Influence of Rhizobium leguminosarum biovar trifolii host specific nodulation genes on the ontogeny of clover nodulation

The infection of white clover seedlings by Rhizobium strains with different host range properties was assessed using various microscopic techniques. Several wild-type and Rhizobium leguminosarum biovar vicias hybrid strains containing defined R. l. bv. trifolii host range genes were used. The morphological changes in the root tissue of uninoculated and rhizobia inoculated white clovers were identified and compared. In particular, changes were observed in the induction of inner cortical cell division, alterations to nodule development and lateral root formation. The responses of the infected roots and the types of structures formed support the hypothesis that lateral roots and nodules may be physiologically homologous structures. To establish a normal pattern of nodulation on white clover roots, both sets of known host specific nodulation genes (operons nod FERL and nod MNX) of R. l. bv. trifolii were required. However, some nodule development occurred when only the nod FERL genes were present in the hybrid strain.