Lecture 19 BIO 311D 2nd Edition Outline of Last Lecture I Review II Plant tropisms III Signaling IV Design an experiment Outline of Current Lecture I Recap hormone experiments II Mutualism with N fixing bacteria III Plant defenses Current Lecture I Recap hormone experiments Auxin controls cell elongation Auxin is distributed away from the light grow at an angle phototropism Darwin and Darwin Light tip removed grow up opaque cap grow up transparent cap grow towards light Auxin in tip of seedling is cut off place tip onto agar Auxin diffuses into agar block place block to one side plant grows at an angle do experiment in the dark Manipulation to the distribution of the auxin simply the different concentrations of auxin affects the growth of the plant the side with auxin source leads to more elongation bending away from the source to the other direction Controls same agar no light An array of light receptors in plant cells Blue light receptors phototropins for phototropism cryptochromes Red Far light receptors called phytochromes With carefully controlled experiment we learn that hototropism is a response to blue light only coleoptile with blue wavelength grows toward light At cellular level Cell elongation Cell division Auxin Cytokinin At plant organ level Shoot tip and root growth Lateral branches grow out Often plant hormones have opposing or interacting effects Ratio of auxin and cytokinin determines the form of the plant growth High auxin and low cytokinin grows a lot of root High cytokinin and low auxin grows a lot of branches High cytokinin and high auxin starts growing can t differentiate and stops too much hormones plant can t differentiate their organs II Mutualism with N fixing bacteria Symbiotic mutualisms of plants Participation in mycorrhizal association increases plant growth How do the two mycorrhizae partners benefit from each other Fungus gets sugars plant gets water Nitrates are usually the limiting factor for plant growth Nitrogen fixing bacteria catalyze the reaction N2 H2 NH3 Rhizobium bacteria are in the soil and also live symbiotically in roots of plants like legumes List some special adaptations of both legume plant and Rhizobium bacteria that this mutualistic relationship would require Plant adaptation tubes to transport sugars to bacteria make nodules through cell division triggered by cytokinin Bacteria adaptation leghemoglobin Plant and bacteria talks to each other constantly through chemical mechanisms What do you expect to find in mutualistic legume plants but NOT in plants that don t associate with nitrogen fixing bacteria Receptor for Rhizobium signal molecule III Plant Enemies and Defenses What do these plants have in common They don t want to be eaten Plant defensive responses can be constitutive always present or induced produced in response to damage or stress Plants such as milkweeds produce sticky latex and other substances to deter herbivores Monarch caterpillars and butterflies are adapted to store and sequester the defensive compounds This beetle has evolved a way to circumvent the milkweed plant s defensive compounds by cutting the veins and draining the leaf tissue before eating
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