their cells lack cilia or flagella. Fungal spores are typically dispersed by wind.