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Trifoliin A, a Rhizobium-binding glycoprotein from white clover, was detected in sterile clover root exudate by a sensitive immunofluorescence assay employing encapsulated cells of Rhizobium trifolii 0403 heat-fixed to microscope slides. Its presence in root exudate was further examined by immunoaffinity chromatography. The binding of trifoliin A to cells was specifically inhibited by the hapten, 2-deoxyglucose. Significantly higher quantities of trifoliin A were detected in root exudate of seedlings grown hydroponically in nitrogen-free medium than in rooting medium containing 15 mM NO-3, a concentration which completely suppressed root hair infection by the nitrogen-fixing symbiont. The presence of trifoliin A in root exudate may make it possible for recognition processes to occur before the microsymbiont attaches to its plant host.

Bacterial attachment is viewed as a cellular recognition event during the infection of legumes by the nitrogen-fixing symbiont, Rhizobium. Studies on the biochemical basis of selective attachment are reviewed, and suggest that this recognition process is accomplished by specific glycoprotein lectin-polysaccharide interactions on the surfaces of the symbionts. An understanding of host specificity may lead to ways to broaden the host range of nitrogen-fixing symbioses.