Attachment of Rhizobium to Legume Roots as the Basis for Specific Interactions

Experiments were conducted to elucidate the basis of the observation that different strains of Rhizobium infect particular legumes. Rhizobia specific for a variety of legumes were grown with ¹³PO^2?₄ and exposed to pea roots (Pisum sativum L.), R. leguminosarum 128C53, which nodulates pea, did not attach to the roots in greater numbers than those strains of rhizobia incapable of infecting pea roots. A complex of R. leguminosarum 128C53 conjugated to a fluorochrome-labeled antibody exhibited a striking attachment to the tips of pea root hairs, where infection normally occurs, but this fluorescent complex also bound to the root hairs of Canavalia en siformis DC., Lupinus polyphyllus Lindl., Trifolium pratense L., and Medicago sativa L., which are not infected by this bacterium. A reproducible, quantitative technique developed for studying interactions between fluorochrome-labeled lectins and rhizobia revealed no relationship between lectin-Rhizobium interactions and the capacity to infect a plant. The data are interpreted as suggesting that simple attachment of Rhizobium to a legume root is not the basis of host-symbiont specificity in this system.