| Xanthomonas campestris | |
|---|---|
.jpg) | |
| Black rot of crucifer leaves caused byXanthomonas campestris pv.campestris | |
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Pseudomonadati |
| Phylum: | Pseudomonadota |
| Class: | Gammaproteobacteria |
| Order: | Lysobacterales |
| Family: | Lysobacteraceae |
| Genus: | Xanthomonas |
| Species: | X. campestris |
| Binomial name | |
| Xanthomonas campestris (Pammel 1895) Dowson 1939 | |
| Type strain | |
| NCPPB 528 | |
| Synonyms | |
Bacillus campestrisPammel 1895 | |
Xanthomonas campestris is agram-negative,obligate aerobic bacterium that is a member of theXanthomonadaceae, a family of bacteria that are commonly known for their association with plant disease.[1] This species includesXanthomonas campestris pv.campestris, the cause of black rot in brassicas (cruciferous vegetables), one of the most important diseases of brassicas worldwide.
These bacteria are facultativesaprophytes, meaning that they are typicallyparasitic while also having the ability to live on dead or decaying organic matter under the proper conditions. Upon initial infection, the bacteria remain in theepiphytic stage; however, the harmfulendophytic stage is reached when the bacteria actually enter the plant host through natural openings.[2] In general, the genes that contribute significantly to the plant-bacteria relationship are theavirulence (avr) genes, the hypersensitivity response andpathogenicity (hrp) genes, and thepathogenicity factors (rpf) genes.[1][3][4] Additionally, the virulence determinants associated with the seedborne diseases that result from this bacterium include extracellular enzymes,polysaccharides,lipopolysaccharides, etc.[3]
Several strains ofXanthomonas campestris produce an exopolysaccharide called xanthan orxanthan gum, which has important uses as a thickener in the food, oil, agricultural, and pharmaceutical industries.[5]
Over 140pathovars ofXanthomonas campestris have been described initially and typically named according to the plant that they were first found to infect.[1] However, several studies have subsequently proposed the reclassification of many of these pathovars in different species within the genusXanthomonas.[6][1][7] This left six pathovars ofX. campestris remaining in this species, which included pathogens ofBrassicaceae plantsX. campestris pv.aberrans, X. campestris pv.armoraciae, X. campestris pv.barbareae,X. campestrispv.campestris, X. campestris pv.incanae, andX. campestrispv.raphani,[8] but still included a small number of other pathovars likeX. campestris pv.plantaginis andX. campestris pv.papavericola.
Further investigation of pathogenicity profiles and multilocus sequencing typing suggested that the list could be narrowed down to just three main pathovars with the different symptoms being black rot, leaf spot, and bacterial blight.[8] BothX. campestris pv.campestris (known for causing black rot of crucifers) andX. campestris pv.incanae (known for causing bacterial blight of garden stocks) are vascular pathogens, and they have been found to invade the plant host through wounds orhydathodes.Xanthomonas campestris pv.campestris also has some limited ability to infect the plant host through the stomata.Xanthomonas pv.raphani has been found to enter the plant through itsstomata to cause infection of the tissue, or ratherparenchyma. This results in bacterial spot on a wider range of hosts, which includes both crucifers and certain solanaceous plants.[1][8][9]
12 pathovars (vitiswoodrowii,vitiscarnosae,vitistrifoliae,bilvae,azadirachtae,durantae,centellae,thespesiae,leeana,merremiae,thirumalacharii, andtrichodesmae) were moved toXanthomonas citri based on phylogenomic evidence in 2022. Pathovars transferred earlier toX. citri includeviticola.[10]
20 pathovars were moved toXanthomonas euvesicatoria in 2023 based on phylogenomic evidence.[11]
Relationships betweenXanthomonas campestris bacteria and plants can be both compatible and incompatible. It is in the compatible relationships, where the bacteria are able to overcome the host's defenses, rather than experience attenuated growth, that disease symptoms will be seen in the plants.[3] This is due to toxins, extracellular enzymes (exported by thetype II secretion system),polysaccharides,lipopolysaccharides, a fatty acid-dependent cell-cell communication system, and proteins (secreted by thetype III secretion system), for example.[1] The genes in the bacterial genome that are responsible for such interactions includeavirulence (avr) and hypersensitivity response andpathogenicity (hrp) genes.[3]
Gene-for-gene patterns control the interactions between theXanthomonas campestris, a bacterial pathogen, and plants.Avr genes are a group of genes that impact the specificity of the interaction between the bacteria and the plant host. When either these bacterial genes or a plant's resistance genes to the pathogen are not present, the interaction will result in disease. Alternatively, when the genes are present, the plant's resistance genes will produce a product that is able to recognize theavr genes of the bacteria, which allows for the plant host to have resistance.[3]Hrp genes are responsible for the determination of the outcome of the interaction between the plant and bacteria. When mutation occurs within these genes, there is impact on both compatible and incompatible interactions. This is the case because there may be an impact on pathogenicity andhypersensitivity response, respectively.[3] Therefore, the plant's ability to limit the spread of the microbial pathogen may be inhibited.
The regulation ofpathogenicity factors (rpf) gene cluster also plays an important role in the plant-bacteria interactions by encoding for a cell-cell signaling system involving diffusible signal factor that is necessary for fullvirulence. This regulation system is involved in the regulation of both the formation and dispersal ofXanthomonas campestris biofilms, which is specifically related to therpfB andrpfF genes within the cluster.[1][4] With a mutation in therpf genes, the synthesis of various extracellular enzymes will be downregulated. This includesendoglucanase,protease, andextracellular polysaccharide (EPS) xanthan, for example, which are important to the virulence of the bacteria.[4]
Like withXanthomonas species in general, the primary route of transmission forXanthomonas campestris is through seeds, which act as the source of inoculum.[3][2] Therefore, the bacteria are initially in theepiphytic stage of the infection cycle, which is when they grow on the surface of the plant, such as on the aerial tissues of leaves and fruit. Various adhesion strategies are utilized by the microbes in order to remain attached to the plant surfaces, and this includes bacterial surface polysaccharides,adhesion proteins, andtype IV pili.[2] Then,biofilm matrices composed primarily of xanthan will form, which will help the bacteria to survive in the harshabiotic conditions of the plant surfaces. These biofilms, along with pigments, also help the bacteria to survive the potential damages to DNA and membrane that result from radiation and light. Eventually, the bacteria enter the host tissue through natural openings such as pores andstomata, or wounds, which is hypothesized to be a result ofchemotaxis.[2] When this happens, theendophytic stage has been reached and colonization in thevascular system orparenchyma takes place. This is when the development of symptoms, such as lesions of leaves, will occur.[2] Progression into the stem and roots can eventually happen as well, which is when there is systematic infection of the plant. Furthermore,Xanthomonas campestris can be spread to other plants when the population of bacteria has increased enough that they emerge on the plant surfaces again. This dispersal includes both environmental and mechanical routes, such as through wind, rain, people, non-specific vectors, seed dispersal, etc.[2]
Xanthomonas campestris is commonly used industrially to produce a water-solubleexo-polysaccharide, known asxanthan gum, fromfermentation of carbon sources like glucose.[5] In this process, a preserved culture of thegram-negative bacterium is expanded through growth and then used as an inoculum inbioreactors with liquidgrowth media. Under select modes of operation, such asbatch fermentation, and proper growth conditions, fermentation then takes place. Therefore, as the microorganism grows, there is production of xanthan, which is secreted from the bacterial cells and ultimately recovered from the media and purified usingalcohol precipitation techniques.[5]
This product is particularly applicable to the food industry as a suspending, thickening, and stabilizing agent. However, it also has applications pertaining to theagricultural industry, theoil industry, thepharmaceutical industry, etc.[5] Because of that, work is being done to investigate advancements that can be made to the current xanthan gum production processes.
