Xanthomonas axonopodis

A Microbial Biorealm page on the genus Xanthomonas axonopodis

Classification

Higher order taxa

Xanthomonas axonopodis

Bacteria; Proteobaceria; Gammaproteobacteria, Xanthomonadales; Xanthomonadaceae; Xanthomonas(1)

Species

NCBI: Taxonomy

Genus species: axonopodis

Description and significance

Xanthomonas axonopodis are proteobacteria and as with all proteobacteria they have a gram negative wall consisting of an outer membrane made up of lipopolysaccharide (LPS) and a phospholipid inner membrane (4). Their appearances are aerobic rod shape with polar flagella. X. axonopodis can be found cultivating on the leaves, stems, and fruit of citrus trees all around the world. They are mostly found in heavily citrus agricultural countries such as the United States, Australia, and Brazil. These bacteria are mainly known as plant pathogens that causes citrus cankers. The leaves, stems, and fruits that are infected with X. axonopodis display lesions which take on yellow halo or ring shaped appearances. They are so persistent that whole orchards have to be destroyed if X. axonopodis are present (5). In 1910, Xanthomonas were first reported and studied in the United States.

Genome structure

Their genome was sequenced in Sao Paulo, Brazil where they found X. axonopodis genome structures to contain a circular dsDNA chromosome with 5 mega base pairs and 4374 genes. Furthermore, the bacterium carries two plasmids, referred to as pXAC33 and pXAC64 (2). The two plasmids are 33 and 64 kilo base pairs long (3).

Cell structure and metabolism

The cell of X. axonopodis is a rod-shaped gram negative membrane with a polar flagella attached onto the membrane (3). The flagella allows the bacteria to move from lesion leaves of citrus trees to others trees infecting its host. Cellualse, protease and pectate lyase from Xanthomonas species have been suggested to play crucial roles in virulence and in bacterial nutrition. Xanthomonas possesses a Type III protein secretion system that is essential for pathogenicity in plants. This system is what causes the plants to become infected by inducing the hypersensitive response in resistant plants. The Type III protein system infects plants by inducing the expression of the host gene Os8N3, resulting in an increase of the host susceptibility (6).

Ecology

This bacteria occurs in agricultural and urban areas. It generally affects citrus trees in high temperature areas, heavy rainfalls, and high winds. Due to wind and rainfall, this bacteria may spread throughout situated areas. It is most likely to infect plants in high temperature and heavy rainfall weather. This causes citrus canker, which may progressively and ultimately lead to economic losses on citrus industries.

Pathology

This bacteria contaminates citrus plants that prevents progressive fruit and health production. It diminishes fruit production of trees, ultimately leading to no fruits and death of plant. This disease may contaminate plants and even equipments. It was originated in 1912 in Florida. It started to spread within Florida and only in Florida. Yet years later in Miami, a new form of this bacteria started to spread and ultimately causing a bigger and deadly problem.

Current Research

Citrus canker was concluded to be a threat to natural environment and economy. It has been tested and researched to produce a feasible elimination of this baterium. Yet on January 11, 2006, USDA has come to the conclusion that the eradication of this baterium is not feasible. Due to the unexpected hurricanes in the years 2004 to 2005, the top United States Department of Agriculture officials came to conclude that this disease has spread drastically and is difficult to eradicate. Because eradication of this baterium is not feasible, officials demand a new approach to decrease, not eliminate, the spread of citrus canker and inhibit further contamination of this bacteria.

For accurate identification of this bacterium, couple PCR methods were developed. A plasmid containing pthA gene was used as a primer for this testing. The A-strain was not able to accurately identify the A strain variant Aw(Wellington strain).

Rep-PCR with BOX and ERIC primers were used to isolate and identify the different canker strain types. They related these identifications with the spread of canker in Florida and Miami. Rep-PCR supported Aw strains among A strains to partake in the spread of this disease in Florida. Strains were isolated and compared with strains found in other geographic areas. Strains found in Manatee County, Florida matches the strains from China and Malaysia. The MI strain (Miami) matches several locations in south and central Florida. These comparisons furthered proved the conclusion and there was a movement of plant material that took place between these countries and counties that caused this disease to spread.

References

http://www.issg.org/database/species/ecology.asp?fr=1&si=219&sts=

http://expasy.org/sprot/hamap/XANAC.html

http://www.doacs.state.fl.us/press/2006/01112006_2.html

http://www.apsnet.org/online/feature/citruscanker/

Edited by student of Rachel Larsen and Kit Pogliano