Xanthomonas axonopodis

A Microbial Biorealm page on the genus Xanthomonas axonopodis

Classification

Higher order taxa

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

X. axonopodis have a parasitic relationship exclusively in plants where they cause citrus cankers on plants. They are most likely originated from southeastern Asia and spread to Japan, South Africa, Australia, the Pacific Islands, South America, and the United States (7). Usually trees that are infected with citrus canker bacteria, such as the X. axonopodis, are burned to prevent the spread of infection because it is difficult to control their rapidly spreading nature. Xanthomonas are an agriculturally devastating bacteria, in 1933 they caused more than 6 million dollars in damage in Florida alone which destroyed about 258,000 grove trees and 3 million nursery trees that had become infected. In 1998, Sao Paulo, Brazil suffered from a citrus canker outbreak where they had no choice but to destroy all infected trees as well as trees that were within a 30 meter radius of the infected trees.

Pathology

Xanthomonas are exclusively pathogenic to a large group of plants such as citrus trees, rice, cotton, beans, and grapes. They cause a highly contagious disease called citrus canker. Citrus canker can destroy an entire crop field, but does not pose any danger to humans or animals. The infection has a wide range of effects which include: defoliation, dieback, severely blemished fruit, reduced fruit quality, and premature fruit drop (8). Once the hosts are infected, X. axonopodis can spread like wildfire because of the many ways it can travel. Some examples of how it’s spread are by human movement of diseased citrus plants, use of contaminated equipment, or by fallen lesion leaves and fruits. Diseased leaves and fruits with lesions have a yellow halo or ring appearance. X. axonopodis would multiply if the lesions were expanding, but would die if it was on a surface facing sunlight. When the bacteria are not on leaves or fruits, it can survive by taking residence inside tree bark until it can transfer itself onto leaves or fruits. However, it can only survive for a few days if its host has fallen to the ground because its natural habitat has changed. This results because the bacteria now has to adapt to a new environment and find new sources for growth and reproduction. The two fundamental host determinants for citrus canker are the stage of leaf expansion and the resistance of mesophyll tissue. The host is most susceptible when its leaves expand ½ to 2/3, at this point the stomata open, but the leaf cuticle is not fully developed. During this time, the bacteria would take advantage of this opening and invade the leaves.

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.