STATUS OF MOLECULAR BIOLOGICAL-RESEARCH ON THE PLANT-PATHOGENIC GENUSCLAVIBACTER

The species of the high-GC Gram-positive genus Clavibacter can be divi ded into three groups: (1) The subspecies within C. michiganensis; (2) the Clavibacter species causing gumming diseases; and (3) the fastidi ous, 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. michiga nensis and C. xyli subspecies can be difficult to detect, since they o ften give symptomless infections. Therefore, serological and DNA-based detection techniques have been developed to aid in disease control. E fforts to determine what factors may be important in pathogenicity hav e 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. michig anensis subspecies has allowed elucidation of other pathogenicity fact ors, e.g., plasmid-encoded extracellular cellulase. For C. toxicus, th e involvement of a phage in annual ryegrass toxicity has been determin ed. Of special interest is C. xyli sap. cynodontis, which has been gen etically 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.