Pigmented variants of Rhizobium

Summary

Non-infective pigmented variants of Rhizobium have been isolated from several cultures after treatment with antibiotics, heat and U.V. irradiation. These variants tolerated much higher doses of heat, U.V. and certain antibiotics than the wild type strains from which they originated. Some of the pigmented variants produced reverse mutants on subculture which had lost their pigment but recovered their infectivity. These reverse mutants also lost their extreme U.V.-resistance. However, many pigmented mutants were very stable and reverted only after U.V. irradiation.

This investigation shows that pigmentation in Rhizobium and loss of ability to form nodules on the roots of leguminous plants are genetically linked and may be pleiotropic effects. So far, more than 10 pigmented variants have been isolated, none of them was infective, while reverse mutants which had lost their pigment always gave rise to nodules.