Rhizobium nodulation genes involved in root hair curling (Hac) are functionally conserved

Five specific transposon-induced nodulation defective (Nod^-) mutants from different fast-growing species of Rhizobium were used as the recipients for the transfer of each of several endogenous Sym(biosis) plasmids or for recombinant plasmids that encode early nodulation and host-specificity functions. The Nod^- mutants were derived from R. trifolii, R. meliloti and from a broad-host-range Rhizobium strain which is able to nodulate both cowpea (tropical) legumes and the non-legume Parasponia. These mutants had several common features (a), they were Nod^- on all their known plant hosts, (b), they could not induce root hair curling (Hac^-) and (c), the mutations were all located on the endogenous Sym-plasmid of the respective strain. Transfer to these mutants of Sym plasmids (or recombinant plasmids) encoding heterologous information for clover nodulation (pBR1AN, pRt032, pRt038), for pea nodulation (pJB5JI, pRL1JI::Tn1831), for lucerne nodulation (pRmSL26), or for the nodulation of both tropical legumes and non-legumes (pNM4AN), was able to restore root hair curling capacity and in most cases, nodulation capacity of the original plant host(s). This demonstrated a functional conservation of at least some genes involved in root hair curling. Positive hybridization between Nod DNA sequences from R. trifolii and from a broad-host-range Rhizobium strain (ANU240) was obtained to other fast-growing Rhizobium strains. These results indicate that at least some of the early nodulation functions are common in a broad spectrum of Rhizobium strains.