Lecture 23 Pathogens and Defenses Plants and plant pathogens have been co evolving since before domestication Agricultural system provides conditions for plant disease epidemics Monoculture growing a single crop over a long period of time in the same area High density planting Analogous to close quarters and crowding big cities Pathogens blemish the product reduce its value cause post harvest loss Where food is scarce a plant pathogen can be the difference between life and death I e Irish potato famine Summary of pathogen host interactions and selective pressures during co evolution see slide 4 lecture 23 Some stages involve infection usually at surface of plant cell and responses relate to receptors usually at surface of plant cell Basla defense 1st Triggered by pathogen PAMP Microbial effortor molecules are pathogens next defense New threat to plant Plant resonds with R gene mediated defense or effector triggered immunity Necrotrophic May infect wider range of host Kill material then feed on dead materials Biotrophs Invaded host cells must stay alive for pathogen to complete life cycle Green patches on plant are live areas being infected and the brown areas are naturally senesced regions Pathogens can enter plants through different routes Penetration through cracks from herbivory natural openings stomes or direct penetration Most pathogens attack a specific plant part Pathogen life cycles can incorporate host features Fungal penetration of a plant surface 1 Adhesion to surface 2 Development of special structure to penetrate 3 Increase in cellular turgor pressure 4 Strengthening of cell wall of fungus so it can penetrate 5 Release of enzymes to degrade weaken host cell wall release of pectinase or cellulase since cell walls are made out of different materials fungi wont degrade itself Fungus developing reinforced cell wall depicts a dark line surrounding the cell Plant pathogens can induce a variety of disease symptoms Blights area of browning and death can be bacterial or fungal Wilts caused by blocking of vascular tissue root stem puckering due to water distress ex spotted wilt usually caused by fungus Developmental reprogramming pathogen interferes with normal growth and development of a plant and causes growth of extra structures ex crown gall tumor Pathogen changes hormonal level of plant example of effector molecule response Effector molecules are molecules made by pathogen to enhance its ability to colonize grow on reproduce in its host Includes enzymes toxins and growth regulators Sometimes these are also toxic to humans Function of enzymes toxins trigger cell death tissue damage Function of growth regulators developmental programming Ergotamine A mycotoxin produced by fungus Some mycotoxins are major health problems for humans and herbivorous animals They can act as hallucinogenics carcinogenics and more Foolish seedling disease plants induced to produce more GA by fungal infection ex of effector molecule response Crown gall Infects plants and induces them to produce more auxin and cytokinin in cells This causes cells to rapidly divide and differentiate meristematic A different agrobacterium strain can cause hairy root disease causes over production of auxin Rapid proliferation of roots Cytokinin growth in shoots Auxin growth in roots look at slide 24 Plants can also be pathogens Plants may grow over other plants EX Mistletoe photosynthetic on it s own however may grow over a tree and steal its nutrients Strangleweed non photosynthetic on its own Slide 30 shows an electron micrograph of a common plant virus These typically weaken plant reduce growth reduce yield Transmitted mainly by herbivory by insect nematodes fungi vegetative propogation Look at charts on slides 31 34 36 38 all pictures Plant response typically R gene mediated Typically stronger than basal response A plant produced toxin PAMP mediated may prevent fungus from penetrating plant cell A B C D slide 35 charts accordingly No defense response basal response Basal response lastly R gene response Plants can acquire systemic resistance by initial exposure to a pathogen R gene response Plant with prior exposure is more resistant Make sure when reviewing this make sure to look at the following objectives 1 Explain the principle of co evolution as it applies to plant pathogen interactions 2 Explain how monoculture of crops make agriculture susceptible to disease epidemics with specific examples 3 Summary the interactive selective pressures between plants and their pathogens 4 Distinguish between biotrophic and necrotrophic pathogens 5 Describe methods of invasion and infection by plant pathogens and the associated plant symptoms 6 Describe the relationships between PAMPs effectors and R proteins and how they mediate plant pathogen interactions 7 Describe some pathogenic plants and viruses and how they infect cause disease and interact with the host plant
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