Functional Analysis of nodD Genes of Rhizobium tropici ClAT899. Pieternel van Rhijn. F. A. Janssens Laboratory of Genetics, KU Leuven, W. de Croylaan 42, B-3001 Heverlee, Belgium. Jos Desair (1), Katrien Viassak (1,2), and Jos Vanderleyden (1). (1) F. A. Janssens Laboratory of Genetics, KU Leuven, W. de Croylaan 42, B-3001 Heverlee, Belgium; (2) CNPAB/EMBRAPA, KM 47, Itaguai 23851, Rio de Janeiro, Brazil. MPMI 7:666-676. Accepted 24 May 1994. Copyright 1994 The American Phytopathological Society.

Phaseolus vulgaris is nodulated by a wide range of rhizo-bia. 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 nodD₁ allele contributes most to the NodD activity. Induction experiments show the same flavonoid specificity for the nodD₁ as Tor the wild-type strain CIAT899. Only the nodD₁ gene of CIAT899 is able to fully complement the nodD₁ mutation of Rhizobium sp. NGR234 for the nodulation on Leucaena, Macroptilium, and Phaseolus. Mutation of nodD₁ of CIAT899 completely abolishes the nodulation of L. leucocephala and M. atropurpureum, and severely reduces the nodulation of P. vulgaris