Early interactions of Bradyrhizobium japonicum and soybean roots: specificity in the process of adsorption

Specificity in the process of adsorption of Bradyrhizobium japonicum to soybean roots (Glycine max (L.) Merr.) was studied with a standardized procedure that quantifies adsorbed rhizobia to young seedlings from dilute suspensions (in the order of 103 bacteria ml−1) of an antibiotic-resistant strain of B. japonicum. Adsorption of this strain was maximal in the late-exponential growth phase. Adsorption was rapid, without an initial lag, and slowed down after 1 h, approaching a plateau after 4 h. This occurred either in absence or presence of Ca2+ plus Mg2+ ions. Adsorption of the antibiotic-resistant strain at a constant concentration of 103 rhizobia ml−1 was increasingly inhibited by the presence of antibiotic-sensitive competitor rhizobia at a concentration ranging from 103 to 108 bacteria ml−1. In the range of 105–107 competitor rhizobia ml−1, a heterologous competitor inhibited adsorption of the antibiotic-resistant strain significantly less than a homologous competitor, indicating the occurrence of symbiont-specific adsorption events. Evidence of adsorption specificity also was shown by competition against a range of heterologous and homologous strains. Specific adsorption did not require the presence of Ca2+ plus Mg2+ ions. By incubating B. japonicum with soybean roots in the presence of a heterologous competitor strain and different concentrations of either N-acetyl-d-galactosamine (the soybean lectin hapten), or lactose (the B. japonicum BJ38 lectin inducer and hapten), it was observed that whereas N-acetyl-d-galactosamine had no effect on specific adsorption, lactose at 0.1 mM stimulated specific adsorption by 105%, and at 10.0 mM inhibited it by 65%.