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Presented By: Garret Morrill

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Many interactions exist in nature. Most are highly competitive or outright antagonistic. How then do mutualistic relationships arise? What is the nature of them? Are they totally friendly or likewise competitive?

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LegumeRhizobium

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Over 18,000 species, only surpassed by the orchid and sunflower families. Many domesticated by humans, including soybeans, alfalfa, peanuts, all beans, etc Provides 1/3 of all the protein to the average human diet. Create root nodules the home of Nitrogen Fixation.

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The number of species are at least as numerous as Legumes, and most likely well over ~18,000 Free-living in soil normally, associate with legumes or Parasponia. Contains N 2 fixation genes nif and fix. Converts Nitrogen gas (N 2 ) into Ammonia (NH 3 ) Found to have genetic histories with other bacterial species like Rickettsiaceae (Pathogen)

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Responsible for the majority of the worlds plant-soluable nitrogen. Gives rise to the expensive (and delicious) structures we harvest. A major aim of research seeks to understand this relationship better.

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NO!

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Fixing N 2 is expensive and largely unnecessary for Rhizobia. Nif and Fix genes lessen efficiency. Specific compatibilities from HR plasmids can increase competitiveness for nodules despite lesser N 2 fixation. Nod, Nol, Noe genes affect initial infection in a similar way, owing to the pathogen-associated history Rhizobia have.

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Have some mechanisms to police Rhizobia behavior. Compatibilities derived from immune functions (T3E response, etc) may control entry to root. Soybeans restrict O 2 permeability of its nodules to punish non- productive Rhizobia. These mechanisms may be only general though or even rare. Many other potential mechanisms havent been explored yet.

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Why does this relationship between Legumes and Rhizobia remain mutually beneficial? Metabolic/Immune regulations developed by Legumes over their evolutionary history with certain Rhizobian species have maintained N 2 fixation levels by selecting for productivity and against cheating.

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Step 1: Obtain two genetically similar legumes with different Rhizobial strains. Measure nitrogen-fixation rates of Rhizobia with their natural hosts

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Step 2: Switch bacterial strains between plants to produce novel lines. Grow the original combinations as well.

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Step 3: Grow each trial separately for several generations. Continue to measure nitrogen fixation and collect genomes each generation. Bacteria

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Step 4: Measure results over time N 2 Fixation OriginalNovel

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Step 4: Measure results over time N 2 Fixation OriginalNovel

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Step 4: Measure bacterial rate of Nitrogen Fixation over time. N 2 Fixation OriginalNovel

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Future studies could include testing hybrid plant response, multiple Rhizobia to compete per nodule, etc Would possibly lead to genetic identifications of legume mechanisms involved in Nodule Policing. Further the research towards optimizing this most important mutualism and others like it.

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Presented By: Garret Morrill