4 research outputs found
Common bacterial blight of bean: a model of seed transmission and pathological convergence
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 g enus (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_Actio ns/ca/CA16107
Common bean resistance to Xanthomonas is associated with upregulation of the salicylic acid pathway and downregulation of photosynthesis
International audienceackground Common bacterial blight (CBB) caused byXanthomonas phaseolipv.phaseoliandXanthomonas citripv.fuscansis one of the major threats to common bean crops (Phaseolus vulgarisL.). Resistance to CBB is particularly complex as 26 quantitative resistance loci to CBB have been described so far. To date, transcriptomic studies after CBB infection have been very scarce and the molecular mechanisms underlying susceptibility or resistance are largely unknown. Results We sequenced and annotated the genomes of two common bean genotypes being either resistant (BAT93) or susceptible (JaloEEP558) to CBB. Reciprocal BLASTp analysis led to a list of 20,787 homologs between these genotypes and the common bean reference genome (G19833), which provides a solid dataset for further comparative analyses. RNA-Seq after inoculation withX. phaseolipv.phaseolishowed that the susceptible genotype initiated a more intense and diverse biological response than the resistant genotype. Resistance was linked to upregulation of the salicylic acid pathway and downregulation of photosynthesis and sugar metabolism, while susceptibility was linked to downregulation of resistance genes and upregulation of the ethylene pathway and of genes involved in cell wall modification. Conclusions This study helps better understanding the mechanisms occurring during the early colonization phase of common bean byXanthomonasand unveils new actors potentially important for resistance and susceptibility to CBB. We discuss the potential link between the pathways induced during bean colonization and genes induced by transcription activator-like effectors (TALEs), as illustrated in otherXanthomonaspathovars
Horizontal gene transfer plays a major role in the pathological convergence of Xanthomonas lineages on common bean
International audienceBackground: Host specialization is a hallmark of numerous plant pathogens including bacteria, fungi, oomycetes and viruses. Yet, the molecular and evolutionary bases of host specificity are poorly understood. In some cases, pathological convergence is observed for individuals belonging to distant phylogenetic clades. This is the case for Xanthomonas strains responsible for common bacterial blight of bean, spread across four genetic lineages. All the strains from these four lineages converged for pathogenicity on common bean, implying possible gene convergences and/or sharing of a common arsenal of genes conferring the ability to infect common bean. Results: To search for genes involved in common bean specificity, we used a combination of whole-genome analyses without a priori, including a genome scan based on k-mer search. Analysis of 72 genomes from a collection of Xanthomonas pathovars unveiled 115 genes bearing DNA sequences specific to strains responsible for common bacterial blight, including 20 genes located on a plasmid. Of these 115 genes, 88 were involved in successive events of horizontal gene transfers among the four genetic lineages, and 44 contained nonsynonymous polymorphisms unique to the causal agents of common bacterial blight. Conclusions: Our study revealed that host specificity of common bacterial blight agents is associated with a combination of horizontal transfers of genes, and highlights the role of plasmids in these horizontal transfers