Genome-Wide Association Study of Resistance to Xanthomonas gardneri in the USDA Pepper (Capsicum) Collection.

Bacterial spot Xanthomonas species cause significant disease outbreaks on tomato and pepper in tropical and subtropical regions throughout the world. Host resistance has been one of the key components of integrated disease management approaches to mitigate plant pathogens. Although a number of resistance genes have been identified in pepper against bacterial spot xanthomonads, emergence of bacterial spot pathogen variants capable of overcoming these sources and changing pathogen distributions reinforce the importance of identifying novel candidates to incorporate into breeding programs. We conducted a genome-wide association study (GWAS) on a diverse U.S. Department of Agriculture collection of pepper germplasm including different species of Capsicum to identify novel sources of disease resistance against a highly virulent X. gardneri strain isolated from a recent outbreak. GWAS identified highly significant single nucleotide polymorphisms associated with defoliation in response to infection with X. gardneri. Functionally relevant candidate genes encoded products involved in disease resistance/susceptibility, hormone signaling, and basal resistance against multiple pathogens in various host-pathogen systems. The X. gardneri-resistant genotypes and quantitative trait loci identified in this study provide alleles that could be used for a resistance gene pyramiding effort against different species of bacterial spot xanthomonads in pepper.

Susceptible to intolerance--a range of hormonal actions in a susceptible Arabidopsis pathogen response.

Ethylene and salicylic acid (SA) are key intermediates in a host's response to pathogens. Previously, we have shown using a tomato compatible interaction that ethylene and SA act sequentially and are essential for disease symptom production. Here, we have examined the relationship between the two signals in the Arabidopsis-Xanthomonas campestris pv. campestris (Xcc) compatible interaction. Preventing SA accumulation by expression of the nahG gene reduced subsequent ethylene production and altered the development of disease symptoms, with plants showing no visible chlorosis. The ethylene insensitive lines, etr1-1 and etr2-1, on the other hand, accumulated SA and exhibited normal but precocious symptom development. Therefore, Arabidopsis, like tomato, was found to exhibit co-operative ethylene and SA action for the production of disease symptoms. However, in Arabidopsis, SA was found to act upstream of ethylene. Jasmonic acid and indole-3-acetic acid levels were also found to increase in response to Xcc. In contrast to ethylene, accumulation of these hormones was not found to be dependent on SA action. These results indicate that the plants response to a virulent pathogen is a composite of multiple signaling pathways.