Mode of infection, nodulation specificity, and indigenous plasmids of 11 fast-growing Rhizobium japonicum strains.

Eleven fast-growing strains of Rhizobium japonicum were characterized with respect to indigenous plasmids and abilities to infect (Inf+) and nodulate (Nod+) cowpea, siratro, wild soybean, and three commercial cultivars of soybean. All strains caused infection via infection threads in root hairs and consistently nodulated cowpea, siratro, and wild soybean in growth pouches. Interactions with commercial cultivars of soybean were strikingly strain specific. Some combinations were Nod-, and infection was delayed in others. The ratios of infections to nodules and the distribution of nodules on primary and lateral roots also varied substantially. A modified in-gel lysis procedure was devised for electrophoretic separation of plasmids from the strains. Plasmids (ranging in size from 35 to greater than 300 megadaltons) were reproducibly detected in all strains.