Abstract
BackgroundThe species Xanthomonas translucens encompasses a complex of bacterial strains that cause diseases and yield loss on grass species including important cereal crops. Three pathovars, X. translucens pv. undulosa, X. translucens pv. translucens and X. translucens pv.cerealis, have been described as pathogens of wheat, barley, and oats. However, no complete genome sequence for a strain of this complex is currently available.ResultsA complete genome sequence of X. translucens pv. undulosa strain XT4699 was obtained by using PacBio long read, single molecule, real time (SMRT) DNA sequences and Illumina sequences. Draft genome sequences of nineteen additional X. translucens strains, which were collected from wheat or barley in different regions and at different times, were generated by Illumina sequencing. Phylogenetic relationships among different Xanthomonas strains indicates that X. translucens are members of a distinct clade from so-called group 2 xanthomonads and three pathovars of this species, undulosa, translucens and cerealis, represent distinct subclades in the group 1 clade. Knockout mutation of type III secretion system of XT4699 eliminated the ability to cause water-soaking symptoms on wheat and barley and resulted in a reduction in populations on wheat in comparison to the wild type strain. Sequence comparison of X. translucens strains revealed the genetic variation on type III effector repertories among different pathovars or within one pathovar. The full genome sequence of XT4699 reveals the presence of eight members of the Transcription-Activator Like (TAL) effector genes, which are phylogenetically distant from previous known TAL effector genes of group 2 xanthomonads. Microarray and qRT-PCR analyses revealed TAL effector-specific wheat gene expression modulation.ConclusionsPacBio long read sequencing facilitates the assembly of Xanthomonas genomes and the multiple TAL effector genes, which are difficult to assemble from short read platforms. The complete genome sequence of X. translucens pv. undulosa strain XT4699 and draft genome sequences of nineteen additional X. translucens strains provides a resource for further genetic analyses of pathogenic diversity and host range of the X. translucens species complex. TAL effectors of XT4699 strain play roles in modulating wheat host gene expressions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2348-9) contains supplementary material, which is available to authorized users.
Highlights
The species Xanthomonas translucens encompasses a complex of bacterial strains that cause diseases and yield loss on grass species including important cereal crops
Bacterial leaf streak (BLS) and black chaff symptoms in the grain spikes on wheat are caused by X. translucens pv. undulosa strains
The reads were assembled into two long contigs using an optimized assembly pipeline, HGAP2 [12]
Summary
The species Xanthomonas translucens encompasses a complex of bacterial strains that cause diseases and yield loss on grass species including important cereal crops. Bacterial pathogens of the genus Xanthomonas cause disease symptoms in a wide range of plant species, including many economically important cereal crops [1]. The species X. translucens represents a complex of strains that are pathogenic on various members of the Poaceae, including wheat, barley, oat, rye and other grass species. X. translucens strains have been classified by pathogenicity types and DNA fingerprinting technologies [3]. Strains causing disease symptoms on barley and wheat are named as X. translucens pv. Undulosa, while strains only pathogenic on barley are called X. translucens pv. Many strains that were isolated from other species, often have been reported to cause disease symptoms on wheat [3]. Thirtythree bacterial strains isolated from diseased ornamental asparagus were identified as X. translucens pv. undulosa using DNA fingerprinting and cross inoculation [5]
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