Parasitic fungi - Eniscuola
Most pathogenic (disease-causing) fungi are parasites of plants. Certain fungi form highly specialized parasitic relationships with insects. The majority of sooty molds are tropical or subtropical, but some species occur in the temperate. The parasitic species transmit diseases and form relations with other organisms, of fungi are pathogenic to man, causing infections that are called mycoses. However, only a few species are routinely found associated with humans and are capable of For example, most host–pathogen interactions are parasitic with the pathogen . The symbiotic relationship between humans and fungi is greatly.
Mating in fungi and Sexual selection in fungi Sexual reproduction with meiosis has been directly observed in all fungal phyla except Glomeromycota  genetic analysis suggests meiosis in Glomeromycota as well. It differs in many aspects from sexual reproduction in animals or plants.
Differences also exist between fungal groups and can be used to discriminate species by morphological differences in sexual structures and reproductive strategies. Fungi employ two mating systems: In sexually reproducing fungi, compatible individuals may combine by fusing their hyphae together into an interconnected network; this process, anastomosisis required for the initiation of the sexual cycle.
Many ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not combine immediately after cell fusion, but remain separate in the hyphal cells see heterokaryosis.
During cell divisionformation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments.
An ascus plural asci is then formed, in which karyogamy nuclear fusion occurs. Asci are embedded in an ascocarpor fruiting body. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. After dispersal, the ascospores may germinate and form a new haploid mycelium. Compatible haploid hyphae fuse to produce a dikaryotic mycelium. However, the dikaryotic phase is more extensive in the basidiomycetes, often also present in the vegetatively growing mycelium.
A specialized anatomical structure, called a clamp connectionis formed at each hyphal septum. As with the structurally similar hook in the ascomycetes, the clamp connection in the basidiomycetes is required for controlled transfer of nuclei during cell division, to maintain the dikaryotic stage with two genetically different nuclei in each hyphal compartment. The parasitized insects are, however, rendered sterile.
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The perpetuation of the insect species and the spread of the fungus are accomplished by the uninfected members of the colonywhich are protected from enemies by the fungus body. Newly hatched scale insects crawl over the surface of the fungus, which is at that time sporulating.
Fungal spores adhere to the young insects and germinate. As the young insects settle down in a new place on the bark to begin feeding, they establish new fungal colonies. Thus, part of the insect colony is sacrificed to the fungus as food in return for the fungal protection provided for the rest of the insects. The insect is parasitic on the tree and the fungus is parasitic on the insect, but the tree is the ultimate victim.
Some fungi are parasitic on insects. For example, Cordyceps militaris invades living insect pupa by drawing nutrients from the pupa that enable the fungus to grow and generate spores for reproduction. The sooty molds constitute another interesting ecological group of fungi that are associated with insects. The majority of sooty molds are tropical or subtropical, but some species occur in the temperate zones.
Aspergillus fumigatus, one of the most important of these opportunists, produces small, airborne spores that are frequently inhaled; in some individuals the fungus starts growing invasively, causing a disease known as aspergillosis, especially in immunocompromised individuals.
A remarkable discovery was that Pneumocystis carinii, the organism causing pneumonia-like symptoms in immunocompromised patients, is a fungus and not a protozoan as had been thought for decades. Why was this pathogen classified as a protozoan? It does not respond to the common drugs used to treat fungal infections, but does respond to anti-protozoan drugs.
This unusual fungus emerged as one of the leading causes of death in AIDS patients in the late twentieth century. Fungi and plants The association of fungi and plants is ancient and involves many different fungi. Plant pathogenic fungi represent a relatively small subset of those fungi that are associated with plants.
Introduction to Fungi
Most fungi are decomposers, utilizing the remains of plants and other organisms as their food source. Other types of associations that will be discussed here include the role of fungi as decomposers, as beneficial symbionts, and as cryptic plant colonizers called endophytes. Most fungi are associated with plants as saprotrophs and decomposers. These fungi break down organic matter of all kinds, including wood and other types of plant material.
Wood is composed primarily of cellulose, hemicellulose, and lignin. Lignin is a complex polymer that is highly resistant to degradation, and it encrusts the more readily degradable cellulose and hemicellulose. Fungi are among the few organisms that can effectively break down wood, and fall into two main types—brown and white rot fungi.
Brown rot fungi selectively degrade the cellulose and hemicellulose in wood, leaving behind the more recalcitrant lignin. The decayed wood is brown in color and tends to form cubical cracks due to the brittle nature of the remaining lignin Fig.
- 24.4: Fungal Parasites and Pathogens
Brown rot residues make up 'humus' in temperate forest soils and are important for mycorrhizal formation see the following paragraph for information on mycorrhizal fungimoisture retention, and for sequestering carbon. Brown rot residues are highly resistant to decomposition and can remain in the soil for up to years.
White rot fungi are more common than brown rot fungi; these fungi degrade cellulose, hemicellulose, and lignin at approximately equal rates. The decayed wood is pale in color, light in weight, and has a stringy texture Fig.
White rot fungi are the only organisms that can completely degrade lignin. Figure 4 Figure 5 An important group of fungi associated with plants is mycorrhizal fungi. Mycorrhiza means 'fungus root', and it refers to a mutually beneficial association a type of symbiosis between fungi and plant roots. There are seven major types of mycorrhizal associations, the most common of which is the arbuscular mycorrhizae, involving members of phylum Glomeromycota associated with roots of most major groups of plants.
Another common type of association is ectomycorrhizae formed between forest trees and members of phyla Basidiomycota and Ascomycota. In this association, the fungus forms hyphae around host root cortical cells—the "Hartig net"— and a sheath of hyphae around the host roots called a "mantle.
A valuable group of ectomycorrhizal fungi are truffles, members of phylum Ascomycota that form underground fruiting bodies. Figure 6 Lichens are examples of a symbiotic association involving a fungus and green algae or less frequently Cyanobacteria. The lichen thallus is composed mostly of fungal hyphae, usually with the alga or cyanobacterium confined to discrete areas of the thallus.
In lichens, reproductive structures of the fungus are often conspicuous, for example disc- or cup-like structures called apothecia Fig. The fungus obtains carbohydrates produced by photosynthesis from the algae or cyanobacteria, and in return provides its partner s with protection from desiccation and ultraviolet light.
Lichens grow in a wide range of habitats on nearly every continent. Think about an inhospitable place, and there's probably a lichen that grows there—on bare rocks, sidewalks, grave stones, the exoskeletons of some insects, and even on cars that remain for a long time in one place!
Figure 7 Some fungi are hidden inside their plant hosts; these are endophytesdefined by their presence inside asymptomatic plants. All plants in natural ecosystems probably have some type of symbiotic association with endophytic fungi Rodriguez et al.
Endophytic fungi have been shown to confer stress tolerance to their host plant, for example, to disease, herbivory, drought, heat, salt and metals. The clavicipitaceous endophytes in the genus Neotyphodium phylum Ascomycota are among the best studied.
These fungi produce alkaloid compounds that protect the grass host from insects that would otherwise feed on them; endophyte-infected turfgrass seed is sold commercially for seeding lawns and other types of grassy recreational areas. Unfortunately, livestock such as sheep, cattle, llamas and horses also are negatively affected by toxins produced by endophytes when they eat infected grass. Afflicted animals develop symptoms including tremors and jerky or uncoordinated movements. Let's now consider the role of fungi as plant pathogens.
Plant pathogenic fungi are parasites, but not all plant parasitic fungi are pathogens. 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 cankersgallswitches' broomleaf 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.
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 oxysporumVerticillium 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 pegmechanical 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.
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 10 Returning to bananas, another devastating disease of this host is black leaf streak, or black Sigatokacaused by Mycosphaerella fijiensis.