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Appl. Environ. Microbiol., Nov 1996, 3991-3996, Vol 62, No. 11
Copyright © 1996, American Society for Microbiology

An Experimental Test of the Rhizopine Concept in Rhizobium meliloti

DM Gordon, MH Ryder, K Heinrich and PJ Murphy
Division of Botany and Zoology, Australian National University, Canberra, ACT 0200, and Department of Crop Protection, University of Adelaide, and Division of Soil Biology, Commonwealth Scientific and Industrial Research Organisation, Glen Osmond, SA 5064, Australia

In some Rhizobium-legume symbioses, compounds known as rhizopines are synthesized by bacteroids and subsequently catabolized by free-living cells of the producing strain. It has been suggested than rhizopines act as proprietary growth substrates and enhance the competitive ability of the producing strain in its interactions with the diverse microbial community found within the rhizosphere. Wild-type, rhizopine-producing Rhizobium meliloti L5-30 and mutant L5-30 strains deficient for either rhizopine synthesis or catabolism were inoculated onto lucerne host plants in competition experiments. These experiments demonstrated that no apparent advantage resulted from the ability to synthesize a rhizopine, whereas the ability to catabolize rhizopine provided a clear advantage when an organism was in competition with a strain without this ability. The results suggest that when an organism is in competition with a catabolism-deficient mutant, the ability to catabolize rhizopine results in enhanced rates of nodulation. The results of the experiments were not consistent with the hypothesis that the sole role of rhizopines is to act as proprietary growth substrates for the free-living population of the producing strain.

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