Introgression of the genes for sesquiterpene carboxylic acid synthesis from a wild tomato, Lycopersicon hirsutum, into the cultivated tomato for increased host-plant resistance to insect pests

Quality and quantity of yield of cultivated tomatoes ( Lycopersicon esculentum L.) are vulnerable to economically significant losses from damage by two lepidopteran (Noctuidae) insect pests, tomato fruitworm (Helicoverpa zea Boddie) and the beet armyworm ( Spodoptera exigua Hübner). L. hirsutum accession LA 1777 is resistant to numerous insect pests of tomatoes and produces large quantities of the sesquiterpene carboxylic acids (SCA): (+)-(E)-α-santalen-12-oic, (−)-(E)-endo-α-bergamoten-12-oic, and (+)-( E)-endo-β-bergamoten-12-oic acid in their trichomes. Larvae of H. zea and S. exigua were continuously exposed to SCA isolated from LA 1777 in artificial insect diet, topically applied to leaves of L. esculentum, or naturally synthesized in trichomes of tomato plants resulting from hybridizing LA 1777 to insect susceptible tomatoes. Results from the bioassays showed that tomato leaflets synthesizing at least 2 mg of total SCA per g of fresh leaf weight are sufficient for host-plant resistance.; F₂ and backcross progeny derived from hybridizing LA 1777 to four accessions of susceptible tomato species (L. esculentum cultivars ‘Chico III’, ‘E6203’, ‘VF145’ and L. hirsutum LA 1033) demonstrated a quantitative mode of inheritance of high levels of SCA necessary for host-plant resistance. Multiple genes with recessive alleles affected the accumulation of high levels of SCA.; F₂ and backcross progeny were assayed for the presence of polymorphic molecular markers using the Amplified Fragment Length Polymorphism (AFLP) protocol. Progeny of each generation were separated into high and low classes for SCA levels and the DNA combined of individual plants within the phenotypic classes. Bulk segregant analysis was used to identify polymorphic AFLP markers present in LA 1777 and absent in ‘Chico III’ and LA 1033. Comparisons of these polymorphic AFLP markers between high-SCA bulks and the zero-SCA plants (‘Chico III’ and LA 1033) revealed 10 AFLP markers associated with presence of SCA. Comparisons of the high-SCA bulks with a set of low-SCA bulks revealed one marker associated with high levels of SCA synthesis. Comparisons of the markers with a TA517 line that produces sesquiterpene precursors to SCA revealed three markers putatively associated with synthesis of the olefin to SCA and six putatively associated with modifications to SCA.