BIOA02 LECTURE 11: Plant Defense

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4 Feb 2012

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BIOA02 Lecture 11: Plant Defense
February 02, 2012
Plants Feed the World but don’t want to
-plants are primary producers and at the base of the food chain
-herbivory most widespread class of herbivores are insects
-numerous insects and also they have mechanism to overcome
the defense system
-large animals also feed on plants including dinosaurs and other
-small and large mammals feed on plants
-there are also intermediate mammals that don’t like to feed on plants
Plant defense mechanism
-two main ways of defense: mechanical defense and chemical defense
-mechanical defense is a structural defense weapon
-chemical defense toxins (plant secondary compounds) incorporated
into plant tissue
-some plants combine both mechanical and chemical defenses
-primary compounds are those that are totally essential for the life of
plants (products of metabolic pathways i.e. photosynthesis)
-secondary compounds are those have no known direct function in life
-scientists thought the secondary compounds were waste
products that were stored away in cells and leaves
-in the 80s, scientist realized these products were under strict
control and so it was realized that they were chemical
weapons/means to fend of herbivores
-expensive to produce
-tight coevolution plant produces a secondary compound in
response to herbivores but some herbivores overcome these
toxins with own chemicals to degrade the toxin and the plant has
to tweak its biochemical pathway to change it a little so the
herbivore can’t feed on the plant again
-four main classes of secondary compounds: phenolics, terpenoids,
glycosides, alkaloids
-phenolics: flavenoids (give vibrant colours and tastes to fruits) toxic
to small animals, oxidants
-terpenoids: found in Mediterranean spices (mint, rosemary, thyme
etc.) can be toxic to small animals
-glycosides: toxins in milkweed sap and pits of apples (bitter taste)
-alkaloids: caffeine, nicotine, morphine deterrent of feeding by
herbivores many are potent drugs
-all toxins evolved against herbivory
-the plants are green because that’s unpalatable to herbivore
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