UC Berkeley

Bacterial spot of tomato (Solanum lycopersicum) is a devastating disease that severely limits yields in important tomato-­growing regions, including the southeastern United States, where the predominant bacterial spot pathogen species is Xanthomonas perforans. Attempts to control the disease with antibiotics and copper-­based pesticides have led to the selection of bacterial strains that are resistant to these treatments. Therefore, we turn to genetic sources of resistance as a sustainable path to reduce crop losses to bacterial spot disease.

This work describes the fine mapping and identification of the RXopJ4 disease resistance locus from the wild tomato relative Solanum pennellii LA716. RXopJ4 resistance depends on recognition of the X. perforans type III effector protein XopJ4. We developed a collection of fourteen molecular markers to map on a segregating F2 population from a cross between the susceptible parent S. lycopersicum FL8000 and the resistant parent RXopJ4 8000 OC7. In the F2 population, a 190-­kb segment on chromosome 6 cosegregated with resistance. This fine mapping enabled the identification of three RXopJ4 candidate genes, all of which encode putative intracellular serine-­threonine protein kinases. Transient coexpression of the XopJ4 effector with each kinase revealed a promising RXopJ4 candidate gene that triggered a hypersensitive response (HR) in Nicotiana benthamiana. Mutations in both XopJ4 and RXopJ4 identified conserved residues required for recognition and the induction of a hypersensitive cell death phenotype. Homozygous transgenic tomato plants containing the RXopJ4 candidate gene have been constructed and will soon be evaluated for disease resistance.

In addition, we undertook a genomic survey of fourteen X. perforans field isolates from all five fresh market tomato production zones of Florida, revealing a preliminary set of core type III effectors common to all isolates. We used this set of core effectors to inform a search for new sources of resistance to bacterial spot disease and identified accessions of Nicotiana and Solanum americanum that recognize core X. perforans effectors. Finally, we performed disease assays on 224 wild tomato accessions and found nine potential sources of bacterial spot disease resistance.