Abstract
Bacterial leaf streak caused by Xanthomonas translucens pv. undulosa (Xtu) is an important disease of wheat (Triticum aestivum) and barley (Hordeum vulgare) worldwide. Transcription activator-like effectors (TALEs) play determinative roles in many of the plant diseases caused by the different species and pathovars of Xanthomonas, but their role in this disease has not been characterized. ICMP11055 is a highly virulent Xtu strain from Iran. The aim of this study was to better understand genetic diversity of Xtu and to assess the role of TALEs in bacterial leaf streak of wheat by comparing the genome of this strain to the recently completely sequenced genome of a U.S. Xtu strain, and to several other draft X. translucens genomes, and by carrying out mutational analyses of the TALE (tal) genes the Iranian strain might harbor. The ICMP11055 genome, including its repeat-rich tal genes, was completely sequenced using single molecule, real-time technology (Pacific Biosciences). It consists of a single circular chromosome of 4,561,583 bp, containing 3,953 genes. Whole genome alignment with the genome of the United States Xtu strain XT4699 showed two major re-arrangements, nine genomic regions unique to ICMP11055, and one region unique to XT4699. ICMP110055 harbors 26 non-TALE type III effector genes and seven tal genes, compared to 25 and eight for XT4699. The tal genes occur singly or in pairs across five scattered loci. Four are identical to tal genes in XT4699. In addition to common repeat-variable diresidues (RVDs), the tal genes of ICMP11055, like those of XT4699, encode several RVDs rarely observed in Xanthomonas, including KG, NF, Y∗, YD, and YK. Insertion and deletion mutagenesis of ICMP11055 tal genes followed by genetic complementation analysis in wheat cv. Chinese Spring revealed that Tal2 and Tal4b of ICMP11055 each contribute individually to the extent of disease caused by this strain. A largely conserved ortholog of tal2 is present in XT4699, but for tal4b, only a gene with partial, fragmented RVD sequence similarity can be found. Our results lay the foundation for identification of important host genes activated by Xtu TALEs as targets for the development of disease resistant varieties.
Highlights
Xanthomonas is a genus of plant associated and plant pathogenic, Gram-negative bacteria
Genome-wide SNP comparison of ICMP11055 with the XT4699 genome and several other available X. translucens genome sequences (Figure 2) showed that, with three exceptions, strains accessioned as X. translucens pv. cerealis (Xtc), Xtt, and Xtu group in distinct clades, and that within the Xtu clade, ICMP11055 resides on a branch separate from the other Xtu strains
The genome of Xtu ICMP11055, a highly pathogenic strain isolated in Iran, was sequenced using SMRT technology and compared with the complete genome of another strain of Xtu, XT4699, isolated from United States, as well as draft or complete genomes of several other X. translucens strains
Summary
Xanthomonas is a genus of plant associated and plant pathogenic, Gram-negative bacteria. Among the molecular determinants that contribute to the complex process of pathogenesis and to host adaptation, type III secreted effector proteins play a key role (Büttner and Bonas, 2010; Jacques et al, 2016) Many such proteins have been identified in plant pathogenic species and pathovars (pathogenic variants of a species that have distinctive host or tissue specificity) of Xanthomonas. In the plant cell nucleus, TALEs individually activate specific corresponding host genes by binding to effectorspecific effector binding elements (EBEs) in the promoters Their DNA binding specificity is conferred by a central repeat region (CRR), which is flanked by an N-terminal domain that contains a type III secretion signal, and a C-terminal domain that harbors nuclear-localization signals and an acidic activation domain. RipTALs display some differences from TALEs; they harbor RVDs not found or found only rarely so far in TALEs, RipTAL repeats are 35 aa in length and exhibit greater overall polymorphism outside the RVD than TALEs, and RipTALs bind best to EBEs immediately preceded (5 ) by a guanine rather than the thymine commonly found upstream of a TALE EBE (Doyle et al, 2013; Lange et al, 2013)
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