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A Sinorhizobium meliloti Lipopolysaccharide Mutant Induces Effective Nodules on the Host Plant Medicago sativa (Alfalfa) but Fails to Establish a Symbiosis with Medicago truncatula

The specific Sinorhizobium meliloti lipopolysaccharide (LPS) mutant Rm6963 (A. Lagares, G. Caetano Anolles, K. Niehaus, J. Lorenzen, H. D. Ljunggren, A. Puhler, and G. Favelukes, J. Bacteriol. 174:5941-5952, 1992) was shown to be mutated in a region corresponding to a cloned 5-kb SstI DNA fragment that was able to complement the lpsB and lpsC mutants of S. meliloti described by Clover et al. (R. H. Clover, J. Kieber, and E. R. Signer, J. Bacteriol. 171:3961-3967, 1989). Sodium dodecyl sulfate polyacryla-mide electrophoresis revealed that the LPS-I and LPS-II fractions of the LPS mutant Rm6963 were shifted to lower molecular weights. While the majority of the Medicago spp. tested established an effective symbiosis with both the S. meliloti wild-type Rm2011 and the LPS mutant Rm6963, the latter induced ineffective nodules on M. truncatula. A light- and electron-microscopic analysis of the ineffective M. truncatula root nodules revealed that the bacteria were released from the infection threads but failed to colonize the plant cells effectively. The plant cytoplasm was filled with numerous vesicles, probably the result of a disturbed bacteroid development. Sections of ineffective M. truncatula root nodules induced by the LPS mutant Rm6963 showed brown, necrotic cells within the central nodule tissue that autofluoresced when viewed under UV light. These observations are best explained by a plant defense response. Evidently, the rhizobial LPS plays a role in plant-microbe signaling during the formation of M. truncatula nodules.