Lecture 2 - Fungi Diversity PDF

PEBO 102 PLANT DIVERSITY: MORPHOLOGY AND FUNCTION Fungal Diversity  Introduction to fungi The position of the fungi in the biological world has been debated for many years. Although they were originally classified with photosynthetic plants, they were separated from these and g...

PEBO 102 PLANT DIVERSITY: MORPHOLOGY AND FUNCTION Fungal Diversity  Introduction to fungi The position of the fungi in the biological world has been debated for many years. Although they were originally classified with photosynthetic plants, they were separated from these and grouped with the algae and protozoa (protista). Later, scientists (mycologists) placed fungi in the Kingdom Mycota.  GENERAL BIOLOGY OF FUNGI Fungi lack chlorophyll and their cell walls contain chitin, mannan and β -glucans – a characteristics of some animal cells. Note that cellulose is generally absent from the cell walls of true fungi. Generally, fungi are divided into two broad groups - microscopic and macroscopic. i) Microscopic fungi (moulds and yeasts). Moulds are multicellular unlike yeasts which are unicellular. ii) Macroscopic fungi (mushrooms, puffballs, jelly fungi etc.) Structural forms These microscopic and macroscopic fungi can either exist as: Unicellular (single-celled and colonial – yeasts) Multicellular forms with several different cell types. Multicellular fungi are primarily filamentous in their growth form (i.e. their bodies consist of long, slender filaments /numerous fine branching threads, called hyphae which together form entangle mass called mycelium. Morphological features of hyphae Hyphae are of two different types according to their structure: 1) Aseptate/Non septate/Coenocytic Aseptate hyphae are continuous or branching filaments (tubes) filled with cytoplasm and multiple nuclei. Aseptate hyphae are known as coenocytic hyphae. 2) Septate Septate hyphae are made up of long chains of cells joined end-to-end and divided by cross-walls called septa which have pores. The septa rarely form a complete barrier, except when they separate the reproductive cells from vegetative form. NB: Cytoplasm flows freely throughout the hyphae, passing through major pores. This cytoplasmic streaming carries synthesised proteins and other nutrients to their actively growing tips. When these nutrients and water are abundant, and temperature is optimum, fungal hyphae grow very rapidly. Hyphae classification based on their function Vegetative hyphae: are those that grow as submerged or surface filaments which digest, absorb and distribute nutrients obtained from the substrate to other parts for growth and development. Aerial hyphae: project above the surface in culture medium. NB: vegetative hyphae may or not produce spores. Reproductive/fertile hyphae: aerial hyphae which produce asexual spores on the surface of the substrate. Nutrition cont’d Mode of acquisition of nutrients Generally, fungi acquire nutrients by any of these three means: Parasitism - obtaining nutrients from another living organism (e.g. Phythophthora sp. being parasitic on cocoa plant). Saprophytism - absorbing nutrients from dead organic matter (e.g. Termitomyces sp. feeding on litter). Symbiotic associations- where some fungi associate with other organisms, especially roots of higher plants and exchange nutrients (e.g. Boletus spp., Glomus, spp. associate with the roots to form structures called mycorrhizae through which the exchanges occur). Habitat Fungi occur both on land (terrestrial) and in water (aquatic-fresh water and marine) REPRODUCTION Fungi reproduces in three different ways: 1. sexual 2. asexual 3. vegetative (budding as seen in yeast)  Life cycle Both sexual reproduction and asexual reproduction occur in fungi. Sexually by forming sexual spores and asexually by forming conidia or asexual spores. During asexual life cycle, spores settle on a substrate, develops an outgrowth known as germ tube which then elongate into a hypha. Through continued growth and branching, an extensive mycelium is produced.  During sexual life cycle, the haploid nuclei of compatible hyphae fuse forming a diploid nucleus (zygote) which subsequently participates in sexual spore development through meiosis. NB: 1) Fungal cells spend most of their life cycle in the haploid state hence their cells already have a chromosome number compatible for sexual union. 2) However, some species seem to have either lost the capability for sexual reproduction or do so only infrequently. Sexual reproduction  Fungal classification The four major groups are: Chytridiomycota, Zygomycota, Ascomycota and Basidiomycota. Chytridiomycota: this group is characterized by motile spores (zoospores) are commonly called chytrids. Most chytrids grow aerobically in soil and mud They may have a unicellular or multicellular body structure and inhabit aquatic environment. Examples- Olpidium and Synchytrium Zygomycota (The coenocytic true fungi) The group is characterized by production of non-motile spores. Their asexual reproduction spore type are called aplanospores. The mycelial organization is coenocytic. Rhizopus and Mucor are among the well-known examples of the Zygomycota and commonly called the black bread moulds. Rhizopus species Mucor species  Ascomycota (The Sac fungi) The characteristic feature of this group is that the sexually produced spores (ascospores) are contained within a sac known as the ascus. The group comprises the following: yeasts, food spoilage moulds, brown fruit rotting fungi, morels and truffles NB. Some morels and truffles are edible but there are poisonous species as well. Most common moulds belong to species within these genera - Aspergillus, Penicillium, Fusarium etc. Basidiomycota (Club fungi) This group includes both microscopic and macroscopic forms. Some species are edible, and others are not. The unique characteristic of the group is the production of basidiospores in a structure called basidium. The basidium is located within the gill region of the mushroom. Among the basidiomycetes include some of the most familiar species of this group are mushrooms (edible) and toadstools (poisonous species/non-edible), puffballs, earth stars, shelf or bracket fungi, stinkhorns, rusts, smuts, jelly fungi, and bird’s-nest fungi. The following are few examples of some mushroom species: Agaricus, Lentinus, Pleurotus, Termitomyces, Volvariella, etc.  Examples of toadstools include Amanita, Chlorophyllum molybdites etc. ECONOMIC IMPORTANCE OF FUNGI Recycling of nutrients/ maintenance of soil fertility Fungi, together with bacteria decompose dead organic material thus releasing nutrients back into the ecosystem. This activity greatly enhances soil fertility, thus promoting crop productivity. Without fungi, this recycling activity would be highly reduced, and we would effectively be lost under piles of dead plant and animal remains, many meters thick. Formation of symbiotic association with plant roots Some fungi are vitally important for plant growth and development through mycorrhizal associations with plant roots. This enhancement of the growth of primary producers is ultimately beneficial to the entire food chain. Fungi are considered natural biofertilizers, since they provide the host with water, nutrients, and pathogen protection, in exchange for photosynthetic products. Mycorrhizae fungi in symbiotic relation with plant roots  Food Fungi are consumed either directly as food or used indirectly in food and beverage industry. Many mushrooms are edible and mushroom cultivation is a commercially viable venture practiced worldwide. E.g. Termitomyces, Volvariella volvacea, Truffle (Tuber spp.) – are rare, wild, edible and very expensive variety considered a delicacy due to its intense aroma and characteristics flavour. Cheeses (P. camemberti, P. roqueforti), beer and wine, bread, some cakes, corn dough (yeast; Saccharomyces cerevisiae) and some soya bean products all benefit from fungal activity.  Medicines Fungi produce antibiotics which are widely used medicinally to control diseases in both human and animal populations. The most famous of all antibiotic drugs Penicillin, is derived from a common fungus called Penicillium chrysogenum (formerly P. notatum). Biocontrol of soil-borne pathogens Some fungi have been used in biological control of pests. E.g. Chinese caterpillar fungus (Cordyceps sinensis), which parasitises insects. The spores of the fungi are sprayed on the crop pests which results in the growth of fungal hyphae in the insects and eventually kills it. Fungi are also used to control nematodes in the soil. E.g. Trichoderma viride is inoculated into the soil and the hyphae penetrates the nematodes ultimately killing them. Chinese caterpillar fungus (Cordyceps sinensis)  Crop Diseases Fungi have great economic importance as they cause the majority of known plant diseases. Most of our common crop plants are susceptible to fungal attack of one kind or another. Fungal diseases can on occasion result in the loss of entire crops if they are not treated with antifungal agents. E.g. Famous Late blight of potato (Phyptophthora infestans) that caused death by starvation in Ireland, Corn smut disease caused by Ustilago maydis destroys the corn kernels. Animal Disease Fungi can also parasitise domestic animals causing diseases, but this is not usually a major economic problem. Ringworm and Candida infections are examples of human fungal infections. A wide range of fungi also live on and in humans, but most exist harmlessly. Spoilage of food and goods Fungi cause spoilage of food (e.g. fruits, tubers, vegetables, preserves) and other consumable goods like fabric, leather, paper, etc. It also causes rotting of wood. Aspergillus spp. are known to produce aflatoxins in most grains and nuts. Chronic dietary exposure to aflatoxins has been shown to cause liver cancer in humans. Late blight of potato (Phyptophthora infestans) Corn smut disease caused by Ustilago maydis Ringworm infections Spoilage of food by fungi