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Tests for a Diffusable Compound Endowing rj₁-incompatible Strains of Rhizobium japonicum with the Ability to Nodulate the rj₁rj₁ Soybean Genotype¹

The nodulation restrictive genotype of soybeans [Glycine max (L.) Merr.] rf₁ rf₁ is nodulated by some strains of Rhizobium japonicum. Inoculation with mixed cultures of rj₁-compatible and rj₁-incompatible strains has shown that the presence of an rj₁-compatible strain facilitates nodulation of rj₁rf₁ plants by an rf₁-incompatible strain. This study was undertaken to determine whether rj₁-compatible strains of R. japonicum produce a diffusable product capable of endowing rf₁-incompatible strains with the ability to nodulate the rj₁rj₁ genotype. An rj₁-incompatible strain remained unchanged in ability to nodulate rj₁rj₁ plants when: (a) grown in culture filtrates of rf₁-compatible strains, (b) cultured in U-tubes across a 0.08 nga pore size polycarbonate membrane from rf₁-compatible strains, or (c) cultured in dialysis tubing suspended in broth cultures of an rf₁-compatible strain. It was concluded that the rf₁-compatible strains tested apparently do not produce a diffusable compound in vitro capable of endowing rf₁-incompatible strains with the ability to nodulate the rf₁rf₁ genotype. The rf₁-compatible Rhizobium appear to exert their effect, in close proximity to the nodulation site.

Key Words: Glycine max (L.) Merr. • Symbiosis • Root nodule • Microsymbiont

² Research geneticist, plant physiologist (support scientist) and microbiologist, Cell Culture and Nitrogen Fixation Lab., Beltsville, Agric. Res. Ctr. West, Beltsville, MD 20705,

Received for publication October 3, 1980.