Interactions between Clavibacter michiganensis and its host plants.

Abstract

Infection by Clavibacter michiganensis, a Gram-positive bacterium, causes bacterial wilt in a variety of agriculturally important plants. In fact, C. michiganensis may be considered the most important bacterial pathogen, with several subspecies specific for alfalfa (C. michiganensis ssp. insidiosus), maize (C. michiganensis ssp. nebraskensis), potato (C. michiganensis ssp. sepedonicus) and tomato (C. michiganensis ssp. michiganensis). The last two are subject to international quarantine regulations, especially C. michiganensis ssp. michiganensis, the causal agent of bacterial canker and wilt in tomato. It is a quarantine organism under the European Union Plant Health legislation (European Union, 1995), causing severe crop failures in all main cultivation areas (Shirakawa et al., 1991). Based on 16S rRNA analysis, it has been shown recently that all subspecies of C. michiganensis form a discrete monophyletic group (Lee et al., 1997). As we have mostly worked with C. michiganensis ssp. michiganensis and the molecular analysis of pathogenicity is most advanced in this system, this minireview will focus on C. michiganensis ssp. michiganensis, the pathogen of tomato. Infection of the host plant usually occurs via wounds, followed by invasion of the xylem vessels, which establishes a systemic vascular disease (Wallis, 1977). After some time, the plants develop wilt symptoms and may perish. An infection at a late stage of plant development resulting in a latent infection is also destructive, as contaminated seeds or, in the case of potato, infected tubers are produced that spread the disease. Chemical control of the disease by antibiotics or copper compounds is not very effective and, in addition, poses severe environmental problems. Also, efforts to provide resistant cultivars of the crop plants have not been successful so far. Therefore, prevention of the disease is only possible by using certified seed or transplants that have been tested to be free of C. michiganensis (Chang et al., 1991; Mansfeld-Giese, 1997). Fortunately, very sensitive and highly specific methods for screening contaminated seeds and transplants have been developed recently (Dreier et al., 1995; Santos et al., 1997).