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Aberrant Nodulation Response of Vigna umbellata to a Bradyrhizobium japonicum NodZ Mutant and Nodulation Signals

¹The Center for Legume Research and ²Department of Microbiology and ³Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville 37996-0845, U.S.A.; ⁴Department of Botany, The University of Tennessee, Knoxville 37996-0845, U.S.A.; ⁵Complex Carbohydrate Research Center, University of Georgia, Athens 30602, U.S.A; ⁶The Ohio State Biotechnology Center, Columbus 43210, U.S.A.; ⁷Case Western Reserve University, Department of Ophthalmology, Cleveland, OH 44106, U.S.A.

The (Brady)rhizobium nodulation gene products synthesize lipo-chitin oligosaccharide (LCO) signal molecules that induce nodule primordia on legume roots. In spot inoculation assays with roots of Vigna umbellata, Bradyrhizobium elkanii LCO and chemically synthesized LCO induced aberrant nodule structures, similar to the activity of these LCOs on Glycine soja (soybean). LCOs containing a pentameric chitin backbone and a reducing-end 2-O-methyl fucosyl moiety were active on V. umbellata. In contrast, the synthetic LCO-IV(C_16:0), which has previously been shown to be active on G. soja, was inactive on V. umbellata. A B. japonicum NodZ mutant, which produces LCO without 2-O-methyl fucose at the reducing end, was able to induce nodule structures on both plants. Surprisingly, the individual, purified, LCO molecules produced by this mutant were incapable of inducing nodule formation on V. umbellata roots. However, when applied in combination, the LCOs produced by the NodZ mutant acted cooperatively to produce nodulelike structures on V. umbellata roots.