Status of molecular biological research on the plant pathogenic genus Clavibacter

The species of the high-GC Gram-positive genus Clavibacter can be divided into three groups: (1) The subspecies within C. michiganensis; (2) the Clavibacter species causing gumming diseases; and (3) the fastidious, xylem-limited subspecies within C. xyli. All members of the taxon are plant pathogens. Basic studies have demonstrated the presence of native plasmids in various species and allowed construction of cloning vectors for a few of these species, Infection of plants by C. michiganensis and C. xyli subspecies can be difficult to detect, since they often give symptomless infections. Therefore, serological and DNA-based detection techniques have been developed to aid in disease control. Efforts to determine what factors may be important in pathogenicity have focused on extracellular polysaccharides in several of the species, but a clear role for these molecules in virulence has not been found. The development of transformation methods for several of the C. michiganensis subspecies has allowed elucidation of other pathogenicity factors, e.g., plasmid-encoded extracellular cellulase. For C. toxicus, the involvement of a phage in annual ryegrass toxicity has been determined. Of special interest is C. xyli sap. cynodontis, which has been genetically engineered for protecting plants from insect damage. Further progress in elucidating the molecular biology of these organisms will require the development of more sophisticated molecular genetic tools.