Draft Genome Sequence of the Flagellated Xanthomonas fuscans subsp. fuscans Strain CFBP 4884

Common bacterial blight of bean: a model of seed transmission and pathological convergence

BACKGROUND

Xanthomonas citri pv. fuscans (Xcf) and Xanthomonas phaseoli pv. phaseoli (Xpp) are the causal agents of common bacterial blight of bean (CBB), an important disease worldwide that remains difficult to control. These pathogens belong to distinct species within the Xanthomonas genus and have undergone a dynamic evolutionary history including the horizontal transfer of genes encoding factors probably involved in adaptation to and pathogenicity on common bean. Seed transmission is a key point of the CBB disease cycle, favouring both vertical transmission of the pathogen and worldwide distribution of the disease through global seed trade.

TAXONOMY

Kingdom: Bacteria; phylum: Proteobacteria; class: Gammaproteobacteria; order: Lysobacterales (also known as Xanthomonadales); family: Lysobacteraceae (also known as Xanthomonadaceae); genus: Xanthomonas; species: X. citri pv. fuscans and X. phaseoli pv. phaseoli (Xcf-Xpp).

HOST RANGE

The main host of Xcf-Xpp is the common bean (Phaseolus vulgaris). Lima bean (Phaseolus lunatus) and members of the Vigna genus (Vigna aconitifolia, Vigna angularis, Vigna mungo, Vigna radiata, and Vigna umbellata) are also natural hosts of Xcf-Xpp. Natural occurrence of Xcf-Xpp has been reported for a handful of other legumes such as Calopogonium sp., Pueraria sp., pea (Pisum sativum), Lablab purpureus, Macroptilium lathyroides, and Strophostyles helvola. There are conflicting reports concerning the natural occurrence of CBB agents on tepary bean (Phaseolus acutifolius) and cowpea (Vigna unguiculata subsp. unguiculata).

SYMPTOMS

CBB symptoms occur on all aerial parts of beans, that is, seedlings, leaves, stems, pods, and seeds. Symptoms initially appear as water-soaked spots evolving into necrosis on leaves, pustules on pods, and cankers on twigs. In severe infections, defoliation and wilting may occur.

DISTRIBUTION

CBB is distributed worldwide, meaning that it is frequently encountered in most places where bean is cultivated in the Americas, Asia, Africa, and Oceania, except for arid tropical areas. Xcf-Xpp are regulated nonquarantine pathogens in Europe and are listed in the A2 list by the European and Mediterranean Plant Protection Organization (EPPO).

GENOME

The genome consists of a single circular chromosome plus one to four extrachromosomal plasmids of various sizes, for a total mean size of 5.27 Mb with 64.7% GC content and an average predicted number of 4,181 coding sequences.

DISEASE CONTROL

Management of CBB is based on integrated approaches that comprise measures aimed at avoiding Xcf-Xpp introduction through infected seeds, cultural practices to limit Xcf-Xpp survival between host crops, whenever possible the use of tolerant or resistant bean genotypes, and chemical treatments, mainly restricted to copper compounds. The use of pathogen-free seeds is essential in an effective management strategy and requires appropriate sampling, detection, and identification methods. USEFUL WEBSITES: https://gd.eppo.int/taxon/XANTPH, https://gd.eppo.int/taxon/XANTFF, and http://www.cost.eu/COST_Actions/ca/CA16107.

Genome sequence of Xanthomonas fuscans subsp. fuscans strain Xff49: a new isolate obtained from common beans in Southern Brazil

The genus Xanthomonas comprises Gram-negative bacteria, many of which are phytopathogens. Xanthomonas fuscans subsp. fuscans is one of the most devastating pathogens affecting the bean plant, resulting in the common bacterial blight of bean (CBB). The disease is mainly foliar and affects a wide variety of bean species, thus acting as the yield-limiting factor for the bean crop. Here, we report the whole-genome sequencing of a new strain of X. fuscans subsp. fuscans, named Xff49, isolated from the infected and symptomatic beans from Capão do Leão, Southern Brazil. The genetic analysis demonstrated the presence of single-nucleotide variants (SNVs) in this strain, potentially affecting the mobilome, cell mobility, and inorganic ion metabolism. In addition, the analysis resulted in the identification of a new plasmid similar to the pAX22 derived from Achromobacter denitrificans, which was named plX, along with plA and plC, previously reported in other strains of X. fuscans subsp. fuscans. Xff49 represents the first Brazilian genome of X. fuscans subsp. fuscans and might provide useful information applicable to the studies of phylogenetics, evolution, and pathogenomics, thereby allowing a better understanding of the genomic features present in the Brazilian strains.

Xanthomonas adaptation to common bean is associated with horizontal transfers of genes encoding TAL effectors

Background

Common bacterial blight is a devastating bacterial disease of common bean (Phaseolus vulgaris) caused by Xanthomonas citri pv. fuscans and Xanthomonas phaseoli pv. phaseoli. These phylogenetically distant strains are able to cause similar symptoms on common bean, suggesting that they have acquired common genetic determinants of adaptation to common bean. Transcription Activator-Like (TAL) effectors are bacterial type III effectors that are able to induce the expression of host genes to promote infection or resistance. Their capacity to bind to a specific host DNA sequence suggests that they are potential candidates for host adaption.

Results

To study the diversity of tal genes from Xanthomonas strains responsible for common bacterial blight of bean, whole genome sequences of 17 strains representing the diversity of X. citri pv. fuscans and X. phaseoli pv. phaseoli were obtained by single molecule real time sequencing. Analysis of these genomes revealed the existence of four tal genes named tal23A, tal20F, tal18G and tal18H, respectively. While tal20F and tal18G were chromosomic, tal23A and tal18H were carried on plasmids and shared between phylogenetically distant strains, therefore suggesting recent horizontal transfers of these genes between X. citri pv. fuscans and X. phaseoli pv. phaseoli strains. Strikingly, tal23A was present in all strains studied, suggesting that it played an important role in adaptation to common bean. In silico predictions of TAL effectors targets in the common bean genome suggested that TAL effectors shared by X. citri pv. fuscans and X. phaseoli pv. phaseoli strains target the promoters of genes of similar functions. This could be a trace of convergent evolution among TAL effectors from different phylogenetic groups, and comforts the hypothesis that TAL effectors have been implied in the adaptation to common bean.

Conclusions

Altogether, our results favour a model where plasmidic TAL effectors are able to contribute to host adaptation by being horizontally transferred between distant lineages.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4087-6) contains supplementary material, which is available to authorized users.

Genome sequencing reveals a new lineage associated with lablab bean and genetic exchange between Xanthomonas axonopodis pv. phaseoli and Xanthomonas fuscans subsp. fuscans

Common bacterial blight is a devastating seed-borne disease of common beans that also occurs on other legume species including lablab and Lima beans. We sequenced and analyzed the genomes of 26 strains of Xanthomonas axonopodis pv. phaseoli and X. fuscans subsp. fuscans, the causative agents of this disease, collected over four decades and six continents. This revealed considerable genetic variation within both taxa, encompassing both single-nucleotide variants and differences in gene content, that could be exploited for tracking pathogen spread. The bacterial strain from Lima bean fell within the previously described Genetic Lineage 1, along with the pathovar type strain (NCPPB 3035). The strains from lablab represent a new, previously unknown genetic lineage closely related to strains of X. axonopodis pv. glycines. Finally, we identified more than 100 genes that appear to have been recently acquired by Xanthomonas axonopodis pv. phaseoli from X. fuscans subsp. fuscans.