FASTIDIOUS GRAM-POSITIVE BACTERIA OF THE GENUS CLAVIBACTER
Department of Biology, University of Turku, Turku, FIN-20520, Finland
Clavibacter is one of the few major Gram-positive genera of plant pathogenic bacteria. Taxonomically, Clavibacter species are related to Mycobacterium and Streptomyces, belonging to the high G+C subdivision of Gram-positive bacteria. Members of this genus share certain general characteristics. They are highly biotrophic and non-saprophytic. Most primarily colonize the vascular system, and because of this are spread in planting materials such as seeds and tubers. These factors combine to make Clavibacter diseases difficult to control and easily spread by certain human activities, such as the cutting of sugarcane during harvesting and the cutting of potato tubers for use as seed. The genus can be divided into three groups: the C. xyli subgroup, the C. michiganensis subgroup, and the group of various species causing the so-called gumming diseases of grasses. The C. xyli subgroup includes C. x. subsp. xyli, which causes ratoon stunting disease in sugarcane, and C. x. subsp. cynodontis, the natural host of which is bermuda grass, but which can be transferred to many other plants. These are the most fastidious, slow-growing and biotrophic species in the genus; in fact C. x. subsp. cynodontis has been referred to as an endophyte because of the lack of disease symptoms shown by colonized plants. The C. michiganensis subgroup contains several important pathogens, particularly C. m. subsp. sepedonicus, which causes potato ring rot, and C. m. subsp. michiganensis, which causes bacterial canker of tomato. These, like ratoon stunt, are economically important diseases that cause losses both directly, due to disease itself, and indirectly, due to the expense of control measures. The gumming diseases are less well characterized except for C. toxicus. This species produces corynetoxin in nematode galls of annual ryegrass, causing serious toxicity problems for livestock in Australia.
As there are no methods for chemical control of diseases caused by Clavibacter, seed certification and sanitation are the most important control measures. Bacterial detection is difficult due to slow growth in culture and lack of clear disease symptoms on indicator plants. Thus molecular methods for detection have been developed for C. m. subsp. sepedonicus, C. m. subsp. michiganensis and C. x. subsp. xyli utilizing antibodies, DNA probes and PCR. Theoretically, PCR should be the most sensitive method, with a detection limit of 1-10 cells per sample. However, the actual detection limit is usually orders of magnitude higher in plant material because plant substances inhibit the PCR reactions. Thus PCR methods have not yet been routinely adapted for screening in seed certification programmes, and immunological methods remain the primary tools for detection.
Recently, important molecular biological advances have been made for this genus. C. m. subsp. sepedonicus, C. m. subsp. michiganensis, C. x. subsp. xyli and C. x. subsp. cynodontis have all been transformed with plasmid vectors. For both of the michiganensis subspecies, this has ailowed it to be shown that an extracellular cellulase is an important virulence factor. For C. x. subsp. cynodontis, strong transcriptional promoters have been isolated, and an integrative vector has been constructed. This allows foreign genes to be transferred into the bacterial chromosome and stably maintained and expressed .
For Gram-negative plant pathogens, the hypersensitive response (HR) has been studied for some years. This plant defence response occurs when non-host plants recognize an invading pathogen and defend themselves by inducing local cell death at the site of invasion. Both C. m. subsp. michiganensis and C. m. subsp. sepedonicus have been shown to give an HR on certain non-host plants, and this is under study to try to understand how plants defend themselves against these pathogens. For the subspecies of C. xyli, however, no HR has been consistently observed on non-host plants. It may be that these unusually biotrophic pathogens are so non-aggressive that they simply do not trigger plant defence mechanisms, which may help to account for their success as pathogens.