Background
Xanthomonads are plant-associated bacteria responsible for diseases on economically important
crops. Xanthomonas fuscans subsp. fuscans (Xff) is one of the causal agents of
common bacterial blight of bean. In this study, the complete genome sequence of strain Xff
4834-R was determined and compared to other Xanthomonas genome sequences.
Results
Comparative genomics analyses revealed core characteristics shared between Xff 4834-R and
other xanthomonads including chemotaxis elements, two-component systems, TonB-dependent
transporters, secretion systems (from T1SS to T6SS) and multiple effectors. For instance a
repertoire of 29 Type 3 Effectors (T3Es) with two Transcription Activator-Like Effectors was
predicted. Mobile elements were associated with major modifications in the genome structure and
gene content in comparison to other Xanthomonas genomes. Notably, a deletion of 33 kbp
affects flagellum biosynthesis in Xff 4834-R. The presence of a complete flagellar cluster
was assessed in a collection of more than 300 strains representing different species and pathovars
of Xanthomonas. Five percent of the tested strains presented a deletion in the flagellar
cluster and were non-motile. Moreover, half of the Xff strains isolated from the same
epidemic than 4834-R was non-motile and this ratio was conserved in the strains colonizing the
next bean seed generations.
Conclusions
This work describes the first genome of a Xanthomonas strain pathogenic on bean and reports
the existence of non-motile xanthomonads belonging to different species and pathovars. Isolation of
such Xff variants from a natural epidemic may suggest that flagellar motility is not a key
function for in planta fitness.
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