Xanthomonas is a gram-negative Plant Pathogenic Bacteria which belong to the phylum Proteobacteria, these are rod shaped and this type of all bacteria cause leaf spot or blight infection in the host, Xanthomonas has single flagellum which is polar in nature. The suitable temperature for the bacterial growth is 25o-30o C. Colonies of bacteria are yellow in color and this yellow color is due to special pigment xanthomonadin which is also important to protect the bacteria from the environmental damages. Xanthomonas genus of the bacteria effect not only crops like cotton, rice but also cause the serious problems in trees like Xanthomonas campestris pv. citri cause citrus canker in citrus, Xanthomonas campestris pv. mangiferae-indicae cause the Bacterial Black Spot of the mango, Xanthomonas campestris pv. fici cause the Bacterial blight of fig and Xanthomonas campestris pv. pruni cause the Bacterial spot of peach.
It is reported that in several conditions, Virulence mechanisms of strain-specific variations in Xanthomonas spp. have been might reflect bacterial adaptations to specific host plants. Bacteria mostly present in the form of epiphytes on the surface of the plant but it invades through wounds, stomata and hydathodes. The main feature of these bacteria is that it can cause infection systemically. When it enters in the tissue of the plant, this pathogen specie multiply in either in intercellular spaces or colonizes in the xylem tissues and spread systematically in the whole plant parts. Bacteria are also supposed as hemibiotrophic organisms because in early life stages they feed on the living host but in later the feed on the dead tissues. On molecular research, bacterial virulence has diagnosed which has main contribution to host-pathogenic interaction. Plant pathogenic bacteria has ability to attach with the plant surface, enter into the intercellular space of the host tissues, where they obtain nutrients and initiate the plant defense responses.
Successful infection of the host plant is established, and this infection is depend upon on the bacterial secretion system which enter the protein or DNA directly into the host cell cytoplasm and this process is known as translocation. The protein which is translocated in the host cell is known as effectors protein. Lots of infectious bacteria excrete the combination of the proteins those give the progress of the disease and multiplication of the bacteria specie. Xanthomonas spp. secretes a characteristic EPS (extracellular polysaccharides) xanthan, by which mucoid appearance of the bacterial colonies are formed. Furthermore, Xanthan might cause wilting of host plants by disturbing the water flow in xylem vessels and this is most common in vascular pathogens. Avirulencant / pathogenic Xanthomonas pathovar have avr genes in their genomes, which shows that interaction with the host is depend upon on the gene for gene theory. Hypersensitive response is occurred in the host when Xanthomonas show incompatibility with the host which leads the local lesions due to cell death of the host. It has been reported that of Xanthomonas campestris pv malvacearum has characteristic plasmids which has important role in the pathogenicity, but it is even not reported in the American and Africans strains of the bacterial pathogens which indicate that variations in the virulence is not related to the plasmid. But in further studies it has been reported that aggressiveness of the pathogen has occurred by plasmids of bacteria. Xanthomonas is included in special group due to its specific characteristics which make it different from the other bacterial species mainly basis on the host range are some are placed in the Xanthomonas campestris.
This article is collectively authored by Tanvir Ahmad and Muhammad Usman. Department of Plant Pathology University of Agriculture Faisalabad.