Plant signals and bacterial components induce modifications in Rhizobium lipopolysaccharide
Abstract
The soil bacterium Rhizobium etli infects its host common bean (Phaseolus vulgaris ) during the establishment of symbiosis. The most abundant molecule of the rhizobial outer membrane, lipopolysaccharide (LPS) is an essential factor for the infection. Compounds exuded from the seed and the root, and plant-induced acidification of the immediate environment, each trigger distinct modifications (LPM) in the LPS. The major inducing compounds of LPM from seed exudate were different than those released from the root and were found to be delphinidin 3-glucoside and glycosides of cyanidin, petunidin, and malvidin. The LPS modifications were detected by loss of antigenicity of monoclonal antibodies (mAbs). A R.etli genetic locus, lpe3 , was isolated and shown to be required for the synthesis of the epitopes of these mAbs. lpe3 coded for at least four genes, lpeABCD. lpeA was not transcriptionally regulated during LPM, and constitutive expression of lpeABCD did not prevent LPM. However, genes encoded by lpe3 were required for an additional LPS property induced by low pH (LPP). Sequences of these genes had similarity to sequences of genes involved in LPS synthesis in other bacteria. Functions encoded by lpe3 are required for wild-type antigenicity of R.efli LPS but may not be involved in LPM. Several mutants of R.efli deficient in LPM were isolated. Mutants CE397 and CE440 had truncated LPSs and lacked LPM induced by anthocyanins. Strain CE440, but not CE397, lacked LPM induced by low pH. Both mutants had symbiotic deficiencies. These studies indicated that LPM induced by anthocyanins required a more complete LIPS structure than detection of LPM by low pH. Growth directly on the seed, or in seed exudate alone, was also found to induce R.etli to produce an extracellular matrix, whose major component appeared to be EPS. Thus, R.etli responds to plant-released signals by modifying the abundance or structures of its outer membrane and extracellular polysaccharides. In doing so, R.etli may generate a great array of functional diversity in its most abundant surface molecules essential for the interaction with P.vulgaris in the environment.
This paper has been withdrawn.