Approach to an Improved Taxonomy of the genus Agrobacterium

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Abstract
SUMMARY: With a view to an improved taxonomy of the genus Agrobacterium, 45 strains. including representatives of all nomen-species, were used to investigate the following features: base composition and compositional distribution of pure deoxyribonucleic acid (DNA); type of flagellation; 3-ketoglycoside formation; phytopathogenicity for tomato and Datura. It is proposed to limit the genus Agrobacterium to two, or possibly three, species: (I) Agrobacterium radiobacter and its phytopathogenic variety A. radiobacter var. rumefaciens; (2) A. rhizogenes; and (3) possibly A. pseudotsugae. More work on the latter two species is required before they can definitely be accepted as separate species of this genus. The DNA of all the strains of the former two species has a Tm value in the narrow range of 93.8°-95. The corresponding with an average molar guanine + cytosine (GC) content of 59.5–62.8%. The variance σ of the compositional distribution of the DNA molecules ranges between 0 and 0.88% with an average of GC). The only available strain of A. pseudotsugae, with 67.7% GC, was mpletely out of this range and its chromosomal DNA was clearly different from that of the other two species. All strains of Agrobacterium proper were peritrichous, frequently with 5–6 flagella. All 8 strains of A. radiobacter and 24 of 28 strains of the variety ‘tumefaciens’ converted lactose into 3-ketolactose; all the other strains were negative in this respet. Several arguments are advanced to include the strain A. stellulatum and A. gypsophilae from this genus. The relationship between Agrobacterium and some other genera is shown graphically in a plot of mean similarity vermus DNA base composition.

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