Graduate Thesis Or Dissertation
 

Taxonomy and host specificity of the genus Agrobacterium

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  • One hundred seventy-eight Agrobacterium isolates, 120 United States (U.S.), 26 Australian and 32 of miscellaneous origin, were characterized for their reactions to 20 biochemical and physiological tests. Isolate pathogenicity and degree of host specificity, utilization of nopaline and octopine, and sensitivity to agrocin-84 also were determined. The U.S. isolates were quite similar, but not identical in character to the 26 Australian isolates. Nearly all the isolates were separated .by 15 diagnostic tests into two homogeneous groups, designated 3-ketolactose positive [K(+)] and 3- ketolactose negative [K(-)]. Sixteen isolates did not conform to either group description. Isolates of the species A. tumefaciens and A. rubi were biochemically, physiologically and pathologically indistinguishable. Five of eight confirmed A. rhizogenes isolates induced tumors in addition to hairy root and were all K(-) in character. A taxonomic proposal for the genus Agrobacterium is presented wherein two species would be recognized. All agrobacteria were inoculated to 11 known crown-gall hosts to determine their host range. The host specificity of all isolates varied greatly with no isolate infecting all host plants. Sixty-six percent of the pathogenic isolates infected six to eight of 11 hosts tested while 3% infected only the host plant from which they were originally isolated (homologous host). However, the host range of an isolate was not influenced by the plant of isolation, Evidence for change in an isolate's host specificity pattern while in culture is presented and discussed relative to its relationship with the Ti plasmid. Tomato and datura were infected by more of the pathogenic agrobacteria (81% each) than any other of the 11 host species tested, thus serving as the best indicators of an isolate's pathogenicity. Low correlations were observed between all combinations of five isolate characters; pathogenicity, 3-ketolactose reaction, nopaline and/or octopine utilization and agrocin-84 sensitivity. Of 32 possible permutation groups of the five characters examined, isolates belonging to 25 groups were identified. Isolates were randomly distributed among all 25 groups regardless of geographic origin or plant species of isolation. Possible reasons for the lack of high correlations between the plasmid-coded characters are discussed.
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