Cooperativity between Rhizobium Mutant Strains: Induction of Nitrogen-fixing Nodules on the Tropical Legume Leucaena leucocephala

Abstract

Summary Strain NGR234 produces highly mucoid (Muc+) colonies on laboratory media and induces nitrogen-fixing nodules on Leucaena plants. However, many transposon Tn5-induced exopolysaccharide mutants which were nonmucoid (Muc−) do not produce normal nodules on Leucaena plants. Instead, these Muc− (unconditional rough) mutants caused root hair curling and formed callus-like structures. When strain ANU265 (a Muc+ Sym plasmid-cured derivative of strain NGR234) was coinoculated with some Muc− callus-forming strains, normal nitrogen-fixing nodules (Nod+ Fix+) were formed. Both Muc− and Muc+ strains were present in the nitrogen-fixing nodules and no genetic exchange was detected between the strains. However, some Muc− strains, when coinoculated with ANU265, produced normal-sized nodules (instead of callus-like structures) that were unable to fix nitrogen. Examination by light microscopy of the callus-like structures produced by the Muc− (callus-forming) mutants on Leucaena plants suggested that they are deformed nodules resulting from early inhibition of the normal infection process. Experiments with a specifically constructed transconjugant strain which produces varying amounts of exopolysaccharide (EPS) supports the conclusion that EPS is essential for correction of the nodulation behaviour of Muc− mutants.