The Rhizobium etli trpB gene is essential for an effective symbiotic interaction with Phaseolus vulgaris

A mutant strain (CTNUX4) of Rhizobium etli carrying Tn5 unable to grow with ammonium as the sole nitrogen source was isolated and characterized, Seque nce analysis showed that Tn5 is inserted into a trpB (tryptophan synthase)- homologous gene. When tested on the roots of Phaseolus vulgaris, strain CTN UX4 was able to induce only small, slightly pink, ineffective (Fix(-)) nodu les. However, under free-living conditions, strain CTNUX4 was unable to pro duce flavonoid-inducible lipo-chitin oligosaccharides (Nod factors) unless tryptophan was added to the growth medium. These data and histological obse rvations indicate that the lack of tryptophan biosynthesis affects the symb iotic behavior of R. etli.