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Biology 4218A final review.docx

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Biology 4218A
Graeme Taylor

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4218 Microorganisms Review Gardiner – Lecture 1 – Invasion by Fungi Survival structures of fungi Diploid Haploid - Oospores – thick wall diploid spore Chlamadospores – asexual spores - Teliospores – thick wall diploid spore originating endogenously (mitosporangic fungi) -Zygospores – diploid resting spore - Conidia – haploid thick or thin walled (also asexual) Zoospores (NOT SURVIVAL SPORES) Pathogen Necrotroph Biotroph Host-specialized Non-specialized Host-specialized How Pathogens find Plants Passive dispersal through air Forcible spore discharge Insect or nematode vectors Chemotactic attractions Fungistasis Three entry routes for plant pathogens 1. Wounds 2. Natural openings 3. Direct Penetration e.g. TMV, Agrobacterium, - via lenticels, stomata - powdery mildews soft rot fungi e.g. Rust fungi, - rice blast pseudomonas Types of Direct Penetration: Penetrations through Penetration through Direct Penetration wounds natural opnings - through wounds - through stroma - direct with haustoria - through natural cracks - through lenticels - subcuticular only between main and latheral - through hydothodes ( holes - appresorium  penetration roots that cause guttation – water peg intracellular mycelium - fungus kills and macerates droplets on surface of - intercellular mycelium cells ahead of its advance leaves) - intercellular mycelium with haustoria * fungi especially are capable of using DIRECT PENETRATION Fungi as plant pathogens: Morphology: Cell walls contain mainly chitin When fungal spores germinate, they develop into microscopical, cylindric, elongated structures with cross walls; the hyphae (singular: hypha) In most cases the hypae join up and form very fine filaments, the mycelium, a kind of felt-like web of varying density Hyphae are distributed within the substrate e.g. soil, organic litter of wood, and are able to extract nutrients and carbohydrates from this substrate Interactions at the plant surface 1. Adhesion The initial interaction between fungal pathogen & plant  when spore lands on plant surface May occur through hyrodrophobic interactions w/ plant cuticle After the spore germinates  germ tube - thin film/sheat/extracellular matrix is secreted to surround the germ tube and attach it to the surface - proteins or glycoprotiens in the film may help with attachment - some of these proteins are hydrophobins that are bound to the fungal germling cell surface. - may also contain degradative enzymes (such as cutinase) that begin to break down the plant surface e.g. Magnaporthe grisea (Rice Blast Fungus) – is an exception The spores (conidia) of this fungus have a structure at their tip that mediates attachement. - Spore tips contain a packet of a mucilage (STM) – acts as a glue to hold the conidia onto the leaf surface (even in the presence of a flowing water). - This fungus also senses surface hardness and surface hydrophobicity (preferred over a hydrophilic surface). 2. Differentiation of infection structures – appressoria found with m.grisea - flattened fungal structure found at the plant that generates enough tugor pressure to drive a penetration peg through the epidermal cell wall form at the end of germ tubes in response to a hard, hydrophobic surface. water is necessary and the fungus responds to chemical signals (waxes and lipids) from the plant surface. the dome shaped appressorium has a specialized cell wall containing the dark pigment melanin. -Mutants that cannot make melanin (called buff, albino or rosy) are not pathogenic!! Germ tube  appresorium  infection plug  can become haustroium Powdery Mildew: spore germination and penetration - spore senses hydrophobic surface (cuticle of plant) and germinates - appresorium forms from germ tube (extended swollen structure) - the appressorium forms a penetration plug which penetrates past the cuticle into the cell wall, and in the host papilla E.g. in class - uredospore w/ adhesion pad on cuticle of plant germ tube appresorium send a penetration plug into pre-existing natural openings of the plant (e.g. stomata)  penetration hypha  substomatal vesicle  infection hypha  haustorial mother cell  haustorium pushes into plant cell (does not actually puncture the plant cell wall) 3. Haustoria Downy mildews – oomycetes Powdery mildews – ascomyctes Rusts – Basidiomycetes Smuts – Basidiomycetes Found in all major divisions of fungi On penetration, fungus increases surface area contact with plasma membrane - enables potential movement of organic carbon Simplest forms are small spheres The largest are complex hand-like structures that occupy a significant portion of the host cell Arise from interceullular hyphae, appressoria or external hyphae Hyphae narrows as it passes through wall of cell then expands Functional exchange takes place within the haustorial complex Metabolic activity within the complex is greater than outside Host appears to function according to signals from fungus 4. Biotrophic infection Intercellular infection hyphae – penetrate host cells at specialized haustorial mother cells to set up intracellular haustoria. These feeding cells are in close contact with the host cells, but separated by a extrahaustorial membrane. The hasutorium cell expresses specific genes (not transcribed in other parts of the fungus). These encoded metabolic functions are: Metabolic transporters for sugars and a.a The EH (interface b/w fungus and plant) is the trade centre bc key site for nutrient acquisition Fungus also produces: Cytokines Invertase (sucrose cleavage) And other enzymes The plant host also contributes proteins, cell wall material and enzymes to the interface region Roles: Acquisition Signalling Communication Avoidance of recognition by host Are localized between the cell wall and plasma membrane of the plant cell – other parts of the pathogen localized in the intercellular space Haustoria are the only location where the pathogen establishes intimate contact with the host cell plasma membrane Host PM surrounding the haustorium, the so-called extrahaustorial membrane (EHM), is fundamentally different in structure and protein content from the regular host PM -- plant cytoplasm -- extrahaustorial membrane -- extrahaustorial matrix -- haustorial wall -- haustorial plasma membrane -- hasutorial cytoplasm (bring glucose, and a.a across from extrahaustorial matrix pas hasutorial wall and haustorial PM) Necrotrophs Biotrophs Biochemical - host cells rapidly killed - host cells NOT rapidly killed and - toxins and cytolytic enzymes - few or no toxins/ cytolytic enzymes Morphological produced - special parasitic structures Features - no special parasitic structures (hasutoria) Host penetration via wounds, natural - host penetration direct or through openings natural openings Ecological - wide host range - narrow host range feature - able to grow saprophytically away - unable to grow away from host from host - attack healthy hosts at all stages of Attack juvenile, debilitated or development senescing tissues 5. Chemical attack Pathogens may use chemicals to overcome plant defenses to successfully penetrate, establish a food relationship and invade the host These chemicals include enzymes, hormones, toxins and extracellular polysaccharides Enzymes – sued to attack host cells Pathogense may mobilize numerous enzymes to attack walls of cells (primary and secondary) or the middle lamella These enzymes include cutinases, pectinases, cellulases, hemicellulases, ligninases, preoteases, lipases, and amylases Enzyme Substrate Utility Cutinase Cutin For direct penetration pectinase Pectin Penetration and spread  cell death cellulase Cellulose Softens walls for fungal spread hemicellulase Hemicellulose -- unclear Ligninase/ laccase Lignin White rot fungi Lipolytic enzymes Fats and oils Fatty acids used as food amylase Starch Glucose used as source of food protease protien --unclear - in general, it is difficult to test whether these enzymes are important for infection because the fungi usually have more than one gene for the enzymes in each group. Single gene mutations usually don’t block the ability of the fungus to infect plants Bacterial pathogens of plants: 1. Compared to the large number of bacterial species, relatively few are capable of infecting plants 2. This suggests that specialized properties are needed for bacteria to interact with plants 3. The bacteria generally gain access to the apoplast through stomates and other natural openings, or through wounds. Enter via.. - wound - natural openings: 1. Stoma, 2. Hydathode, 3. Bacteria in nectar through nectrathode Viral pathogens: Inanimate molecules Can survive on tools, or in soil - TMV stable for many years Some can only last a few hours May survive in plant structures such as seeds, tubers or in vectors Plant to plant transmissions require a wound - virus placed on leaf or in soil cannot gain access
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