ETHYLENE PREVENTS NODULATION OF VICIA-SATIVA SSP NIGRA BY EXOPOLYSACCHARIDE-DEFICIENT MUTANTS OF RHIZOBIUM-LEGUMINOSARUM BV VICIAE

Exopolysaccharide-deficient mutants (Exo mutants) of Rhizobium legumin osarum bv. viciae are usually impaired in root nodule formation on the ir 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. Unde r these circumstances, Exo mutants induced delayed formation of partia lly infected nodules. Exo mutants did not induce abnormally large amou nts of ethylene in host roots nor showed abnormal production of lipo-o ligosaccharide Nod signals; thus, impaired nodulation could not be asc ribed 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 th at impaired nodulation is indeed caused by the absence of EPS. Our res ults are consistent with the following hypothesis: In addition to othe r nodulation-related phenomena, rhizobial Nod signals also induce ethy lene formation in host plant roots. By influencing root cell growth, e thylene inhibits proper root infection by rhizobia. In case of delayed nodulation, for instance, due to EPS deficiency, ethylene formation p recedes root infection and as a result nodulation is impaired.