“…This specificity forms the basis for the grouping of legumes and rhizobia into cross inoculation groups; that is, groups of legume species nodulated by a single species of Rhizobium [1][2][3][4]. Cross inoculation groups were thought to be mutually exclusive, but promiscuity (nodulation in more than one cross-inoculation group) has been demonstrated [5][6][7]. The mechanistic basis of bacterium-host specificity is unknown.…”