Quantitative trait loci regulating the fatty acid profile of acylsugars in tomato

Acylsugars are secondary metabolites with proven insect resistance properties that are produced by many Solanaceous species including Solanum pennellii, a wild relative of tomato. The acylsugar chemotypes of S. pennellii varies greatly within its natural range and might be the product of plant/insect coevolution. The S. pennellii accession LA716 was used to transfer increased levels of acylsugar production into the cultivated tomato, resulting in the acylsugar tomato breeding line CU071026. S. pennellii accession LA716 produces high levels of acylsugars with chemotypes that differ greatly from those produced by CU071026 or the trace acylsugars of cultivated tomato. Understanding the genetic regulation of acylsugar chemistry will aid efforts to breed acylsugar production into cultivated tomato, allowing for alteration of both acylsugar base moieties and fatty acid profiles. This study uses a BC1F1 population produced from the cross of S. pennellii LA716 and CU071026 with CU071026 as the recurrent parent to identify QTL that change the fatty acid profile of acylsugars. Multiple QTL and epistatic interactions between QTL were detected including three QTL on chromosomes 2, 5, and 7, which significantly alter the percentage of extended iso-odd branched fatty acids and straight chain fatty acids on the acylsugars. We also report the introgression of one of these QTL, FA 2, into CU071026, resulting in a new tomato line with significantly increased i11:0 as a percentage of the fatty acids in its acylsugars. Candidate genes for these QTL and the impacts of altering acylsugar fatty acid are discussed.     

Introgression of acylsugar chemistry QTL modifies the composition and structure of acylsugars produced by high-accumulating tomato lines

Acylsugars are important insect defense compounds produced at high levels by glandular trichomes of the wild tomato, Solanum pennellii. The ability to produce acylsugars at elevated levels was bred into the tomato line CU071026. This study utilized a marker-assisted backcross approach to individually introgress into CU071026 and to fine map the three quantitative trait loci (QTL) fatty acid 5 (FA5QTL), fatty acid 7 (FA7QTL), and fatty acid 8 (FA8QTL), which were previously associated with changes in acylsugar chemistry. Additional breeding with and fine mapping the previously introgressed QTL, fatty acid 2 (FA2QTL), was also conducted. The effect of these four QTL on acylsugar quality and quantity in the presence of the five introgressions of CU071026 was evaluated. Incorporation of the QTL altered acylsugar chemotype by modulating the length, orientation, and/or relative proportion of fatty acid acyl groups. The resulting quantities of acylsugar produced in most of the new lines were similar to that of CU071026; however, introgression of FA5QTL reduced acylsugar levels. The acylsugar lines containing each QTL were characterized for acylsugar level, trichome abundance, and acylsugar chemistry through gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry. The novel acylsugar chemotype lines created can contribute to elucidation of the mechanism of insect resistance mediated by acylsugars and help with identification of yet-unknown genes contributing to acylsugar synthesis and diversity.     

Combination of Acylglucose QTL reveals additive and epistatic genetic interactions and impacts insect oviposition and virus infection

Acylsugars are important insect defense compounds produced at high levels by glandular trichomes of the wild tomato, Solanum pennellii. Marker-assisted selection was used to select for plants containing the three AGQTL named AG3QTL, AG4QTL, and AG11QTL from self-pollinated populations derived from an interspecific backcross population of CU071026 x (CU071026 x S. pennellii LA716). High acylglucose-accumulating lines were selected from these populations that possess these three AGQTL and the fewest number of extraneous S. pennellii LA716 introgressions. Incorporation of these three acylglucose QTL in the presence of the five standard S. pennellii introgressions of CU071026 altered acylsugar level and sugar moiety, demonstrating epistatic interactions between the acylglucose QTL on both of these traits. Comparison of the lines generated from the two breeding techniques indicated the three acylglucose QTL are essential but not necessarily sufficient for the production of elevated levels of acylglucose acylsugars. Fine-mapping of AG3QTL, AG4QTL, and AG11QTL resulted in less than 1 Mbp intervals for the locations of AG4QTL and AG11QTL; proposals of the causal genes underlying these acylglucose QTL are discussed. Characterization of the fatty acid profile of lines selected out of the interspecific backcross populations revealed an increase in the proportion of acylsugar n-C10 fatty acid acyl chains, possibly governed by one or more of the three acylglucose QTL. Characterization of the acylsugar profile of acylglucose lines selected from the interspecific backcross populations also demonstrated interactions among the acylglucose QTL to further modulate the diversity of acylsugars accumulated. Evaluation of an acylglucose line and controls against the tomato insect pest Frankliniella occidentalis demonstrated that levels of resistance differed among these lines and that the acylsugars accumulated by the acylglucose line were effective at reducing both F. occidentalis oviposition and incidence of Tomato spotted wilt orthotospovirus. However, of some of the acylglucose lines and hybrids tested against Spodoptera exigua did not indicate differences for larval weight gain and survival.     

Quantitative trait loci increasing acylsugars in tomato breeding lines and their impacts on silverleaf whiteflies

Solanum pennellii LA716, a wild relative of tomato, produces acylsugars, an insect resistance compound with activity against many tomato insect pests. Breeding of cultivated tomato using S. pennellii LA716 as a donor parent has led to the development of the elite acylsugar-producing tomato breeding line CU071026. CU071026 contains five introgressed S. pennellii genomic regions, and produces acylsugars at moderate levels that are effective against insect pests. A BC1F1 population was created by crossing the F1 CU071026?×?S. pennellii LA716 with CU071026 as the recurrent parent; this BC1F1 population was used to identify additional regions of the S. pennellii genome important for further improvement of acylsugar production. This population was genotyped with 94 markers in the segregating regions and phenotyped for level of acylsugar production. Using QTLNetwork 2.1 for the detection of quantitative trait loci (QTL) and epistatic interactions, this study identified five QTL for total acylsugar level. Additionally, two epistatic interactions between QTL were found to control significant levels of total acylsugar production. Two of the QTL identified were further evaluated in silverleaf whitefly (Bemisia tabaci) field cage trials using acylsugar breeding lines that differ for the presence/absence of these QTL. While high levels of silverleaf whitefly resistance were observed in all acylsugar breeding lines, lines containing the additional QTL on either chromosomes 6 or 10 had increased levels of total acylsugar production and reduced incidence of whitefly. Acylsugar lines containing the chromosome 6 QTL also had increased density of the type IV glandular trichomes which produce and exude acylsugars.     

Quantitative trait loci regulating sugar moiety of acylsugars in tomato

Acylsugars are broad-spectrum insect resistance sugar esters produced at very high levels by some accessions of the wild tomato, Solanum pennellii. Transferring acylsugar production from S. pennellii LA716 to cultivated tomato through traditional breeding developed the benchmark acylsugar breeding line CU071026. The base moiety of acylsugars (sucrose vs. glucose) can vary among S. pennellii accessions. Additionally the accession S. pennellii LA716 produces almost exclusively acylglucoses, but the breeding line CU071026 derived from S. pennellii LA716 produces exclusively acylsucroses. This study uses a BC1F1 and a BC1F2 population derived from the cross CU071026 × (CU071026 × S. pennellii LA716) to identify and confirm the action of three quantitative trait loci (QTL) on chromosomes 3, 4, and 11. The QTL on chromosomes 3 and 11 are both required for acylglucose production, while addition of the chromosome 4 QTL affects the level of acylglucose produced in the presence of the QTL on chromosomes 3 and 11. A three-way interaction between these acylglucose QTL was confirmed with a post hoc ANOVA. Identification of these three QTL provides a blueprint for breeding to shift acylsucrose production to acylglucose production in tomato breeding lines. The implications of these QTL and two additional QTL affecting total acylsugar level in the BC1F2 are discussed.     

QTL analysis of pest resistance in the wild tomato Lycopersicon pennellii: QTLs controlling acylsugar level and composition

Some accessions of Lycopersicon pennellii, a wild relative of the tomato Lycopersicon esculentum, are resistant to a number of important pests of cultivated tomato due to the accumulation of acylsugars, which constitute 90% of the exudate of type-IV trichomes in L. pennellii LA716. An interspecific F₂ population, created by the cross L. esculentum x L. pennellii LA 716, was surveyed for acylsugar accumulation and subjected to RFLP/QTL analysis to determine the genomic regions associated with the accumulation of acylglucoses, acylsucroses, and total acylsugars, as well as with acylglucoses as a percentage of total acylsugars (mole percent acylglucoses). Data were analyzed using MAPMAKER/QTL with and without a log₁₀ transformation. A threshold value of 2.4 (default value for MAPMAKER/QTL) was used, as well as 95% empirically derived threshold values. Five genomic regions, two on chromosome 2 and one each on chromosomes 3, 4 and 11, were detected as being associated with one or more aspects of acylsugar production. The L. esculentum allele is partially dominant to the L. pennellii allele in the regions on chromosomes 2 and 11, but the L. pennellii allele is dominant in the region on chromosome 3. Throughout this study, we report the comparative effects of analytical methodology on the identification of acylsugar QTLs. Similarities between our results and published results for the genus Solanum are also discussed.R. W. Doerge · S.-C. Liu · J. P. Kuai contributed equally to the paper, and we ordered randomly     

Identification of quantitative trait loci associated with acylsugar accumulation using intraspecific populations of the wild tomato, Lycopersicon pennellii

 Lycopersicon pennellii LA716, a wild relative of tomato, is resistant to a number of insect pests due to the accumulation of acylsugars exuded from type IV trichomes. These acylsugars are a class of compounds including both acylglucoses and acylsucroses. Intraspecific populations between L. pennellii LA716 and L. pennellii LA1912, the latter an accession that assorts for low-level acylsugar accumulation, were created to study the inheritance of type IV trichome density, acylsugar accumulation levels, percentage of acylsugars that are acylglucoses, and leaf area. The F₂ population was subsequently used to determine genomic regions associated with these traits. The relative proportion of acylglucoses and acylsucroses was found to be largely controlled by a single locus near TG549 on chromosome 3. One locus on chromosome 10 showed significant associations with acylsugar levels. In addition, 1 locus on chromosome 4 showed significant associations with leaf area. Ten additional loci showed modest associations with one or more of the traits examined, 5 of which have been previously reported. Received: 13 March 1997 / Accepted: 19 September 1997     

Differential and Synergistic Functionality of Acylsugars in Suppressing Oviposition by Insect Herbivores

Acylsugars are secondary metabolites exuded from type IV glandular trichomes that provide broad-spectrum insect suppression for Solanum pennellii Correll, a wild relative of cultivated tomato. Acylsugars produced by different S. pennellii accessions vary by sugar moieties (glucose or sucrose) and fatty acid side chains (lengths and branching patterns). Our objective was to determine which acylsugar compositions more effectively suppressed oviposition of the whitefly Bemisia tabaci (Gennadius) (Middle East—Asia Minor 1 Group), tobacco thrips, Frankliniella fusca (Hinds), and western flower thrips, Frankliniella occidentalis (Pergande). We extracted and characterized acylsugars from four S. pennellii accessions with different compositions, as well as from an acylsugar-producing tomato breeding line. We also fractionated the acylsugars of one S. pennellii accession to examine the effects of its components. Effects of acylsugars on oviposition were evaluated by administering a range of doses to oviposition sites of adult whiteflies and thrips in non-choice and choice bioassays, respectively. The acylsugars from S. pennellii accessions and the tomato breeding line demonstrated differential functionality in their ability to alter the distribution of whitefly oviposition and suppress oviposition on acylsugar treated substrates. Tobacco thrips were sensitive to all compositions while western flower thrips and whiteflies were more sensitive to acylsugars from a subset of S. pennellii accessions. It follows that acylsugars could thus mediate plant-enemy interactions in such a way as to affect evolution of host specialization, resistance specificity, and potentially host differentiation or local adaptation. The acylsugars from S. pennellii LA1376 were separated by polarity into two fractions that differed sharply for their sugar moieties and fatty acid side chains. These fractions had different efficacies, with neither having activity approaching that of the original exudate. When these two fractions were recombined, the effect on both whiteflies and thrips exceeded the sum of the two fractions’ effects, and was similar to that of the original exudate. These results suggest that increasing diversity of components within a mixture may increase suppression through synergistic interactions. This study demonstrates the potential for composition-specific deployment of acylsugars for herbivore oviposition suppression, either through in planta production by tomato lines, or as biocides applied by a foliar spray.     

Identification of QTLs controlling acylsugar fatty acid composition in an intraspecific population of Lycopersicon pennellii (Corr.) D’Arcy

 Acylsugars exuded by type IV glandular trichomes are responsible for insect resistances found in many Lycopersicon pennellii accessions. Acylsugars are complex mixtures composed of polyacylated sugars (glucose or sucrose) esterified to branched and straight-chain 4 : 0 to 12 : 0 fatty acids. The biogeneses of these unusual fatty acid constituents have their origins in branched-chain amino acid pathways. However, the mechanism of fatty acid elongation in these systems and the genetic control of carbon flux from amino acid to fatty acid pathways remain unclear. In this study, we used an intraspecific F₂ population derived from the cross between L. pennellii LA716 and L. pennellii LA1912 to examine the genetic basis of acylsugar fatty acid composition. Six QTLs were detected which, combined, explain 23–60% of the variance observed for each of the nine segregating fatty acid constituents. Both correlation data and QTL analysis data indicate that branched medium-chain fatty acids are synthesized through elongation of short-chain precursors in two-carbon increments. The proportion of iso-branched acylsugar fatty acids that have an even-carbon chain length was found to be primarily determined by a single locus that maps to a location 5.5 cM above TG117 on chromosome 8. QTL function in several cases can be inferred from discrete patterns of fatty acid composition; in other cases, control of acylsugar fatty acid composition appears to be complex. Received: 7 April 1998 / Accepted: 28 December 1998     

Inheritance of acylsugar contents in tomatoes derived from an interspecific cross with the wild tomato Lycopersicon pennellii and their effect on spider mite repellence

Acylsugars present in Lycopersicon pennellii are responsible for the high levels of pest resistance often found in this wild tomato taxon. We investigated the inheritance of acylsugar contents in segregating populations of the interspecific tomato cross L. esculentum x L. pennellii and estimated correlations between leaflet acylsugar contents and the levels of mite repellence. Acylsugar contents were quantified with the Sommogy-Nelson colorimetric method in the acessions L. esculentum 'TOM-584' (P(1), low acylsugars), L. pennellii 'LA-716' (P(2), high acylsugars), in the interspecific F(1) (P(1) x P(2)) and in the F(2 )(P(1) x P(2)) generations. Mite resistance was assessed by a repellence test. Broad-sense heritability of acylsugar contents was moderately high (h(2)(b) = 0.476). Frequency distributions in the P(1), P(2), F(1) and F(2) can be explained by the action of a single major locus, with near-complete dominance of the L. esculentum allele for low-acylsugar content over the L. pennellii allele for high content. Indirect selection for high levels of acylsugars in leaflets led to correlated increases in the levels of mite repellency, indicating that acylsugars may be the main factor involved in mite resistance.     

Marker-assisted transfer of acylsugar-mediated pest resistance from the wild tomato,Lycopersicon pennellii,to the cultivated tomato,Lycopersicon esculentum

Acylsugars exuded from type IV trichomes mediate the multiple pest resistance found in the wild tomato species, Lycopersicon pennellii. A marker-assisted selection breeding program was used to attempt the transfer of the ability to accumulate acylsugars to cultivated tomato. RFLP and PCR-based markers were used through three backcross generations to select plants containing 5 target regions associated by QTL analysis with acylsugar accumulation. The BC1F1 plant selected possessed all 5 target regions and accumulated acylsugars at a moderate level similar to that of the interspecific F1 control. The BC2F1 and BC3F1 selections contained complementary subsets of the 5 target regions and did not accumulate acylsugars. BC3F1 plants with complementary subsets of the 5 target regions were intermated to produce populations segregating for the 5 target regions. From 1000 BC3F1-intermated plants, three plants were found which accumulated acylsugars at low levels and contained 3 to 5 of the target regions. The recovery of acylsugar accumulation in progeny of the intermated BC3F1 plants supports the involvement of at least some of the 5 target regions in acylsugar biosynthesis. However, since the levels of acylsugars accumulated by these plants were lower than that of the interspecific F1, it is likely that another, as of yet unidentified, region is necessary for accumulation of higher levels of acylsugars.     

NaCl tolerance in <Emphasis Type="Italic">Lycopersicon pennellii</Emphasis> introgression lines: QTL related to physiological responses

The growth and ion content of salt sensitive Lycopersicon esculentum Mill. cv. M82 and salt tolerant L. pennellii Correll accession LA716 were examined under both control and stress conditions (150 mM NaCl). L. esculentum grew more vigorously than L. pennellii under optimal conditions, however, L. pennellii was able to maintain its growth better than cultivated tomato when the plants were exposed to salinity. Sodium content of both L. esculentum and L. pennellii increased as a result of NaCl stress. In addition, both species showed reduced potassium and calcium content due to salinity. The physiological traits were also measured in a population of 52 L. pennellii introgression lines grown under both normal and stress conditions. A total of 311 quantitative trait loci (QTL) were identified for the studied traits: plant height, stem diameter, leaf number, leaf and root fresh and dry mass, and sodium, potassium and calcium contents. Some of the loci (124) were identified under both control and stress conditions while 86 QTL were identified only under non-stress conditions and 101 loci were identified only under NaCl stress.     

Functionally Divergent Alleles and Duplicated Loci Encoding an Acyltransferase Contribute to Acylsugar Metabolite Diversity in Solanum Trichomes

Glandular trichomes from tomato (Solanum lycopersicum) and other species in the Solanaceae produce and secrete a mixture of O-acylsugars (aliphatic esters of sucrose and glucose) that contribute to insect defense. Despite their phylogenetic distribution and diversity, relatively little is known about how these specialized metabolites are synthesized. Mass spectrometric profiling of acylsugars in the S. lycopersicum x Solanum pennellii introgression lines identified a chromosome 11 locus containing a cluster of BAHD acyltransferases with one gene (named Sl-ASAT3) expressed in tip cells of type I trichomes where acylsugars are made. Sl-ASAT3 was shown to encode an acyl-CoA-dependent acyltransferase that catalyzes the transfer of short (four to five carbons) branched acyl chains to the furanose ring of di-acylsucrose acceptors to produce tri-acylsucroses, which can be further acetylated by Sl-ASAT4 (previously Sl-AT2). Among the wild tomatoes, diversity in furanose ring acyl chains on acylsucroses was most striking in Solanum habrochaites. S. habrochaites accessions from Ecuador and northern Peru produced acylsucroses with short (≤C5) or no acyl chains on the furanose ring. Accessions from central and southern Peru had the ability to add short or long (up to C12) acyl chains to the furanose ring. Multiple ASAT3-like sequences were found in most accessions, and their in vitro activities correlated with observed geographical diversity in acylsugar profiles.     

Development of PCR-based markers from the tomato glutamate oxaloacetate transaminase isozyme gene family as a means of revitalising old isozyme markers and recruiting new ones

Sequences annotated as aspartate aminotransferases (synonymous with glutamate oxaloacetate transaminases) in the SOL Genomics Network unigene database were used to design 10 pairs of PCR primers for genetic marker development. These primer pairs generated nine CAPS markers, two SCAR markers and one SSR marker, which were bin-mapped using a set of tomato introgression lines (IL) derived from Lycopersicon esculentum cv. M82 and Lycopersicon pennellii LA716. Based on their bin locations, these markers are largely dispersed throughout the tomato genome and appear to have tagged all four of the glutamate oxaloacetate transaminase (Got) isozyme marker genes placed on the classical genetic map of tomato. Orthologous relationships with Arabidopsis aspartate aminotransferase (Asp) genes suggest the existence of at least two additional functional Got genes in tomato that have also been tagged by these markers and likewise an additional functional Asp gene in Arabidopsis. The Got-2 isozyme marker has often been used for the marker-assisted breeding of the I-3 gene for Fusarium wilt resistance introgressed from L. pennellii LA716. The Got-2 CAPS marker that we have developed offers a facile PCR-based alternative to the isozyme marker for the marker-assisted breeding of I-3. However, all of the PCR-based markers we have developed have the potential to assist the breeding of linked traits introgressed from wild relatives of tomato.     

Salt tolerance in <Emphasis Type="Italic">Solanum pennellii</Emphasis>: antioxidant response and related QTL

Background  

Excessive soil salinity is an important problem for agriculture, however, salt tolerance is a complex trait that is not easily bred into plants. Exposure of cultivated tomato to salt stress has been reported to result in increased antioxidant content and activity. Salt tolerance of the related wild species, Solanum pennellii, has also been associated with similar changes in antioxidants. In this work, S. lycopersicum M82, S. pennellii LA716 and a S. pennellii introgression line (IL) population were evaluated for growth and their levels of antioxidant activity (total water-soluble antioxidant activity), major antioxidant compounds (phenolic and flavonoid contents) and antioxidant enzyme activities (superoxide dismutase, catalase, ascorbate peroxidase and peroxidase) under both control and salt stress (150 mM NaCl) conditions. These data were then used to identify quantitative trait loci (QTL) responsible for controlling the antioxidant parameters under both stress and nonstress conditions.     

Profiling,isolation and structure elucidation of specialized acylsucrose metabolites accumulating in trichomes of <Emphasis Type="Italic">Petunia</Emphasis> species

Introduction

Acylsugar specialized metabolites function as defenses against insect herbivores, and are the most abundant specialized metabolites produced in Solanaceous trichomes. Metabolite profiling provides the foundation for determining the genetic basis of specialized metabolism and its evolution.

Objectives

To profile and identify acylsugar specialized metabolites in three Petunia species: P. axillaris, P. integrifolia and P. exserta.

Methods

Metabolites were profiled using ultra-high performance liquid chromatography/time-of-flight mass spectrometry (UHPLC/TOF MS). Metabolites were purified using solid phase extraction and HPLC, and structures were established using NMR spectroscopy.

Results

Twenty-eight distinct acylsucrose formulas, representing a sampling of more than 100 different detected chemical forms, were purified from three Petunia species and structures have been proposed based on one- and two-dimensional NMR data. 15 of the 28 purified acylsugars were sucrose pentaesters that possess a malonyl group on the fructose ring. These malonate esters can be readily distinguished from other acylsugars based on distinct masses of pseudomolecular ions and fragment ions generated using multiplexed collision-induced dissociation. Chemical diversity of acylsugars was observed between Petunia species, particularly with respect to the lengths of acyl chains and specific acylation positions.

Conclusions

These findings suggest substrate selectivity of various acyltransferases in Petunia species.

     

RAPD markers associated with salt tolerance in an interspecific cross of tomato (Lycopersicon esculentum×L. pennellii)

This study was conducted to identify randomly amplified polymorphic DNA (RAPD) markers associated with quantitative trait loci (QTLs) conferring salt tolerance during germination in tomato. Germination response of an F₂ population (2000 individuals) of a cross between UCT5 (Lycopersicon esculentum, salt-sensitive) and LA716 (L. pennellii, salt-tolerant) was evaluated at a salt-stress level of 175 mM NaCl+17.5 mM CaCl₂ (water potential ca. –9.5 bars). Germination was scored visually as radicle protrusion at 6-h intervals for 30 consecutive days. Individuals at both extremes of the response distribution (i.e., salt-tolerants and salt-sensitives) were selected. The selected individuals were genotyped for 53 RAPD markers and allele frequencies at each marker locus were determined. The linkage association among the markers was determined using a “Mapmaker” program. Trait-based marker analysis (TBA) identified 13 RAPD markers at eight genomic regions that were associated with QTLs affecting salt tolerance during germination in tomato. Of these genomic regions, five included favorable QTL alleles from LA716, and three included favorable alleles from UCT5. The approximate effects of individual QTLs ranged from 0.46 to 0.82 phenotypic standard deviation. The results support our previous suggestion that salt tolerance during germination in tomato is polygenically controlled. The identification of favorable QTLs in both parents suggests the likelihood of recovering transgressive segregants in progeny derived from these genotypes. Results from this study are discussed in relation to using marker-assisted selection in breeding for salt tolerance. Received: 16 June 1997 / Revision received: 11 August 1997 / Accepted: 2 September 1997     

Identification of drought-responsive microRNAs in tomato using high-throughput sequencing

Drought is a major abiotic stress affecting crop productivity and quality. As a class of noncoding RNA, microRNA (miRNA) plays important roles in plant growth, development, and stress response. However, their response and roles in tomato drought stress is largely unknown. Here, by using high-throughput sequencing, we compared the miRNA profiles before and after drought treatment in two tomato genotypes: M82, a drought-sensitive cultivated tomato (Solanum lycopersicum), and IL2-5, a drought-tolerant introgression line derived from M82 and the tomato wild species S. pennellii (LA0716). A total of 108 conserved and 208 novel miRNAs were identified, among them, 32 and 68 were significantly changed in expression after stress. Further, 1936 putative target genes were predicted for those differentially-expressed miRNAs. Gene ontology and pathway analysis showed that many of the target genes were involved in stress resistance, such as genes in GO terms including response to stress, defense response, response to stimulus, phosphorylation, and signal transduction. Our results suggested that miRNAs play an essential role in the drought response of tomato. This work will help to further characterize specific miRNAs functioning in drought tolerance.     

Use of introgression lines and zonal mapping to identify RAPD markers linked to QTL

RAPD primers were identified as giving parent-specific bands when screened with a set of introgression lines containing introgressed regions of Lycopersicon pennellii that encompass 5 quantitative trait loci affiliated with the production and composition of acylsugars, compounds associated with insect resistance. Primers giving L. pennellii introgression specific bands were zonally mapped to identify bands affiliated with the quantitative trait target and flanking regions using subsets of 7 to 16 F2 individuals which contained small overlapping segments (zones) of the L. pennellii genome spanning those regions. Seventeen RAPD primers, agt-related primers, and an agt clone were then used in mapping the complete F2 population of 144 individuals. This work resulted in the identification of RAPD markers for three of the 5 quantitative trait loci and the construction of an integrated RAPD/RFLP genomic map for tomato (Lycopersicon esculentum x L. pennellii LA716) of 111 RAPD and 8 acylglucose transferase related markers added to a framework map of 150 RFLP markers.