FUNCTIONAL-ANALYSIS OF NODD GENES OF RHIZOBIUM-TROPICI CIAT899

Phaseolus vulgaris is nodulated by a wide range of rhizobia. A remarkable feature of Phaseolus is the release of a large variety of flavonoid molecules (M. Hungria, C. M. Joseph, and D. Phillips, 1991. Plant Physiol. 97:751-758; M. Hungria, C. M. Joseph, and D. Phillips, 1991. Plant Physiol. 97:759-764). Here we describe how three bean symbionts, R. tropici CIAT899, R. etli CE3, and Rhizobium sp. NGR234, differ in their response with pure flavonoid molecules and with HPLC-fractionated root exudates of Phaseolus and Leucaena. Whereas R. etli CE3 and Rhizobium sp. NGR234 respond with all flavonoid molecules earlier identified in bean exudates, R. tropici CIAT899 shows only induction with naringenin. In contrast, when fractionated exudates of Phaseolus and Leucaena are used, no remarkable differences in induction profile are observed. Our previous studies (P. J. S. van Rhijn, B. Feys, C. Verreth, and J. Vanderleyden, 1993. J. Bacteriol. 175:438-447) indicated that R. tropici CIAT899 contains five nodD alleles. The present study indicates that the nodD(1) allele contributes most to the NodD activity. Induction experiments show the same flavonoid specificity for the nodD(1) as for the wild-type strain CIAT899. Only the nodD(1) gene of CIAT899 is able to fully complement the nodD(1) mutation of Rhizobium sp. NGR234 for the nodulation on Leucaena, Macroptilium, and Phaseolus. Mutation of nodD(1) of CIAT899 completely abolishes the nodulation of L. leucocephala and M. atropurpureum, and severely reduces the nodulation of P. vulgaris.