J.W. Scott

Ty-3, a begomovirus resistance locus near the Tomato yellow leaf curl virus resistance locus Ty-1 on chromosome 6 of tomato

Resistance to begomoviruses including bipartite Tomato mottle virus (ToMoV) and monopartite Tomato yellow leaf curl virus (TYLCV) has been introgressed to cultivated tomato (Solanum lycopersicum) from S. chilense accessions LA1932, LA2779, and LA1938. Previous research demonstrated that three regions on chromosome 6 were associated with the resistance, two of which were required for a line to provide a high level of resistance. In the present study, we identified a large S. chilense introgression spanning markers from C2_At2g39690 to T0834 in LA2779-derived advanced breeding lines resistant to both TYLCV and ToMoV. A begomovirus resistance locus, Ty-3, was mapped to the marker interval between cLEG-31-P16 and T1079 on the long arm of chromosome 6. In addition to the Ty-3 locus, the large introgression also spans the Ty-1 region near the Mi gene, suggesting the possible coexistence and linkage of resistance alleles at both Ty-1 and Ty-3 loci in these lines. In contrast, LA1932-derived advanced breeding lines possess a much shorter introgression from cLEG-31-P16 to C2_At5g41480, which also carries a begomovirus resistance locus that is probably allelic at the Ty-3 locus. The PCR-based markers tightly linked to the Ty-3 locus as well as the markers near the Ty-1 region have been used in our breeding program for efficient selection of begomovirus resistance in the past three growing seasons and will be useful resources for tomato breeders around the world.

Evidence for the preemptive nature of tomato race 3 of Xanthomonas campestris pv. vesicatoria in Florida

ABSTRACT Until recently, tomato race 1 (T1) of Xanthomonas campestris pv. vesicatoria was the only race causing bacterial spot of tomato in Florida. In 1991, tomato race 3 (T3) was first identified in 3 of 13 tomato production fields surveyed. By 1994, T3 was observed in 21 of 28 fields and was the only race identified in 14 fields. In field studies, tomato genotypes with resistance to either T1 or T3 or susceptibility to both were co-inoculated with strains of both races. Lesions on 10 plants in each of three replications for each genotype were sampled three times during the experiment; bacterial isolations were made from each lesion, and tomato race identifications were made for each strain. At the third sampling date, T3 was isolated from 97% of the lesions on the susceptible genotype Walter and the T1-resistant genotype Hawaii 7998, while T3 was isolated from 23% of the lesions and T1 from the remaining 77% on the T3-resistant genotypes PI 128216 and PI 126932. In surface population studies done in growth rooms, suspensions of T1 and T3 were applied alone and in combination to the leaf surfaces of susceptible and resistant genotypes. T1 populations were reduced more than 10-fold when applied in combination with T3, compared with populations that developed when T1 was applied alone. T3 populations were not affected when applied in combination with a T1 strain. In greenhouse studies with the T3-resistant genotype Hawaii 7981, disease was significantly reduced in plants inoculated with T3 in combination with T1, compared with plants inoculated with T1 alone. These results clearly demonstrate the competitive nature of T3 in the presence of T1 and help explain the emergence of T3 as a prevalent race in Florida.

Monogenic resistance in tomato to Fusarium oxysporum f. sp. lycopersici race 3

SummaryResistance to fusarium wilt, incited by Fusarium oxysporum (Schlecht.) f. sp. lycopersici (Sacc.) Snyder & Hansen race 3 in tomato (Lycopersicon esculentum Mill.) was discovered in LA 716, a L. pennellii accession. A resistant BC1F3 breeding line, E427, was developed from LA 716. E427 was crossed with the susceptible cv. Suncoast and F1, BCP1, BCP2 (to Fla 7155, a susceptible parent) F2, F3, and BCP2S1 seeds were obtained. Segregation for resistance following root dip inoculation over three experiments indicated a single dominant gene controlled resistance. Five of the 12 BCP1S1s segregated more susceptible plants, whereas one of the 12 segregated more resistant plants than expected (P<0.05). Three of 23 F3 lines segregated more susceptible plants than expected while 1 of the 23 had more resistant plants than expected (P<0.05). Segregation in all other lines fit expected ratios. Five of the 23 F3s were homozygous resistant which was an acceptable fit to expectations (P=0.1−0.5). The gene symbol I3 is proposed for resistance to race 3 of the wilt pathogen. Deviations from expected ratios in data reported here and for other breeding lines indicate an effect of modifier genes and/or incomplete penetrance. Plant age at inoculation and seed dormancy did not affect results.

Identification of QTL associated with resistance to bacterial spot race T4 in tomato

Bacterial spot of tomato (Solanum lycopersicum L.), caused by several Xanthomonas sp., is a serious but difficult disease to control by chemical means. Development of resistance has been hindered by emergence of races virulent to tomato, by the quantitative inheritance of resistance, and by a low correlation between seedling assays and resistance in the field. Resistance to multiple races, including race T4, has been described in the S. lycopersicum var. cerasiformae accession PI 114490. We used molecular markers to identify associations with quantitative trait loci (QTL) in an elite inbred backcross (IBC) population derived from OH 9242, PI 114490 and Fla. 7600, a breeding line with tomato accession Hawaii 7998 (H7998) in its pedigree. Race T4 resistance has also been described in the advanced breeding lines Fla. 8233, Fla. 8517, and Fla. 8326, and a selective genotyping approach was used to identify introgressions associated with resistance in segregating progeny derived from crosses with these lines. In the IBC population, loci on chromosomes 11 and 3, respectively, explained as much as 29.4 and 4.8% of resistance variation. Both these loci were also confirmed by selective genotyping: PI 114490 and H7998 alleles on chromosome 11 each provided resistance. The PI 114490 allele on chromosome 3 was confirmed in the Fla. 8517 population, and an allele of undetermined descent was confirmed at this locus in the Fla. 8326 population. A chromosome 12 allele was associated with susceptibility in the Fla. 8517 population. Additional loci contributing minor effects were also implicated in the IBC population or by selective genotyping. Selection for the major QTL in a marker-directed phenotyping approach should significantly improve the efficiency of breeding for resistance to bacterial spot race T4, although as yet undetected QTL would be necessary to carry out strict marker assisted selection.

Molecular Mapping of Hypersensitive Resistance from Tomato 'Hawaii 7981' to Xanthomonas perforans Race T3

Bacterial spot of tomato (Solanum lycopersicum) is caused by four species of Xanthomonas. The disease causes significant yield losses and a reduction in fruit quality. Physiological races have been described with tomato race 3 (T3) corresponding to strains of Xanthomonas perforans. The breeding line Hawaii 7981 (hereafter H7981) shows a hypersensitive reaction (HR) to race T3 strains conditioned by the interaction of the host resistance locus Xv3 and the bacterial effector avrXv3. The Xv3 gene is required for H7981-derived resistance to be effective under field conditions, though its expression is subject to genetic background. The segregation of HR in F(2) populations derived from H7981 crossed to processing tomato parents OH88119 and OH7870 was studied in 331 progeny, with the two independent crosses providing validation. We screened 453 simple-sequence repeat, insertion/deletion, and single-nucleotide polymorphism markers and identified 44 polymorphic markers each for the OH88119 and OH7870 populations covering 84.6 and 73.3% of the genome, respectively, within 20 centimorgans (cM). Marker-trait analysis using all polymorphic markers demonstrated that Xv3-mediated resistance maps to chromosome 11 in the two independent crosses. Allelism tests were conducted in crosses between lines carrying Xv3 derived from H7981, Rx-4 derived from plant introduction (PI) 128216, and resistance derived from PI 126932. These allelism tests suggested that the loci conditioning HR to race T3 strains are linked within 0.1 cM, are allelic, or are the same gene.

Genotype by environment interaction of tomato blossom-end scar size

SummaryLarge blossom-end scar is a disorder in tomato fruit which reduces its marketability. The disorder is affected by genotype and by several environmental factors and therefore the genotype by environment interaction was studied by stability analysis. Blossom-end scar size was recorded for 4 tomato cultivars grown in 6 fields. The blossom-end scar size (BSC) was affected by the genotype, the field and their interaction. Stability analysis revealed that most of the interaction resulted from different stability of the cultivars. Heterogeneity of the slopes was significant (P<0.0013). The stability slopes were 0.29, 0.74, 1.11 and 1.85 for BR-214, FA-38, Hayslip and Suncoast, respectively. The stability slopes seemed to associate with the means of the cultivars over all environments, which were 1.57, 2.92, 3.84 and 5.43, respectively. Analysis of a blossom-end scar index (BSI), which also takes fruit size into account, revealed stability similar to BSC. It was concluded, that selection for small BSC under most conditions would yield cultivars with small and stable BSC under most growing environments, however differences between genotypes in non-inducing environments are expected to be small.

Diallel analysis over two environments for blossom-end scar size in tomato

SummaryTo investigate genetic regulation of blossom-end scar size in tomato (Lycopersicon esculentum Mill.), a half diallel cross including 10 parents was grown under warm fall conditions in Bradenton, Florida, and cool winter conditions in Hazeva, Israel. The parents were a random sample representing all available fresh market tomato breeding lines commercially grown under subtropical field conditions. A randomized, complete block design with three replications was used and the blossom-end scar index (BSI), a measure for scar size relative to truit size, was measured on 40 and 25 fruits per plot in Bradenton and Hazeva, respectively. Analysis of variance for BSI indicated highly significant (P=0.001) variation among parents and among F1s in both environments. In Bradenton and Hazeva both, the difference in average BSI between parents and F1s was not significant suggesting insignificant overall dominance effects. Further partitioning of variation within F1s indicated that general combining ability (GCA) effects were highly significant in both environments, whereas specific combining ability (SCA) effects were significant at P=0.001 in Bradenton, but only at P=0.05 in Hazeva. Estimated variance components for GCA and SCA effects indicated that BSI inherited mainly additively in both environments. Analysis combined over environments indicated that variation in sensitivity to environments was 5-fold higher among parents than among F1s. The genotype x environment variance component was not large enough to justify testing over more than one environment for population improvement purposes and early testing of hybrids. Evaluation of inbred lines, however, may have to be done in more than one environment, especially for ‘Suncoast’-derived material. Under temperatures in Hazeva, genotype differences were more pronounced and heritabilities higher than under high temperatures in Bradenton. The estimated overall single plot heritability was 0.63. Breeding lines with a pointed blossom-end morphology (e.g. NC 8276 and NC 140 in this study) generally had small blossom-end scars, and intercrossing of these lines or crossing with non-pointed, moderately smooth breeding lines generally resulted in smooth hybrids.