Many aromatic compounds are toxic when supplied at concentrations employed in most growth media. This effect was demonstrated when rhizobia and agrobacteria were grown in auxanographic plates in which cells were seeded in agar and exposed to a gentle gradient of substrate concentration. An auxanographic nutritional survey with representative strains revealed that and cowpea sp. could utilize a relatively large proportion of the aromatic and hydroaromatic compounds tested; and species displayed intermediate nutritional versatility; was relatively fastidious. The hydroaromatics quinate and shikimate were not toxic. Most of the strains examined grew at the expense of one or both of these substrates. Quinate was metabolized via protocatechuate and 3-oxoadipate. All of the strains examined were able to grow at the expense of protocatechuate and therefore must contain six structural genes for the enzymes required to convert this aromatic compound to common intermediary metabolites. Conservation of pathways for aromatic catabolism against a background of wide evolutionary divergence among the suggests that pressures for selection of the traits were exerted throughout the evolutionary history of the organisms. A probable selective pressure is competition for nutrients in the soil. In addition, the ability of agrobacteria and rhizobia to respond to aromatic compounds may have selective value in bacterial-plant interactions.


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