Symbiotic mutants of Rhizobium meliloti that uncouple plant from bacterial differentiation.

Spontaneous mutants at a new symbiotic locus in Rhizobium meliloti SU47 are resistant to several phages and are conditionally insensitive to a monoclonal antibody to the bacterial surface, apparently because they are deficient in a wild-type exopolysaccharide. On alfalfa, the mutants do not curl root hairs, but penetrate the epidermis directly, forming nodules that contain no visible infection threads or "bacteroids," have a few bacteria in superficial intercellular spaces only and not within the nodule cells, and fail to fix nitrogen (Fix-). Evidently, infection threads are not essential for cell proliferation and nodule formation, which are here induced by a bacterial signal at a distance and uncoupled from the bacterial differentiation that normally goes on as well.