A bacterium pathogenic on bean
Xanthomonas axonopodis pv. phaseoli and its variant fuscans (called also X. fuscans subsp. fuscans) are causal agents of common bacterial blight of bean. This disease has a worldwide distribution. It remains one of the most deterrents to bean production justifying that these pathogens are classified in quarantine lists in countries that are still disease-free. Disease control only relies on prophylactic measures that are partially inefficient. The development of new and innovative control methods depends on a better knowledge of the bacterial determinants of the host colonization, pathogenicity and trans-mission by seeds.
What is the identity of the pathogen?
Xanthomonads belong to the γ-proteobacteria. Bacteria of the genus Xanthomonas are mobile by their polar flagellum and produce abundant exopolysaccharides, among which xanthan. They are characterized by their yellow pigment, the xanthomonadin. These bacteria cause diseases on a wide range of plant species. However each strain causes disease on a limited number of host plants. Strains of the same species causing the same type of symptom on the same host range, are classified in the same pathovar.
The pathovar phaseoli is divided into four genetic lineages, one of which gathers fuscous strains that produce a brown diffusible pigment (fuscans). Fuscous strains are generally more aggressive than the non fuscous ones and were first isolated in Europe.
What do we know on bean colonization by X. axonopodis pv. phaseoli ?
Contaminated seed is the major inoculum source for common bacterial blight in temperate climates. From a contaminated seed (primary focus) bacteria may colonize the surface of shoot tissues (epiphytic development) or may invade emerging seedlings (endophytic development). X. axonopodis pv. phaseoli is particularly well-adapted to epiphytic survival in the phyllosphere because it is able to aggregate in biofilms that protect it against environmental stresses. Symptoms appear when environmental conditions are favourable (high relative humidity and warm temperature) and when its population sizes are sufficient (106 ufc/g of fresh weight). The major mode of dispersal of the bacterium is by splashing water. Bacterial transmission to seeds occurs whatever the presence or the absence of symptoms. Colonization of bean by X. axonopodis pv. phaseoli in the absence of symptoms depends on a functional T3SS (type 3 secretion system) but not the epiphytic survival. Secretion mutants do not colonize endophytically the host plant and do not contaminate seeds by the vascular pathway. However they are still able to transmit to seeds by the floral route but with a reduced efficiency compared to the wild type.
Why sequencing strain CFBP4834 of X. axonopodis pv. phaseoli ?
Sequencing the genome of a strain of pathovar phaseoli is of great interest because it will allow a better understanding of ecological and epidemiological features of this bacterium, among which the transmission by seeds. For this host with a short cycle, we have developed biotests allowing analysis of host colonization by this pathogen in elementary processes: phyllosphere colonization and transmission to and by seeds.
We have worked with strain CFBP4834-R and we have got data on its epidemiology, colonization and transmission by seeds. This strain is a spontaneous mutant resistant to rifamycin and is very aggressive on beans. This strain can be genetically modified and numerous mutants in candidate genes have already been constructed by direct mutagenesis in this strain. They are mutants in genes for which homologous genes are present in closed bacteria with sequenced genomes (X. axonopodis pv. citri, pv. vesicatoria, X. campestris pv. campestris, X. oryzae pv. oryzae). The knowledge of the genome of X. axonopodis pv. phaseoli will allow studies of functional genomics. A targeted approach will allow the study of the role of candidate genes that are potentially important for colonization of host plant (T3SS effectors, sensors, adhesions…) and that are specific to the pathovar phaseoli. A transcriptomic approach will allow identification of genes potentially implicated in specific processes of the biological cycle of the bacterium, its epiphytic competences and its transmission by seeds. Comparative genomics will allow identification of genes implicated in host specificity by comparison with other X. axonopodis pathovars closed to phaseoli (citri and vesicatoria) and with other bacteria pathogenic on bean (Pseudomonas syringae pv. phaseolicola and P. syringae pv. syringae). It will be also possible to construct in silico phylogenies and look for markers specific to the species or the pathovar.
To learn more about it
Alavi, S. M., Poussier, S. and Manceau, C. 2007. Characterization of ISXax1, a novel insertion sequence restricted to Xanthomonas axonopodis pv. phaseoli (variants fuscans and non-fuscans) and Xanthomonas axonopodis pv. vesicatoria. Appl. Environ. Microbiol. 73, 1678-1682.
Alavi, S. M., Sanjari, S., Durand, F., Brin, C., Manceau, C. and Poussier, S. 2008. Assessment of the genetic diversity of Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans as a basis to identify putative pathogenicity genes and a Type III Secretion System of the SPI-1 family by multiple suppression subtractive hybridizations. Appl. Envir. Microbiol. 74, 3295-3301.
Darrasse, A., Bureau, C., Samson, R., Morris, C. E. and Jacques, M. A. 2007. Contamination of bean seeds by Xanthomonas axonopodis pv. phaseoli associated with low bacterial densities in the phyllosphere under field and greenhouse conditions. Eur. J. Plant Pathol. 119, 203-215.
Darsonval, A., Darrasse, A., Durand, K., Bureau, C., Cesbron, S., and Jacques, M.-A. 2009. Adhesion and fitness in the bean phyllosphere and transmission to seed of Xanthomonas fuscans subsp. fuscans. Mol. Plant Microbe Interact. 22, 747-757.
Darsonval, A., Darrasse, A., Meyer, D., Demarty, M., Durand, K., Bureau, C., Manceau, C. and Jacques M.-A. 2008. Type III secretion system of Xanthomonas fuscans subsp. fuscans is involved in phyllosphere colonization process and in transmission to seeds of susceptible bean. Appl. Environ. Microbiol. 74, 2669-2678.
Jacques, M. A., Josi, K., Samson, R. and Darrasse, A. 2005. Xanthomonas axonopodis pv. phaseoli is aggregated in stable biofilm population sizes in the phyllosphere of field-grown beans allowing resistance to hydric stress. Appl. Environ. Microbiol. 71, 2008-2015.