Population genetics of Xanthomonas citri pv. citri causing citrus bacterial canker in Saudi Arabia as a basis for improving dise

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Population genetics of Xanthomonas citri pv. citri causing citrus bacterial canker in Saudi Arabia as a basis for improving dise



Disease management of bacterial plant pathogens requires the development of tools that allow the precise identification and diagnosis of bacteria at the subspecies, pathotype and strain levels. Similar to genotyping methods developed for examining the epidemiology of human pathogens at a small scale, methods for the epidemiological typing of Xanthomonas have been recently developed. Xanthomonas pathogens can have devastating effects on agriculture and the associated economy. Xanthomonas citri pv. citri, a quarantine organism in several citrus-producing countries that causes Asiatic citrus canker, is a striking example of such a pathogen. The disease caused by this organism is found in Saudi Arabia, where attempts to eradicate it have failed, and it is now considered endemic in different citrus-producing areas of the country. Two pathogenic variants of this bacterium co-exist in Saudi Arabia. A broad-host-range type of the bacterium, X. citri pv. citri-A (Xcc-A), or pathotype A, can infect and induce canker lesions in almost all commercial citrus varieties, from the susceptible Mexican lime to the moderately resistant mandarins. A restricted host range type that is mostly limited to Mexican lime under field conditions, X. citri pv. citri-A* (Xcc-A*), or pathotype A*, is more narrowly distributed worldwide but is also found in Saudi Arabia. This mixture of X. citri pv. citri strains with different host ranges is epidemiologically unique and could allow epidemiological comparisons of these two pathotypes to be made under the same environmental conditions. Furthermore, this situation could favor new genetic variants due to recombination following horizontal genetic exchange between the pathological variants. Recent phylogenetic analyses based on Multilocus Sequence Analysis (MLSA) and amplified fragment length polymorphism (AFLP) analysis clearly separated the broad-hostrange Xcc-A strains from the limited host range Xcc-A* strains. Genetic typing of bacterial populations will provide valuable data for monitoring epidemics and assessing population genetics and evolutionary parameters that may contribute to efficiently controlling the disease. This work will produce a distribution map of the pathogenic variants that will be helpful for disease management under the conditions that exist in Saudi Arabia.

1 Conduct surveys in the Kingdom of Saudi Arabia in different areas where citrus species are known to be infected by X. citri pv. citri. These surveys in citrus production areas will allow mapping of the disease distribution and collection of samples. 2 Establishment of a bacterial collection of X. citri pv. citri strains from Saudi Arabia and preservation of the cultures as frozen and/or freeze-dried cultures. Appropriate sampling will be conducted to address population genetics questions 3 Analysis of the genetic diversity of the bacterial collection. MLVA profiles based on 14 microsatellites will be produced for the entire collection of strains. Additional genetic analyses using a second MLVA scheme with minisatellites and the sequences will be conducted. 4 Perform pathogenicity tests on a representative subsampling of the obtained genetic diversity to describe the host range of the different strains and their aggressiveness and to potentially associate some genetic markers with specific pathological phenotypes. 5 Population genetic analyses: Monitor epidemics and assess population genetics and evolutionary parameters that will contribute to efficient control of the disease. 6 Elaboration of control strategies adapted to the citrus canker situation in Saudi Arabia.

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