Objectives for Today's Class
Reference: Agrios Chapter 6

Structural Defense

Without the use of induced antibiotics to protect themselves, plants must rely on various structural and anatomical attributes for either avoid infection entirely or prevent infection initially.

• Preexisting defense structures
  • Surface waxes
  • Structure of Epidermal cell walls
  • Position of stomata and lenticles
  • Thick cell walls
  • Leaf hairs

• Defense Structures Formed in Response to Infection by the Pathogen

  Once infection occurs, the plant will/may respond by producing structural barriers to further ingress of the parasite. The mechanism are either at the tissue level, termed Histological Defense Structures by Agrios) or cellular/subcellular level (cellular defense reaction).

• Histological Defense Structures Leaf XS and Early Blight
  • Formation of Cork Layers

    What group(s) of organisms would most likely be inhibited by this type of defense?

  • Formation of Abscission layer

    This is a designed response by the plant to excise a portion of itself to benefit the reset. Sacrifice few to save many - George S. Patton. Is this good strategy?

  • Formation of tyloses and Geranium Bacterial Blight

    Is this effective? What are the disadvantages/advantages for the host?

  • Deposition of gums

    What group of hosts use this type of defense? Why is it not used by more?

• Cellular Defense Structures

  This type of defense may be more hypothetical, teleological and/or observational than effective.

  • Callose thickening of the cell wall

  • Cytoplasmic Defense Reaction When cytoplasmic contents surround the site of infection is this a defense reaction or a pizza man delivery?

  • Necrotic Defense Reaction - HYPERSENSITIVITY The ultimate defense is ultimate susceptibility? What group(s) of organisms might be best suited or control by this mechanism?

Metabolic (Biochemical) Defense

• Preexisting Biochemical Defense
  • Inhibitors released by the plant in its environment

    Some plants exude onto their surface compounds which inhibit the germination of fungi (Tomato- Cercospora). Other plants (Onion) produce phenolic compounds that protect them from infection. The interest in chemical defense started with red scaled onions and protocatechuic acid. The observation of the red coloration; while it recognized that there was something chemically different between resistant and susceptible plants, lead to a lot of misconceptions which are still extant. As it turns out, the red color is not necessary for resistance. The red color, or tissue darkening sometimes even to purple or black, is the result of oxidation of phenolic compounds involved. Sometimes the oxidized phenols have very little anti-microbial properties. None-the-less color, or lack thereof, has been take as an indication of resistance.

  • Receptor sites

    If the parasite needs a particular recognition site and the host does not provide it then perhaps infection will be avoided.

    If the host is lacking a specific attachment/receptor site for a toxin then no toxic activity will occur.

  • Lack of essential nutrients

• Preformed killer factors

  phenols, tannins, chitinases, glucanases

  Chitinases are receiving a lot of attention as possible resistance factors and a great deal of Biotech. Research is aimed at providing increased chitinase capacity for cells. What in vivo conditions are necessary for chitinase to be effective? Are these conditions being considered by the biotech. pundits?

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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