Abstract
Bacterial leaf spot of lettuce, caused by Xanthomonas hortorum pv. vitians, is an economically important disease worldwide. For instance, it caused around 4 million CAD in losses in only a few months during the winter of 1992 in Florida. Because only one pesticide is registered to control this disease in Canada, the development of lettuce cultivars tolerant to bacterial leaf spot remains the most promising approach to reduce the incidence and severity of the disease in lettuce fields. The lack of information about the genetic diversity of the pathogen, however, impairs breeding programs, especially when disease resistance is tested on newly developed lettuce germplasm lines. To evaluate the diversity of X. hortorum pv. vitians, a multilocus sequence analysis was performed on 694 isolates collected in Eastern Canada through the summers of 2014 to 2017 and two isolates in 1996 and 2007. All isolates tested were clustered into five phylogroups. Six pathotypes were identified following pathogenicity tests conducted in greenhouses, but when phylogroups were compared with pathotypes, no correlation could be drawn. However, in vitro production of xanthan and xanthomonadins was investigated, and isolates with higher production of xanthomonadins were generally causing less severe symptoms on the tolerant cultivar Little Gem. Whole-genome sequencing was undertaken for 95 isolates belonging to the pathotypes identified, and de novo assembly made with reads unmapped to the reference strain’s genome sequence resulted in 694 contigs ranging from 128 to 120,795 bp. Variant calling was performed prior to genome-wide association studies computed with single-nucleotide polymorphisms (SNPs), copy-number variants and gaps. Polymorphisms with significant p-values were only found on the cultivar Little Gem. Our results allowed molecular identification of isolates likely to cause bacterial leaf spot of lettuce, using two SNPs identified through genome-wide association study.
Highlights
It has been previously demonstrated [46] that resistance to bacterial leaf spot in lettuce cultivars is conferred by the resistance gene Xar1
Observed moderate to high resistance to bacterial leaf spot when the cultivar Little Gem was artificially inoculated with X. hortorum pv. vitians isolates collected in California and Florida, respectively
The results in this study indicate that there are currently at least five pathotypes of Xanthomonas hortorum pv. vitians in lettuce fields in the province of Quebec, Canada, based on pathogenicity tests conducted in the greenhouse on lettuce cultivars Little Gem, Paris
Summary
Bacteria often use a cell-communication system, known as quorum sensing, to coordinate colonization of hosts [3] and inject effector proteins in plant cells using diverse secretion systems [4] (for a more complete review, see Giraldo and Valent [5]). Members of the bacterial genus Xanthomonas, which possess type I to VI secretion systems [6], are divided into 27 species known to cause disease on many mono and dicotyledonous plants. Those species are further subdivided into pathovars to differentiate strains according to their pathogenicity on different host species. Pathovars can be further divided into pathotypes to differentiate strains within pathovars according to their pathogenicity on varieties of the same host species
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