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8 How plants defend themselves I - Copy

8 How plants defend themselves I - Copy - PP 315 590J...

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PP 315 / 590J  Lecture 8 How plants defend themselves I Objectives Visualize and understand how pathogens colonize a plant through host tissue to form recognizable  patterns. Colonization occurs by interrupting normal plant processes. Plants have developed response  mechanisms to avoid, tolerate, or resist pathogen colonization. How Pathogens Interfere with Host Development Colonization After a pathogen successfully gains entry into a susceptible host and establishes an infection site, the  pathogen may begin to grow through host tissues. This growth through host tissues is called  colonization At the macroscopic or organism level, the extent of colonization may be characterized into four general  patterns of colonization.  Patterns of Host Colonization 1) Local  - local diseases involve a limited amount of host tissue invaded around each infection site. The  best example is a typical leaf spot disease.  This may be the result of containment of the pathogen by the host and/or limited growth by the pathogen.  The ultimate size of the lesion is probably most determined by the host and by environmental factors.  2) Unrestricted  - colonization continues to spread (enlarge) as long as there remains tissue of the type  originally attacked. Examples include: root rots, soft rots, some leaf spots.  3) Extensive (blights)  - Lesion continues across tissue types. Examples include: late blight, fire blight.  4) Systemic colonization  - colonization of most or all of the plant usually via the vascular system. May or  may not involve wilting. For example, loose smut, caused by  Ustilago tritici  on wheat and  Ustilago nuda  on  barley has systemic mycelial growth, but causes no wilting. Vascular wilts caused by fungi and bacteria  are the most obvious diseases caused by pathogens that exhibit systemic colonization and wilting.  Viruses typically are systemic inside their hosts and cause systemic symptoms. (1) (2) (3) (4) These four patterns of colonization represent the visible end result of growth through host tissues. 
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How fungi grow through host tissues    intracellular  - growth through cells    intercellular  - growth between cells    vascular  - growth in xylem or phloem; systemic growth    subcuticular  - growth below cuticle; penetration of epidermal cells via specialized cells known as  haustoria    epiphytic  - growth outside of cuticle; penetration of epidermis via specialized cells known as haustoria Fungi that are intercellular, subcuticular, and epiphytic obtain their nutrients from their hosts via a  specialized feeding cell known as an  haustorium .
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