Lecture 19 BIO 311D 2nd EditionOutline of Last Lecture I. ReviewII. Plant tropismsIII. Signaling IV. Design an experiment Outline of Current Lecture I. Recap hormone experimentsII. Mutualism with N-fixing bacteriaIII. Plant defenses Current LectureI. 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 placeblock 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 At plant organ levelAuxin Cell elongation Shoot tip and root growthCytokinin Cell division Lateral branches grow out - Often plant hormones have opposing or interacting effectsRatio of auxin and cytokinin determines the form of the plant growthHigh auxin and low cytokinin: grows a lot of rootHigh cytokinin and low auxin: grows a lot of branches High cytokinin and high auxin: starts growing can’t differentiate and stops (too muchhormones, 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 requirePlant adaptation: tubes to transport sugars to bacteria; make nodules through cell division triggered by cytokininBacteria 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 compoundsThis beetle has evolved a way to circumvent the milkweed plant’s defensive compounds, by cutting the veins and draining the leaf tissue before
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