The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv.vitians in three lettuce cultivars

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 resulting in resistance.Little was known about the inheritance of resistance;however,resistance to other lettuce pathogens is often determined by resistance gene candidates(RGCs)encoding nucleotide-binding leucine-rich repeat(NB-LRR)proteins.Therefore,we determined the inheritance of BLS resistance in the cultivars La Brillante,Little Gem and Pavane and mapped it relative to RGCs.The reaction to Xcv was analyzed in nine F_(1),F_(2) and recombinant inbred line populations of lettuce from HR3compatible or HR3HR crosses.The HR in La Brillante,Pavane and Little Gem is conditioned by single dominant genes,which are either allelic or closely linked genes.The resistance 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 loci in the RGC2 family.The Xar1 gene confers a high level of BLS resistance in the greenhouse and field that can be introgressed into commercial lettuce cultivars to reduce BLS losses using molecular markers.

Molecular markers reliably predict post-harvest deterioration of fresh-cut lettuce in modified atmosphere packaging

Fresh-cut lettuce is popular,but highly perishable product.Genetic studies of two bi-parental populations derived from crossing parents with rapid and slow rates of deterioration showed that the deterioration rate is a heritable trait(broad spectrum heritability,H2 of 0.56–0.87).The major genetic determinant of the deterioration rate in both populations was the quantitative trait locus(QTL),qSL4,located on linkage group 4.This QTL explained 40–74%of the total phenotypic variation of the trait in the two populations.Saturating the qSL4 region with single-nucleotide(SNP)markers allowed detection of six haplotypes in a set of 16 lettuce accessions with different rates of deterioration.Three of the haplotypes were always associated with very rapid rates of deterioration,while the other three haplotypes were associated with slow rates of deterioration.Two SNPs located 53 bp apart were sufficient to separate the 16 accessions into two groups with different rates of deterioration.The accuracy of markers-trait association was subsequently tested on 350 plants from seven F2 families that originated from crossing parents with different rates of deterioration.The H2 of deterioration rate in these seven families ranged from 0.64 to 0.90.The SNP-based analysis accurately identified individuals with rapid,intermediate,and slow rates of deterioration in each family.Intermediate rate of deterioration was found in individuals having heterozygous alleles at qSL4,indicating an additive effect of the alleles.The assay can be used for fast,accurate,and reliable identification of deterioration rate after processing for salad.