Yuval Eshed

The Making of a Compound Leaf: Genetic Manipulation of Leaf Architecture in Tomato

The most distinctive morphogenetic feature of leaves is their being either simple or compound. To study the basis for this dichotomy, we have exploited the maize homeobox-containing Knotted-1 (Kn1) gene in conjunction with mutations that alter the tomato compound leaf. We show that misexpression of Kn1 confers different phenotypes on simple and compound leaves. Up to 2000 leaflets, organized in compound reiterated units, are formed in tomato leaves expressing Kn1. In contrast, Kn1 induces leaf malformations but fails to elicit leaf ramification in plants with inherent simple leaves such as Arabidopsis or in tomato mutant plants with simple leaves. Moreover, the tomato Kn1 ortholog, unlike that of Arabidopsis, is expressed in the leaf primordia. Presumably, the two alternative leaf forms are conditioned by different developmental programs in the primary appendage that is common to all types of leaves.

A genomic library of Lycopersicon pennellii in L. esculentum: A tool for fine mapping of genes

The cultivated tomato contains only a small fraction of the genetic variation present in its wild relatives. In order to use the wild germplasm in tomato breeding and genetic studies we developed a new kind of genetic resource which is composed of 50 L. esculentum lines each containing a single introgression from the green fruited species L. pennellii (LA 716). Each of the introgression lines is nearly isogenic to the cultivated tomato; these lines provide complete coverage of the wild species genome. The lines contain on the average an introgression of 33 cM from a total genome size of 1200 cM. The size and identity of the introgressed segments was determined based on RFLP analysis of 350 markers. This resource can be viewed as a genomic library of the wild species in the cultivated background. It covers the entire genome with single independent ‘inserts’ per line and therefore every phenotypic difference between the introgression lines can be associated with the unique introgressed segment. The development and potential application of this resource are discussed.

Advanced backcross QTL analysis in tomato. I. Identification of QTLs for traits of agronomic importance from Lycopersicon hirsutum

Abstractu2002Advanced backcross QTL (AB-QTL) analysis is a new strategy for studying the effect of unadapted alleles on the agronomic performance of elite cultivated lines. In this paper we report results from the application of the AB-QTL strategy to cultivated tomato using the wild species Lycopersicon hirsutum LA1777 as the donor parent. RFLP genomic fingerprints were determined for 315 BC2 plants and phenotypic data were collected for 19 agronomic traits from approximately 200 derived BC3 lines which were grown in replicated field trials in three locations worldwide. Between 1 and 12 significant QTLs were identified for each of the 19 traits evaluated, with a total of 121 QTLs identified for all traits. For 25 of the QTLs (20%) corresponding to 12 traits (60%), the L. hirsutum allele was associated with an improvement of the trait from a horticultural perspective, despite the fact that L. hirsutum is overall phenotypically inferior to the elite parent. For example, L. hirsutum has fruit that remains green when ripe (lack of red pigment) yet alleles were found in this species that significantly increase red color when transferred into cultivated tomatoes. Wild alleles were also associated with increases in total yield and soluble solids (up to 15%) and brix×red yield (up to 41%). These results support the idea that one cannot predict the genetic potential of exotic germplasm based on phenotype alone and that marker-based methods, such as the AB-QTL strategy, should be applied to fully exploit exotic germplasm.

Mapping and introgression of a tomato yellow leaf curl virus tolerance gene, TY-1.

The whitefly-transmitted tomato yellow-leaf curl gemini-virus (TYLCV) is a major pathogen of tomatoes. The wild tomato species Lycopersicon chilense, which is resistant to the virus, was crossed to the cultivated tomato, L. esculentum. The backcross-1 selfed (BC1S1) generation was inoculated and a symptomless plant was selected. This plant was analyzed using 61 molecular markers, which span the tomato genome, to determine which L. chilense chromosome segments were introgressed. A BC2S1 population was cage-inoculated with viroliferous whiteflies (Bemisia tabaci), the natural insect vector of the virus, and subjected to RFLP analysis. Markers on chromosomes 3 and 6 were significantly associated with the level of tolerance; the association of chromosome-6 markers was further substantiated in two additional BC2S1 populations. A tolerant BC2S1 plant which was homozygous for L. chilense introgressions in chromosomes 3, 6 and 7 was crossed to generate a BC3S1 population which was planted in an infested field. A TYLCV-tolerance gene with partial dominance, TY-1, was mapped to chromosome 6; two modifier genes were mapped to chromosomes 3 and 7. Field and whitefly-mediated cage inoculations of nearly-isogenic lines in BC3S3 supported our conclusion that TY-1 is the major TYLCV-tolerance locus.

Advanced backcross QTL analysis of tomato. II. Evaluation of near-isogenic lines carrying single-donor introgressions for desirable wild QTL-alleles derived from Lycopersicon hirsutum and L. pimpinellifolium

Abstractu2002Improved-processing tomato lines were produced by the molecular breeding strategy of advanced backcross QTL (AB-QTL) analysis. These near-isogenic lines (NILs) contained unique introgressions of wild alleles originating from two donor wild species, Lycopersicon hirsutum (LA1777) and L. pimpinellifolium (LA1589). Wild alleles targeted for trait improvement were selected on the basis of previously published replicated QTL data obtained from advanced backcross populations for a battery of important agronomic traits. Twenty three NILs were developed for 15 genomic regions which were predicted to contain 25 quantitative trait factors for the improvement of seven agronomic traits: total yield, red yield, soluble solids, brix×red yield, viscosity, fruit color, and fruit firmness. An evaluation of the agronomic performance of the NILs in five locations worldwide revealed that 22 out of the 25 (88%) quantitative factors showed the phenotypic improvement predicted by QTL analysis of the BC3 populations, as NILs in at least one location. Per-location gains over the elite control ranged from 9% to 59% for brix×red yield; 14% to 33% for fruit color; 17% to 34% for fruit firmness; 6% to 22% for soluble-solids content; 7% to 22% for viscosity; 15% to 48% for red yield, and 20% to 28% for total yield. The inheritance of QTLs, the implementation of the AB-QTL methodology for characterizing unadapted germplasm and the applicability of this method to other crops are discussed.

Lycopersicon esculentum lines containing small overlapping introgressions from L. pennellii

SummaryThe objective of this project was to introgress small overlapping chromosome segments which cover the genome of L. pennellii into Lycopersicon esculentum lines. The interspecific hybrid was backcrossed to L. esculentum, and a map of 981 cM, based on 146 molecular markers covering the entire genome, was produced. A similar backcross 1 population was selfed for six generations, under strong selection for cultivated tomato phenotypes, to produce 120 introgression lines. The introgression lines were assayed for the above-mentioned molecular markers, and 21 lines covering 936 cM of L. pennellii, with an average introgression of 86 cM, were selected to provide a resource for the mapping of new DNA clones. The rest of the lines have shorter introgressions consisting of specific regions with an average size of 38 cM. The proportion of the L. pennellii genome in the introgression lines was lower than expected (252 cM) because of strong selection against the wild-parent phenotype. The mean introgression rate for ends of linkage groups in the 120 lines was 3 times higher than for other regions of the genome. The introgression lines can assist in RFLP-based gene cloning by allowing the rapid selection of DNA markers that map to specific chromosome segments. The introgression lines also provide a base population for the mapping and breeding for quantitative traits such as salt and drought tolerance that characterize the wild species L. pennellii.

Introgressions fromLycopersicon pennellii can improve the soluble-solids yield of tomato hybrids.

RFLP-defined chromosome segments covering the entire tomato genome were introgressed from the wild green-fruited speciesLycopersicon pennellii into the cultivated tomato (L. esculentum cv M82; Eshed et al. 1992). SixL. pennellii chromosome segments were selected for a detailed evaluation based on previous observations of their effects on the two yield components, fresh tomato yield and total soluble-solids content (Brix). Differences in the quantitative traits measured between M82 and the introgression lines, or their hybrids with different inbred parents, can be attributed to the alien chromosome segments. Replicated field trials, grown at wide and dense spacing, identified three quantitative trait loci (QTLs) for solublesolids content on chromosomes 1, 5 and 7. In plants heterozygous for the chromosome-5 locus there was a 50% increase in soluble-solids yield in wide but not in dense spacing. Plants heterozygous for the chromosome-1 QTL/s were tested over a 2-year period, in three genetic backgrounds, and showed a significant 16% elevation in soluble-solids yield only in dense spacing. These results demonstrate that wild tomato germplasm can be used to improve the yield of the cultivated crop.

Two tightly linked QTLs modify tomato sugar content via different physiological pathways

Abstract. Congenic lines that differ in a single defined chromosome segment are useful for the study of complex phenotypes, as they allow isolation of the effect of a particular quantitative trait locus (QTL) from those of the entire genome. We conducted high-resolution QTL mapping of a 9-cM introgression, originating from the wild tomato species Lycopersicon pennellii, in two extremely different genetic and physiological backgrounds. In the indeterminate glasshouse background we identified only a single QTL (Brix9-2-5) that affects the total soluble solids of the fruit [mainly sugars, measured in Brix units (B)]. This QTL was previously delimited within the gene for an apoplastic invertase, Lin5, that modulates sugar partitioning to the fruit. Analysis of the effects of the same chromosome segment in determinate, open-field tomatoes, revealed two QTLs, 0.3xa0cM apart: the fruit-specific Brix9-2-5 that affects B only, and the shoot-specific PW9-2-5, which accounts for an altered growth habit resulting in increases in plant weight, yield, and B. This study highlights the power of the congenic approach for dissecting developmental pathways leading to complex phenotypes.

Mapping of Ve in tomato : A gene conferring resistance to the broad-spectrum pathogen, Verticillium dahliae race 1

Abstractu2002The soil-borne fungi Verticillium spp. cause vascular wilt disease in a wide range of crop plants. In tomato, resistance to Verticillium dahliae race 1 is conferred by a single dominant gene, Ve. Previous efforts to map Ve in tomato have yielded confusing results, locating it on different chromosomes, which subsequently raised the possibility that Verticillium resistance may be controlled by a number of loci. We used three different mapping populations to obtain an unambiguous map location of Ve: a recombinant inbred (RI) line population; an F2 population segregating for Verticillium resistance; and a population of 50 introgression lines (IL). In all of the mapping populations Ve was positioned on the short arm of chromosome 9 tightly linked to the RFLP marker GP39. This linkage was confirmed by screening for GP39 in different breeding lines with known resistance or susceptibility to Verticillium. A perfect match was found between GP39 and the Verticillium response of the lines, indicating the potential of GP39 in the rapid detection of Verticillium resistance and as a starting point for map-based cloning of Ve. This approach is particularly relevant for Verticillium dahliae race 1, since in the present work we also show that the isolate that infects tomato is responsible for wilt disease in other important crop plants.

TAO1, a Representative of the Molybdenum Cofactor Containing Hydroxylases from Tomato

Aldehyde oxidase and xanthine dehydrogenase are a group of ubiquitous hydroxylases, containing a molybdenum cofactor (MoCo) and two iron-sulfur groups. Plant aldehyde oxidase and xanthine dehydrogenase activities are involved in nitrogen metabolism and hormone biosynthesis, and their corresponding genes have not yet been isolated. Here we describe a new gene from tomato, which shows the characteristics of a MoCo containing hydroxylase. It shares sequence homology with xanthine dehydrogenases and aldehyde oxidases from various organisms, and similarly contains binding sites for two iron-sulfur centers and a molybdenum-binding region. However, it does not contain the xanthine dehydrogenase conserved sequences thought to be involved in NAD binding and in substrate specificity, and is likely to encode an aldehyde oxidase-type activity. This gene was designated tomato aldehyde oxidase 1 (TAO1). TAO1 belongs to a multigene family, whose members are shown to map to clusters on chromosomes 1 and 11. MoCo hydroxylase activity is shown to be recognized by antibodies raised against recombinant TAO1 polypeptides. Immunoblots reveal that TAO1 cross-reacting material is ubiquitously expressed in various organisms, and in plants it is mostly abundant in fruits and rapidly dividing tissues.