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From ancient times to the present: How did Xanthomonas evolve into the pathogen we know today?
Xanthomonas, derived from the Greek words "xanthos" (yellow) and "monas" (solid), is a genus of bacteria, many of whose members cause plant diseases. This genus contains at least 27 plant-related Xanthomonas species, which together infect more than 400 plant species. Each species typically has a specific host and/or tissue range and colonization strategy, and these tissue characteristics are closely related to the evolution of the pathogen.
Changes in grass taxonomy
The development of the genus Xanthomonas has gone through many taxonomic and phylogenetic studies. Bacterium vesicatorium was first described in 1921 as a pathogen of peppers and tomatoes. Later, Dowson reclassified it as Xanthomonas campestris and formally proposed the genus Xanthomonas. With the development of genomics, Xanthomonas has been studied more deeply, and the taxonomic map of these species has become clearer based on the results of DNA:DNA hybridization.
Xanthomonas spp. is evolutionarily related to the opportunistic human pathogen Stenotrophomonas maltophilia, and this strain was previously known as Xanthomonas maltophilia.
Among the phylogenetic relationships among these species, new research suggests that banana and maize/maize pathotypes should be reorganized based on recent phylogenetic data, further clarifying the evolution and classification of Xanthomonas.
Morphology and growth characteristics
Xanthomonas bacteria are straight rod-shaped cells with a size of 0.4 to 1.0 microns wide and 1.2 to 3.0 microns long, and have the ability to move with monopolar flagella. In terms of growth characteristics, it often forms sticky, raised and yellow colonies. This yellow color is derived from a pigment called xanthomonadin. Different Xanthomonas species show adaptation to ambient temperature, with an optimal growth temperature of 25-30°C and the ability to grow between 4 and 37°C.
Xanthomonas plant pathogen
Xanthomonas species cause bacterial spot and wilt diseases on leaves, stems and fruits of many plants. Particular pathogenic species exhibit a high degree of specificity, and some species are divided into multiple pathogenic variants. Take Xanthomonas citri subsp. citri as an example. This bacterium is an important disease cause of citrus plants and causes considerable losses to commercial crops. Meanwhile, bacterial blight of rice caused by Xanthomonas or yzae pv. oryzae is particularly devastating in major rice-producing areas of Asia.
The pathogenic process begins with the bacteria attaching to the surface of the plant host, subsequently entering the host tissue and colonizing wounds or natural openings, eventually reappearing on the surface and spreading.
The focus of controlling these diseases is to limit the introduction of pathogens. Many plant species have developed resistance to the disease, which may be the most economical control measure.
Pathological mechanisms and prevention and treatment strategies
Xanthomonas attach via surface polysaccharides, attachment proteins, and type IV flagella, and can form biofilms to resist environmental stress. The xanthomonadins produced by these bacteria are effective against UV radiation. Pathogenicity usually involves the type III secretion system (T3SS), which can inject up to 30 effector proteins that interfere with the plant's immune system. To prevent infection, in addition to cultivating disease-resistant varieties, appropriate chemical control measures should be selected.
Industrial applications
Xanthomonas species also produce an edible polysaccharide called xanthan gum, which is widely used in food and other industrial fields. This product also plays a major role as a biofilm matrix in the Xanthomonas disease cycle.
Exploration of Xanthomonas resources
Isolates of most Xanthomonas species are available from the National Collection of Plant Pathogenic Bacteria in the UK, in addition to many international cultural collections such as the ICMP in New Zealand and the CFBP in France. The genomes of multiple Xanthomonas have been sequenced and data support is provided in various data sets.
With the in-depth study of Xanthomonas species, our understanding of this type of plant pathogens has gradually become clearer. By reviewing the evolution of this pathogen, we cannot help but wonder, can future plant protection strategies be more effective in combating these pathogens that threaten plant growth?
