The nodDABC Genes of Rhizobium leguminosarum biovar trifolii Confer Root-Hair Curling Ability to a Diverse Range of Soil Bacteria and the Ability to Induce Novel Root Swellings on Beans

Abstract

Cloned DNA fragments coding for the nodDABC genes of Rhizobium leguminosarum biovar trifolii strain ANU843 were introduced into Rhizobium strains possessing Sym plasmid deletions. These strains were able to: (a) synthesise four butanol-soluble Nod metabolites; (b) affect the normal growth pattern of plant root hairs of a wide range of host and non-host legumes; and (c) induce many root outgrowths on Phaseolus plants. The four Nod metabolites produced by these strains were labelled by supplying cultures with ¹⁴C-acetate in the presence of a flavonoid inducer of nod gene expression. In contrast, more than ten Nod metabolites were synthesised by wild-type strains or constructed strains containing the full complement of R. leguminosarum biovar. trifolii nodulation and host specific nodulation genes. Strain ANU845 containing nodDABC did not induce infection threads or nodule initiation sites but distorted and curled cells in plant root hairs. However strain ANU845 induced root outgrowths on beans (Phaseolus vulgaris) that appeared to result from a proliferation of the epidermal tissue. Transfer of plasmids bearing nodDABC to various Gram-negative bacteria, Agrobacterium tumefaciens, Pseudomonas aeruginosa, Lignobacter sp., Azospirillum brasilense and Escherichia coli, and different non-nodulating mutant rhizobia conferred on these strains the ability to cause root-hair curling and distortions. Several strains induced root-hair curling on clover and a range of other non-host legumes. We suggest that the expression of nodDABC in a range of soil bacteria may extend or alter the effects of these soil bacteria on the roots of host plants.