Genome, transmission, and quantification studies of Xanthomonas translucens, the pathogen causing bacterial leaf streak in small grains

Abstract

Bacterial leaf streak (BLS) of wheat and barley is an economically important disease in most small grain growing regions worldwide and is primarily caused by Xanthomonas translucens pv. undulosa and X. translucens pv. translucens, respectively. BLS is generally described as a seedborne disease, however, the pathogen may overwinter in a range of host plants. X. translucens pv. undulosa has a broad host range and infects several cultivated small grains along with perennial and annual weedy grasses. The advancement of high-throughput sequencing has facilitated a growing database of whole genome sequences of X. translucens strains. In this study we sequenced and assembled the complete genomes of five X. translucens pv. undulosa strains originating from wheat, cultivated wild rice, and three weedy grasses. This study details the first X. translucens pv. undulosa genomes isolated from non-wheat hosts. We compared these genomes to publicly available strains and found all the X. translucens pv. undulosa were closely related, sharing ≥99.25% average nucleotide identity, but non-clonal. Genome structures and the presence of Type III secreted effectors varied among strains, but no unique features were identified to differentiate those originating from wheat and non-wheat hosts. Despite being considered a seedborne disease, the records of seed transmission are circumstantial and/or inconsistent. In this study, wheat and barley seed were inoculated with a rifampicin-resistant strain of X. translucens pv. undulosa and X. translucens pv. translucens, respectively, with the aim of tracking the movement of the pathogens from the host’s seed into developing plant tissues. Rifampicin-resistant strains were recovered from both wheat and barley at multiple growth stages through maturity. Under mist-irrigated field conditions, pathogen recovery occurred from all sampled tissue from the third leaf to spikes at soft dough, in both hosts. Recovery was reduced under dryland conditions, especially from wheat. Quantitative PCR can be used to detect and quantify X. translucens from wheat and barley seed but does not distinguish living cells from dead cells. We tested a protocol for conducting a viability PCR assay, which quantifies viable X. translucens cells, and then used the assay to examine pathogen populations on different wheat seed lots. We found that the viable populations of X. translucens cells on wheat seed decreased over one year, although the final concentration of viable cells exceeded 1  103 CFU/g, the reported threshold needed to cause BLS. The genome, transmission, and quantification studies of X. translucens contribute to our understanding of pathogen diversity and BLS epidemiology and may inform the development of management strategies.