418_assign_4 - association e Describe two differences...

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Name _________________________________ BioMI 418 Microbial Ecology Exercise 4 (25 points) due Friday March 28, 2008 “Lifestyle alternatives for rhizobia: mutualism, parasitism, and forgoing symbiosis”: 1) Rhizobia are studied extensively due in part to their agricultural and economic importance. Most studies focus on mutualism: when the bacteria are associated with a legume host. More and more ecologists are describing other rhizobial lifestyles. To start, do a little background research and answer the following questions about rhizobia. a) Bacteria that fix nitrogen are referred to as ________________ trophs . b) Rhizobia are members of which major bacterial phylum ? c) Describe the mutualism between a rhizobium species and its host plant: How does the plant benefit? How does the bacterium benefit? d) Cell-cell signalling systems are used to establish a symbiotic association between a rhizobium and its host plant. Describe the roles of “Nod Factors” and “plant flavonoids” in establishing the
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Unformatted text preview: association. e) Describe two differences between “determinant” and “indeterminate” nodules. 2) The article describes three life strategies seen in rhizobia. When multiple strains of rhizobium infect a single plant, this sets up a scenario for “cheaters” (also known as parasitic rhizobia) to enter the plant. a) Describe how a cheater differs from a non-cheating strain. b) How does cheating benefit the parasitic rhizobium? c) The authors describe a cleaver laboratory experiment where they manipulated the N 2 available to individual nodules of a single plant. Some nodules were exposed to atmospheric levels of N 2 while some had no available N 2 . How did the plant sanction the nodules that were unable to fix nitrogen? d) Describe a way in which a cheater could avoid plant sanctions. 3) What circumstances would allow free-living (non-symbiotic) rhizobia to out compete (and proliferate more than) a symbiotic form?...
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418_assign_4 - association e Describe two differences...

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