Novel characteristics (Intra and Ex planta) of indigenous serogroups of rhizobium meliloti Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/j6731697m

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  • This thesis presents novel findings regarding symbiotic, physiological, and antigenic characteristics of representatives from an indigenous soil-borne population of Rhizobium meliloti. Nodules were sampled from 'Vernal', 'Anchor', and 'Saranac' alfalfa, and the isolates analyzed serologically. Rhizobium meliloti serogroup 31 was identified as a dominant nodule occupant, with a greater and more consistent nodule occupancy rate on Vernal (60%) compared to Anchor (24%) or Saranac (36%). The symbiotic effectiveness of the parent isolate of serogroup 31 was evaluated on each cultivar over four harvests in a greenhouse study. Only inoculated Vernal responded with an increase in shoot dry weight and N₂ assimilated relative to N supplemented plants between harvests two and three. Field isolates of serogroup 31 from nodules on Vernal produced homogeneous, effective responses on Vernal and Anchor. In contrast, serogroup 31 isolates from Anchor nodules were heterogeneous in effectiveness on the parent host, with poorly effective isolates shown to be substantially more effective on Vernal. Isolates of Rhizobium meliloti serogroups 31 and 17 were grown in yeast extract mannitol broth (YEM) containing NaCl or polyethylene glycol (PEG). At water potentials below -0.5 MPa imposed by PEG, strain 31 had a lower specific growth rate than strain 17 which was accompanied by irregular cell morphology. In contrast, neither strain was affected significantly over the range of water potentials created by NaCl. Upshock of water stressed cells (-1.0 MPa PEG) into normal YEM (-0.15 MPa) resulted in a faster recovery of growth by strain 31 than strain 17. Strain 31 also required significantly fewer days to nodulate alfalfa compared with strain 17 when transferred from YEM at -1.0 MPa PEG onto the roots of alfalfa seedlings in plant growth medium (-0.1 MPa). Supplemental calcium (O.lmM) reduced the differences between strains in their response to water stress. Several novel phenomena were shown to hinder the production of fluorescent antibody conjugates specific to serogroup 31. An indirect fluorescent methodology, using the biotin-avidin system, was developed to successfully detect and specifically discriminate members of serogroup 31 from other strains of R. meliloti. Using this technique, members of serogroup 31 were found to comprise 17% of the total R. meliloti population in soil samples with a recent history of alfalfa cultivation.
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