Evidence countering the role of polygalacturonase in invasion of root hairs of leguminous plants by Rhizobium spp.

Abstract

Polygalacturonase has been postulated as being involved in the invasion of the host legume by Rhizobium spp. The role of this enzyme in the invasion of Glycine max by Rhizobium japonicum was investigated. Enzyme activity of several Rhizobium spp. was assayed viscometrically and no constitutive polygalacturonase was found. The enzyme could not be induced by growth with either pectin or galactose as the sole carbon source. Extract and exudate from the roots of nodulating and nonnodulating strains of Glycine max that had been incubated with effective strains of R. japonicum failed to show any higher levels of polygalacturonase activity than did uninoculated control plants. Previously reported experiments were duplicated. Levels of polygalacturonase activity from the extract and exudate of white clover inoculated with R. trifolii Cl 226 were as reported in the literature. Contrary to published reports, however, no greater levels of enzyme activity were found from effective plant–bacteria associations than from either ineffective associations or uninoculated control plants. It was concluded from these experiments that polygalacturonase does not play a major role in the invasion of Glycine max by Rhizobium japonicum.