Abstract
Xanthomonas citri pv. glycines is a major pathogen of soybean in Korea. Here, we analyzed pathogenicity genes based on a comparative genome analysis of five Korean strains and one strain from the United States, 8ra. Whereas all six strains had nearly identical profiles of carbohydrate-active enzymes, they varied in diversity and number of candidate type III secretion system effector (T3SE) genes. The five Korean strains were similar in their effectors, but differed from the 8ra strain. Across the six strains, transcription activator-like effectors (TALEs) showed diverse repeat sizes and at least six forms of the repeat variable di-residue (RVD) sequences, with differences not correlated with the origin of the strains. However, a phylogenetic tree based on the alignment of RVD sequences showed two distinct clusters with 17.5 repeats, suggesting that two distinct 17.5 RVD clusters have evolved, potentially to adapt Xcg to growth on distinct soybean cultivars. The predicted effector binding elements of the TALEs fell into six groups and were strongly overlapping in sequence, suggesting evolving target specificity of the binding domains in soybean cultivars. Our findings reveal the variability and adaptability of T3SEs in the Xcg strains and enhance our understanding of Xcg pathogenicity in soybean.
Highlights
Xanthomonas citri pv. glycines (Xcg), previously classified as X. axonopodis pv. glycine [1], causes bacterial pustules in soybean and is one of the most prevalent soybean (Glycine max) pathogens in the Republic of Korea [2]
We found that the strains had similar proportions of identified, 58–59% were glycoside hydrolases (GHs), 27.5% were glycosyltransferases (GTs), 2% were polysaccharide lyases (PLs), 8% were carbohydrate esterases (CEs), and 2% were carbohydrate-binding modules (CBMs)
Genome-wide studies of pathogenicity effectors and analysis of the pathogenic variability of representative strains in Korea should allow us to understand the extent of variation among distinct races of
Summary
Xanthomonas citri pv. glycines (Xcg), previously classified as X. axonopodis pv. glycine [1], causes bacterial pustules in soybean and is one of the most prevalent soybean (Glycine max) pathogens in the Republic of Korea [2]. Glycines (Xcg), previously classified as X. axonopodis pv. Glycine [1], causes bacterial pustules in soybean and is one of the most prevalent soybean (Glycine max) pathogens in the Republic of Korea [2]. Xcg invades and multiplies extracellularly within the apoplast, causing localized leaf spots or leaf streaks [3]. In the Republic of Korea, a report estimated that 89.7% of soybean fields were infected by Xcg in 2010 [2]. The recessive disease resistance locus rxp (resistance to xanthomonas phaseoli) confers partial resistance to Xcg [5] and has been introduced into many commercial soybean cultivars. The Xcg avirulence gene that triggers rxp-mediated resistance is unknown
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