Clavibacter michiganensis subsp. michiganensis: first steps in the understanding of virulence of a Gram-positive phytopathogenic bacterium
Introduction
Besides fungi and viruses, bacterial pathogens are significant factors which reduce yields of agriculturally important plants worldwide. Research has generated a vast body of information on the mechanisms which govern plant–microbe interaction, but these advances in the understanding of pathogenicity of the phytopathogenic bacteria are almost entirely restricted to Gram-negative organisms. Reasons for this are, that they are easier to handle and genetic tools are readily available. Among the phytopathogenic bacteria most are Gram-negative, however, of the few Gram-positive phytopathogens some are quite important, since they cause great losses in the cultivation of crop plants. One example is Clavibacter michiganensis subsp. michiganensis which is the subject of this review. More information on the other subspecies of C. michiganensis can be found in recent review articles by Burger and Eichenlaub (2003) and Jahr et al. (1999).
Section snippets
Clavibacter michiganensis subsp. michiganensis is a pathogen of tomato
The bacterial genus Clavibacter belongs to the Gram-positive, plant-pathogenic actinomycetes (family Microbacteriaceae) (Stackebrandt et al., 1997) and is only represented by a single species, C. michiganensis. This species is subdivided into five subspecies according to their different host ranges (for reviews see Jahr et al., 1999, Burger and Eichenlaub, 2003). C. michiganensis subsp. sepedonicus is responsible for ring rot of potato (Solanum tuberosum; Manzer and Genereux, 1981), C.
Exopolysaccharides and extracellular enzymes
To explain the mechanism by which Cmm is inducing bacterial wilt, several hypotheses have been discussed. As the bacteria are primarily located in the xylem of infected plants, a physical impairment of water transport may cause a severe water stress resulting in wilting. The production of exopolysaccharides (EPS) may be a prominent factor contributing to plugging of xylem vessels. Like most soil- and plant-associated bacteria, Cmm produces high molecular weight, acidic EPS (Rai and Strobel,
Control of Cmm and detection methods
In contrast to Gram-negative plant-pathogenic bacteria, an incompatible reaction between Cmm and a tomato cultivar has not yet been found and all efforts to obtain resistant tomato cultivars by breeding so far have not been satisfactory. As a consequence spreading of Cmm has to be prevented and therefore Cmm is a quarantine organism in the EU (European Union, 1995) and many other tomato producing countries.
The only possibility to reduce agricultural losses by Cmm-infections is by removing and
The development of genetic tools and methods for Cmm
In order to shed some light on the molecular mechanisms by which Cmm is causing the wilting disease in tomato plants it is necessary to identify the genes involved in pathogenicity and to characterize the function and role of their gene products. However, the genetic analysis of pathogenicity of Gram-positive bacteria is severely hampered by the lack of genetic tools and methods since those developed for Gram-negative bacteria are mostly of no use in Gram-positives.
Investigating Cmm for the
Identification of pathogenicity genes
During the phase of the development of genetic tools and methods for Cmm it became necessary to generate a plasmid-free derivative of strain NCPPB382, in order to prevent incompatibility reactions between the cloning vector and the parental plasmids resident in the strain. It was found that a shift of the growth temperature from 26 to 32 °C resulted in plasmid curing in Cmm (Meletzus and Eichenlaub, 1991). Thus, a plasmid-free derivative, designated CMM100, was isolated. Interestingly, a plant
Plasmids of Cmm are transmissible
As mentioned earlier a rise in growth temperature from 24–26 to 30–32 °C can result in loss of the plasmids and we observed that during handling of Cmm in the laboratory plasmid pCM2 carrying the pathogenicity gene pat-1 is frequently lost.
Since our experimental strain NCPPB382 was isolated in the UK in 1956 it was an important question whether the pathogenicity genes were still residing on plasmids and how pathogenicity genes celA and pat-1 and plasmids were distributed in more recent natural
New information from the Clavibacter michiganensis subsp. michiganensis genome project
The genome of C. michiganensis subsp.michiganesis strain NCPPB382 is about 3.4 Mb in size (including the two plasmids) as determined by pulsed field gel electrophoresis (PFGE). In the course of these experiments also several other strains of Cmm and C. michiganensis subsp. sepedonicus (Cms) strain Cs7 were analyzed and all have similar genome sizes of about 3.3–3.4 Mb (Gartemann and Eichenlaub, unpublished). The [G+C] content is nearly identical with 72.2 and 72.6% for Cmm and Cms, respectively,
Outlook
It was fortunate that in Cmm the genes directly responsible for induction of disease in the host plant were located on plasmids and thus quite easily identified. However, all other genes that are required for other aspects of the plant–microbe interaction, like host recognition, infection, suppression of plant defence, and effective colonization map on the chromosome. The genome project on Cmm will provide the sequence information required for the selection of genes representing candidates in
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2022, Food Chemistry: XCitation Excerpt :In addition, a previous study showed that FcEPS extracted from E. sakazakii M1 was mainly composed of fucose, galactose, and glucose, in which the content of fucose reached 42.72 mol% (Xiao et al., 2021). C. michiganensis subsp. is a plant pathogenic bacterium that spreads through the xylem, leading to bacterial wilt and canker in tomato and other plants (Gartemann et al., 2003). The EPS produced by C. michiganensis subsp. may protect bacteria by inhibiting the plant defense system and is conducive to the adhesion of bacteria on the plant surface, thus promoting the infection and colonization of host plants.