LOTUS-CORNICULATUS NODULATION SPECIFICITY IS CHANGED BY THE PRESENCE OF A SOYBEAN LECTIN GENE

Plant lectins have been implicated as playing an important role in med iating recognition and specificity in the Rhizobium-legumme nitrogen-f ixing symbiosis. To test this hypothesis, we introduced the soybean le ctin gene Le1 either behind its own promoter or behind the cauliflower mosaic virus 35S promoter into Lotus corniculatus, which is nodulated by R. loti. We found that nodulelike outgrowths developed on transgen ic L. corniculatus plant roots in response to Bradyrhizobium japonicum , which nodulates soybean and not Lotus spp. Soybean lectin was proper ly targeted to L. corniculatus root hairs, and although infection thre ads formed, they aborted in epidermal or hypodermal cells. Mutation of the lectin sugar binding site abolished infection thread formation an d nodulation. Incubation of bradyrhizobia in the nodulation (nod) gene -inducing flavonoid genistein increased the number of nodulelike outgr owths on transgenic L. corniculatus roots. Studies of bacterial mutant s, however, suggest that a component of the exopolysaccharide surface of B. japonicum, rather than Nod factor, is required for extension of host range to the transgenic L. corniculatus plants.