Lecture 24 Guest Lecture on Plant Defenses Plants are autotrophs and necessary organisms in the food chain because they provide energy for higher trophic levels Plants are linked throughout the food web through herbivores mostly insects Plants need to defend themselves because every plant will be attacked by an herbivore at some life stage Wide variety of herbivores Including Chewers Beetles larvae etc Miners Leave a maze like pattern with a dark line of waste in the center Suckers Suck out Phloem which is where the carbohydrates are Gallers On the stems and or leaves of most plants Root feeders Feed on the roots of plants Seed predators Eat the embryo Seed predators eat the seeds preventing new plants from growing Only type of herbivore that kills the plant Co evolution of plants and herbivores 1 Herbivores select for plant defensive traits 2 Those defenses select for offensive traits in the herbivores stronger mouthparts 3 Plants become better defended and herbivores become better at overcoming defenses 4 Co evolution requires specificity the evolution of each trait is due to the other 5 Co evolution requires reciprocity both traits must evolve Note than evolution can occur without another organism such as evolution within one species to develop flashier coloring Types of plant defenses 1 Physical Trichomes spines latex milky white sap substance thick cuticle Consider a flower that has derived spines It is tough for insects to maneuver around them They also fill the insect gut with useless crap and little nutrients 2 Chemical Secondary metabolites not directly involved in growth reproduction or development Defense compounds that inhibit enzymes disrupt protein synthesis and DNA repair or weaken cell structure include codeine morphine cocaine nicotine strychnine and more Defense chemicals that inhibit cellular respiration include cyanide and digitalis Essential oils may also act as repellants citronella menthol and camphor Those that inhibit digestion include tannis cannabinoids and more 3 Indirect Attacked plants emit volatiles airborne chemicals that attract enemies of herbivores like parasitoid wasps Some plants house ant colonies that defend plants from herbivores Like from elephants that eat acacia trees The importance of plant survival for humans 1 Agriculture A large percentage of our crops are lost to herbivorous insects US farms spend 7 8b annually on chemical pesticides Alternatives to chemical pest control include associational plants planting a repellant crop in with the regular crop natural enemies genetic engineering Some bacteria genes encode a pesticidal protein 2 Medicine Of the top 150 prescription drugs 118 come from natural sources Ex Periwinkle Pacific Yew etc Produce anticancer drugs 80 of the world s population relies on products of natural medicine
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