Introduction to Fungi
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What is the difference between a parasite and a pathogen? Plant parasitic fungi obtain nutrients from a living plant host, but the plant host doesn't necessarily exhibit any symptoms. In this sense, endophytic fungi discussed in the preceding paragraph are plant parasites because they live in intimate association with plants and depend on them for nutrition.
Plant pathogenic fungi are parasites and cause disease characterized by symptoms. Biotrophic fungal pathogens obtain nutrients from living host tissues, often via specialized cells called haustoria that form inside host cells Fig. Necrotrophic pathogens obtain nutrients from dead host tissue, which they kill through the production of toxins or enzymes.
Most biotrophic fungi have fairly narrow host ranges—they are specialized on a limited number of plant hosts. Necrotrophic fungi can be either generalists, growing on a wide range of host species, or specialized on a restricted range of hosts. Some plant pathogenic fungi change the way that their hosts grow, either by affecting the level of growth regulators produced by the plant, or by producing growth regulators themselves.
Examples of changes in plant growth caused by plant pathogenic fungi include cankers , galls , witches' broom , leaf curl and stunting. Figure 8. We can further divide plant pathogenic fungi by the stage of the plant host that is attacked, for example, seeds, seedlings, or adult plants, and by what part of the plant is affected—roots, leaves, shoots, stems, woody tissues, fruits or flowers.
A group of fungi including species of Fusarium , Rhizoctonia and Sclerotium cause seed rot and infect plants at the seedling stage. These pathogens can attack a wide range of plants. Often, seedling pathogens cause damping-off symptoms because they occur in wet soils. Many of the same fungi that kill seedlings can also infect the roots of mature plants and cause root and crown rot diseases.
Infection often occurs through wounds, and results in lesions or death of part or all of the root system and crown. Some common root rots of trees are caused by members of phylum Basidiomycota in the genera Armillaria and Heterobasidion.
Armillaria spp. Species of Heterobasidion survive as saprotrophs in dead tree stumps and roots, but can also infect living hosts through root contact. These fungi cause decay in the roots and crown; infected trees become weakened and die, or may blow over in high winds. Wood rot fungi, most of which are also members of Basidiomycota, infect trees through wounds, branch stubs and roots, and decay the inner heartwood of living trees. Extensive decay weakens the tree, and reduces the quality of wood in trees harvested for timber see the discussion of "white rot" and "brown rot" fungi above.
Vascular wilt pathogens kill their host by infecting through the roots or through wounds and growing into the xylem, where they produce small spores that get carried upward until they are trapped at the perforated ends of the xylem vessels. The spores germinate and grow through the pores. The fungus is transported throughout the plant in this manner. The first symptom of vascular wilt is a loss of turgidity in the plant leaves, often on one side of the plant or a single branch. If the stems of infected plants are cut open, vascular discoloration is evident.
Among the important vascular wilt fungi are Fusarium oxysporum , Verticillium albo-atrum and V. One of the most famous vascular wilts is Panama disease of bananas, caused by Fusarium oxysporum forma specialis f. This fungus nearly wiped out banana production in Latin America in the early twentieth century. Most bananas that were being grown for export were a single cultivar, 'Gros Michel', which turned out to be highly susceptible to Panama disease. There is no effective method for controlling Panama disease and it rapidly spread throughout banana plantations around the world.
The banana industry was saved by the discovery of the cultivar 'Cavendish' that is resistant to the strain of Panama disease that killed 'Gros Michel'. Leaf spot pathogens infect through natural plant openings such as stomates or by penetrating directly through the host cuticle and epidermal cell wall. In order to penetrate directly, fungi produce hydrolytic enzymes—cutinases, cellulases, pectinases and proteases—for breaking down the host tissue.
Alternatively, some fungi form specialized structures called appressoria sing. Turgor pressure builds up in the appressorium, and in combination with an infection peg , mechanical force is exerted to breach the host cell walls. Once inside the plant leaf, the fungus must obtain nutrients from the cells, and this is often accomplished by killing host cells necrotrophs.
Brief Introduction to Fungi
Death of host cells is evident as an area of dead cells called a lesion Fig. Figure 9. Many leaf-spotting fungi produce toxins that kill host cells and this often produces a lesion surrounded by a yellow halo Fig. If enough of the leaf surface is killed, or if the infected leaves drop prematurely, the plant's ability to produce photosynthates is severely impaired. Figure Returning to bananas, another devastating disease of this host is black leaf streak, or black Sigatoka , caused by Mycosphaerella fijiensis. Unlike the root-infecting pathogen that causes Panama disease, the black Sigatoka pathogen can be controlled by applications of a protective fungicide to banana leaves.
American chestnut trees were once a prominent hardwood tree in the eastern U. Cankers develop when the pathogen kills the phloem and vascular cambium in a woody host. If the canker encircles the trunk or branch of a tree, that plant part will die. The canker-causing fungus can often be identified based on the fruiting bodies that form in the canker. In contrast to cankers, galls result from abnormal growth of a plant, usually due to an increase in cell size and cell division. Although galls are often associated with insect pests, some fungal pathogens induce galls; two common examples are the black knot pathogen , Apiosporina morbosa on Prunus spp.
Gymnosporangium is a type of rust fungus. Rust fungi are biotrophic pathogens—they infect, grow, and sporulate in living plant tissue. Even though biotrophs require living host tissue for their growth and reproduction, they can be devastating pathogens by reducing the photosynthetic surface and increasing water loss in the host plant. Rust fungi attack a wide range of plants, and often require two, unrelated hosts in order to complete their life cycles. Rust fungi are so-named because of the abundant orange spores that are formed on plants that are infected by these fungi; infected plants often look as though they are rusting.
One historically important rust fungus is black stem rust of wheat, a disease that was well known to the ancient Romans. Black stem rust , caused by Puccinia graminis f. Since the barberry host is required for the pathogen to complete its life cycle, early control measures in the United States and Canada were aimed at eliminating this host, not the economically important host, wheat. We now know that this method of eradication was of limited success because rust spores can be carried long distances—for example, from northern Mexico to the U.
Now let's take a closer look at fungi and the types of structures that they form. A key characteristic of fungi that has contributed to their successful exploitation of diverse ecological niches is the formation of a filamentous thallus called the mycelium. A mycelium is composed of branching, microscopic tubular cells called hyphae Fig.
The fungal cell wall in the Kingdom Fungi is composed of chitin and glucans in Ascomycota, Basidiomycota and Chytridiomycota as well as chitosan and other components in Zygomycota Kirk et al.
Hyphae can have cross walls called septa , or lack cross walls nonseptate; aseptate; coenocytic. The type of hyphae— septate or aseptate —is characteristic of specific groups of fungi. In fungi that form septate hyphae, there are perforations at the septa, called septal pores, which allow the movement of cytoplasm and organelles from one compartment to the next.
The type and complexity of the septal pore is characteristic of specific groups of fungi. Hyphae grow from a germinating spore or other type of propagule, and these are described in more detail in the section "Fungal Reproduction. As a result of apical growth, hyphae are relatively uniform in diameter, and mycelium that grows in an unimpeded manner forms a circular colony on solid substrates that support fungal growth; agar, a gelatinous material derived from seaweed, amended with different types of nutrients is commonly used to grow fungi in culture Fig.
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Some fungi grow exclusively or mostly as yeasts , defined as single-celled fungi that reproduce by budding or fission. In contrast to apical growth that is characteristic of hyphae, yeasts exhibit wall growth over the entire cell surface, often resulting in a nearly spherical cell Fig. There are also fungi that can switch between mycelial growth and yeast-like growth, dependent upon the environmental conditions.
The ability to grow in different forms is called dimorphism, and is exhibited by some members of phyla Ascomycota, Basidiomycota and Zygomycota. Most of the organelles present in fungal cells are similar to those of other eukaryotes. Fungi have been found to possess between 6 and 21 chromosomes coding for 6, to nearly 18, genes.
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Genome sizes range from 8. Many fungi Ascomycota have a life cycle that is predominantly haploid, while others Basidiomycota have a long dikaryotic phase. Fungi frequently reproduce by the formation of spores. A spore is a survival or dispersal unit, consisting of one or a few cells, that is capable of germinating to produce a new hypha.
Unlike plant seeds, fungal spores lack an embryo, but contain food reserves needed for germination. Many fungi produce more than one type of spore as part of their life cycles.
Characteristics of Fungi – Biology 2e
Fungal spores may be formed via an asexual process involving only mitosis mitospores , or via a sexual process involving meiosis meiospores. The manner in which meiospores are formed reflects the evolutionary history and thus the classification for the major groups phyla of fungi. Many fungi produce spores inside or upon a fruiting body. Many people are familiar with the mushroom, a type of fruiting body produced by some Basidiomycota. You may recognize other fungal fruiting bodies such as puffballs, or shelf fungi. These are examples of large, conspicuous fruiting bodies, but there is an even greater diversity of microscopic fruiting bodies produced by various fungi.
What all fruiting bodies have in common is that they produce spores and provide a mechanism for dispersing those spores. Fruiting bodies will be discussed in more detail within the fungal groups. Many fungi are able to reproduce by both sexual and asexual processes. Sexual and asexual reproduction may require different sets of conditions e. In some fungi, two sexually compatible strains must conjugate mate in order for sexual reproduction to occur. The terms ' anamorph ' and ' teleomorph ' are used to convey the asexual and sexual reproduction morphological types, respectively, in a particular fungus.