Location: Crop Improvement and Protection ResearchTitle: The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars
|TRENT, MARK - Former ARS Employee|
|TRUCO, MARIA JOSE - University Of California|
|ANTONISE, RUDIE - Keygene Nv|
|MICHELMORE, RICHARD - University Of California|
Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2014
Publication Date: 12/23/2014
Citation: Hayes, R.J., Trent, M., Truco, M., Antonise, R., Michelmore, R.W., Bull, C.T. 2014. The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars. Horticulture Research. 1:14066.
Interpretive Summary: Lettuce yields can be reduced by the disease bacterial leaf spot caused by the bacteria Xanthomonas campestris pv. vitians. Resistant cultivars are the most feasible method to reduce disease losses, although commercially useable cultivars with complete resistance do not exist. The cultivars La Brillante, Pavane, and Little Gem express an incompatible host-pathogen interaction known as a hypersensitive response (HR) to some California strains of Xcv. The HR restricts the bacteria’s ability to grow on the plant, resulting in resistance to the disease. These three cultivars are not suitable for commercial production in California and the resistance found in these cultivars needs to be bred into new improved cultivars. Determining the inheritance of the HR will facilitate breeding new resistant cultivars. Little was known about the genes controlling resistance to this bacteria; however, resistance to several other lettuce diseases are associated with or determined by nucleotide-binding leucine-rich repeat (NB-LRR) genes that are found in all plants. In this research we determined that the HR in La Brillante, Pavane, and Little Gem is conditioned by single dominant genes. These genes in each cultivar are either the same gene, or separate genes that are located close together on the same chromosome. The gene in La Brillante was named Xanthomonas resistance 1 (Xar1) and is located in a region of the lettuce genome on chromosome 2 containing numerous NB-LRR genes. The gene can be bred into cultivars adapted to California, which produces about 75% of the US lettuce supply, to reduce crop losses.
Technical Abstract: Lettuce yields can be reduced by the disease bacterial leaf spot (BLS) caused by the pathogen Xanthomonas campestris pv. vitians (Xcv) and host resistance is the most feasible method to reduce disease losses. The cultivars La Brillante, Pavane, and Little Gem express an incompatible host-pathogen interaction as a hypersensitive response (HR) to California strains of Xcv while most commercial cultivars express a compatible interaction resulting in susceptibility. Little was known about the inheritance of BLS resistance; however, resistance to several other lettuce pathogens has often been shown to be genetically linked to or determined by resistance gene candidates (RGCs) encoding nucleotide-binding leucine-rich repeat (NB-LRR) proteins. The objectives of this research were to determine the inheritance of BLS resistance in the cultivars La Brillante, Little Gem, and Pavane and to map BLS resistance relative to RGCs. Intact leaves of nine F1, F2, and recombinant inbred line (RIL) populations of lettuce from HR × compatible or HR × HR crosses were infiltrated with Xcv to determine the segregation of the HR and to map the HR. Salinas 88 × La Brillante RILs expressing either an HR or compatible reaction were evaluated for BLS disease severity in replicated greenhouse and field experiments. The HR in La Brillante, Pavane, and Little Gem is conditioned by single dominant genes, which are either allelic or closely linked genes. The gene in La Brillante was designated Xanthomonas resistance 1 (Xar1) and mapped to lettuce linkage group 2. Xar1 is present in a genomic region that contains numerous NB-LRR encoding RGCs and functional pathogen resistance genes in the RGC2 family. The Xar1 gene confers a high level of BLS resistance in the greenhouse and field that can be bred into commercial lettuce cultivars using phenotypic or molecular breeding methods.