Ethylene Prevents Nodulation of Vicia sativassp. nigra by Exopolysaccharide-Deficient Mutantsof Rhizobium leguminosarum bv. Viciae. Wilbert A. T. van Workum. Institute of Molecular Plant Sciences, Leiden University, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands. Anton A.N. van Brussel, Teun Tak, Carel A. Wijffelman, and Jan W. Kijne Institute of Molecular Plant Sciences, Leiden University, Wassenaarseweg 64, 2333 AL Leiden, The Netherlands. MPMI 8:278-285. Accepted 2 December 1994. Copyright 1995 The American Phytopathological Society.

Exopolysaccharide-deficient mutants (Exo mutants) of Rhizobium leguminosarum bv. viciae are usually impaired in root nodule formation on their host plants. However, we found that Vicia sativa ssp. nigra (vetch) could be nodulated by such mutants if ethylene production by the host plant root, resulting from rhizobial inoculation, was minimized. Under these circumstances, Exo mutants induced delayed formation of partially infected nodules. Exo mutants did not induce abnormally large amounts of ethylene in host roots nor showed abnormal production of lipo-oligosaccharide Nod signals; thus, impaired nodulation could not be ascribed to these features. The nodulation ability of R. leguminosarum bv. viciae Exo mutants only affected in EPS synthesis could be restored completely by coinoculation with a Nod Exo+ strain, indicating that impaired nodulation is indeed caused by the absence of EPS. Our results are consistent with the following hypothesis: In addition to other nodulation-related phenomena, rhizobial Nod signals also induce ethylene formation in host plant roots. By influencing root cell growth, ethylene inhibits proper root infection by rhizobia. In case of delayed nodulation, for instance, due to EPS deficiency, ethylene formation precedes root infection and as a result nodulalion is impaired.