Roger T. Chetelat

Uniform ripening Encodes a Golden 2-like Transcription Factor Regulating Tomato Fruit Chloroplast Development

Pretty or Sweet The grocery-store tomato that looks beautiful but tastes like tart cardboard arises from selection processes favoring phenotypes that make commercial production more reliable. Significant in that selection process was a mutation that reduced the mottled color variations of unripe green tomatoes, leaving them a uniform, pale, green. Powell et al. (p. 1711) analyzed the molecular biology of the mutation. The uniform ripening mutation turns out to disable a transcription factor called Golden 2-like (GLK2). GLK2 expression increases the fruits photosynthetic capacity, resulting in higher sugar content. Controlling when tomatoes turn from green to red requires knocking out the gene that adds flavor. Modern tomato (Solanum lycopersicum) varieties are bred for uniform ripening (u) light green fruit phenotypes to facilitate harvests of evenly ripened fruit. U encodes a Golden 2-like (GLK) transcription factor, SlGLK2, which determines chlorophyll accumulation and distribution in developing fruit. In tomato, two GLKs—SlGLK1 and SlGLK2—are expressed in leaves, but only SlGLK2 is expressed in fruit. Expressing GLKs increased the chlorophyll content of fruit, whereas SlGLK2 suppression recapitulated the u mutant phenotype. GLK overexpression enhanced fruit photosynthesis gene expression and chloroplast development, leading to elevated carbohydrates and carotenoids in ripe fruit. SlGLK2 influences photosynthesis in developing fruit, contributing to mature fruit characteristics and suggesting that selection of u inadvertently compromised ripe fruit quality in exchange for desirable production traits.

A Pollen Factor Linking Inter- and Intraspecific Pollen Rejection in Tomato

Reds Versus Greens Self-incompatibility (SI) allows plants to prevent inbreeding. Crosses with distant relatives (outbreeding) can also be problematic and is prevented by unilateral interspecific incompatability (UI). In the nightshade family, SI functions within green-fruited species, whereas crosses between green-fruited and red-fruited species (which includes tomato) results in UI. Li and Chetelat (p. 1827) found a gene, related to known SI genes within this family, that differs in transcript length and function between individuals that are red-fruited and those that are green-fruited. A survey of species shows that the green-fruited species have a functional allele of this gene, whereas the transcript of this gene in red-fruited species, which are self-compatible, produce a putatively nonfunctional protein. These findings suggest that cultivated tomato may have lost the ability to pollinate other species within the same family, owing to the loss of this protein. The inability to cross with distant relatives in the nightshade family is linked to mechanisms preventing self pollination. Self-incompatibility (SI)—intraspecific pollen recognition systems that allow plants to avoid inbreeding—in the Solanaceae (the nightshade family) is controlled by a polymorphic S locus where “self” pollen is rejected on pistils with matching S alleles. In contrast, unilateral interspecific incompatibility (UI) prevents hybridization between related species, most commonly when the pollen donor is self-compatible (SC) and the recipient is SI. We observed that in Solanum, a pollen-expressed Cullin1 gene with high similarity to Petunia SI factors interacts genetically with a gene at or near the S locus to control UI. Cultivated tomato and related red- or orange-fruited species (all SC) exhibit the same loss-of-function mutation in this gene, whereas the green-fruited species (mostly SI) contain a functional allele; hence, similar biochemical mechanisms underlie the rejection of both “self” and interspecific pollen.

Interspecific reproductive barriers in the tomato clade: opportunities to decipher mechanisms of reproductive isolation

The tomato clade within the genus Solanum has numerous advantages for mechanistic studies of reproductive isolation. Its thirteen closely related species, along with four closely allied Solanum species, provide a defined group with diverse mating systems that display complex interspecific reproductive barriers. Several kinds of pre- and postzygotic barriers have already been identified within this clade. Well-developed genetic maps, introgression lines, interspecific bridging lines, and the newly available draft genome sequence of the domesticated tomato (Solanum lycopersicum) are valuable tools for the genetic analysis of interspecific reproductive barriers. The excellent chromosome morphology of these diploid species allows detailed cytological analysis of interspecific hybrids. Transgenic methodologies, well developed in the Solanaceae, allow the functional testing of candidate reproductive barrier genes as well as live imaging of pollen rejection events through the use of fluorescently tagged proteins. Proteomic and transcriptomics approaches are also providing new insights into the molecular nature of interspecific barriers. Recent progress toward understanding reproductive isolation mechanisms using these molecular and genetic tools is assessed in this review.

Distribution, ecology and reproductive biology of wild tomatoes and related nightshades from the Atacama Desert region of northern Chile

Over the past 20xa0years, several expeditions were made to northern Chile to collect populations of wild tomatoes (Solanum chilense, S. peruvianum) and allied nightshades (S. lycopersicoides, S. sitiens), and obtain information about their geographic distribution, ecology and reproductive biology. Restricted mainly to drainages of the Andean and the coastal cordillera, populations are geographically fragmented. The two nightshade species are rare and threatened by human activities. Adaptation to extreme aridity and soil salinity are evident in S. chilense and S. sitiens (the latter exhibits several xerophytic traits not seen in the tomatoes) and to low temperatures in S. lycopersicoides and S. chilense. All tested accessions are self-incompatible, with the exception of one S. peruvianum population collected at the southern limit of its distribution. Several distinguishing reproductive traits—anther color, attachment, and dehiscence, pollen size, and flower scent—suggest S. sitiens and S. lycopersicoides attract different pollinators than S. chilense and S. peruvianum. The four Solanum spp. native or endemic to Chile provide a variety of novel traits which, through hybridization and introgression with cultivated tomato, could facilitate development of improved varieties, as well as research on a variety of basic topics, including plant-pollinator interactions, abiotic stress responses, and evolution of reproductive barriers.

The Role of a Pollen-Expressed Cullin1 Protein in Gametophytic Self-Incompatibility in Solanum

We previously isolated a pollen factor, ui6.1, which encodes a Cullin1 protein (CUL1) that functions in unilateral interspecific incompatibility (UI) in Solanum. Here we show that CUL1 is also required for pollen function in self-incompatibility (SI). We used RNA interference (RNAi) to reduce CUL1 expression in pollen of Solanum arcanum, a wild SI tomato relative. Hemizygous T0 plants showed little or no transmission of the transfer DNA (T-DNA) through pollen when crossed onto nontransgenic SI plants, indicating that CUL1-deficient pollen are selectively eliminated. When crossed onto a related self-compatible (SC) accession lacking active S-RNase, pollen transmission of the T-DNA followed Mendelian ratios. These results provide further evidence for functional overlap between SI and UI on the pollen side and suggest that CUL1 mutations will reinforce SI-to-SC transitions in natural populations only if preceded by loss of pistil S-RNase expression.

Solanum sect. Lycopersicon

In this review, we examine the plant group Solanum sect. Lycopersicon – a clade of 13 species, including the domesticated tomato (Solanum lycopersicum L.) and its wild relatives – along with four allied species in the immediate outgroups Solanum sects. Juglandifolia and Lycopersicoides. We summarize the geographic distribution and morphological characters of these plant groups, describing their evolutionary relationships in the context of a new taxonomic revision at the species level of all these groups. We provide an overview of the role that wild tomato species have played in the development of cytogenetic stocks, in classical and molecular genetic studies as well as in crop improvement through traditional and advanced tools. We discuss how the very narrow genetic basis of cultivated tomato germplasm has forced tomato geneticists and breeders to rely on the wealth of genetic variation present in the wild relatives to address the many breeding challenges. The numerous molecular mapping studies conducted using interspecific crosses have clearly demonstrated that the breeding value of exotic (wild) tomato germplasm goes far beyond its phenotype. These studies also show that we are still far from being able to fully exploit the breeding potential of the thousands of accessions stored in seed banks around the world, in addition to those that may still be found in natural habitats. Over the past decades, tomato breeders have been at the forefront of establishing new principles for crop breeding based on the use of wild species to improve modern cultivars. In this respect, among all model systems, the wild and domesticated species of the tomato clade have pioneered development of novel populations such as “exotic libraries.” These genetic resources, combined with the increasing knowledge deriving from the many “omics” tools, including the tomato genome sequence, are expected to further improve the efficiency with which wild tomato relatives will contribute to the improvement of this important crop.

Fine Mapping of ui6.1, a Gametophytic Factor Controlling Pollen-Side Unilateral Incompatibility in Interspecific Solanum Hybrids

Unilateral incompatibility (UI) is a prezygotic reproductive barrier in plants that prevents fertilization by foreign (interspecific) pollen through the inhibition of pollen tube growth. Incompatibility occurs in one direction only, most often when the female is a self-incompatible species and the male is self-compatible (the “SI × SC rule”). Pistils of the wild tomato relative Solanum lycopersicoides (SI) reject pollen of cultivated tomato (S. lycopersicum, SC), but accept pollen of S. pennellii (SC accession). Expression of pistil-side UI is weakened in S. lycopersicum × S. lycopersicoides hybrids, as pollen tube rejection occurs lower in the style. Two gametophytic factors are sufficient for pollen compatibility on allotriploid hybrids: ui1.1 on chromosome 1 (near the S locus), and ui6.1 on chromosome 6. We report herein a fine-scale map of the ui6.1 region. Recombination around ui6.1 was suppressed in lines containing a short S. pennellii introgression, but less so in lines containing a longer introgression. More recombinants were obtained from female than male meioses. A high-resolution genetic map of this region delineated the location of ui6.1 to ∼0.128 MU, or 160 kb. Identification of the underlying gene should elucidate the mechanism of interspecific pollen rejection and its relationship to self-incompatibility.

Transmission and recombination of homeologous Solanum sitiens chromosomes in tomato

The goal of the present experiments was to transfer the chromosomes of Solanum sitiens (syn. Solanum rickii) into cultivated tomato (Lycopersicon esculentum). By crossing an allotetraploid L. esculentum × Solanum sitiens hybrid to sesquidiploid L. esculentum × S. lycopersicoides, a trigenomic hybrid (2n+14=38) was obtained. Analysis of the latter by GISH (genomic in situ hybridization) indicated it contained a full set of 12 S. sitiens chromosomes, plus two extras from S. lycopersicoides. This and other complex hybrids were pollinated with Lycopersicon pennellii-derived bridging lines to overcome unilateral incompatibility. A total of 40 progeny were recovered by embryo rescue, including diploids and aneuploids (up to 2n+8). In order to determine the origin of chromosomes and the location of introgressed segments, progeny were genotyped with RFLP markers. S. sitiens-specific markers on all chromosomes, except 6 and 11, were detected in the progeny. Several S. sitiens chromosomes were transmitted intact, either through chromosome addition (i.e., trisomics) or substitution (i.e., disomics). Recombination between S. sitiens and L. esculentum was detected on most chromosomes, in both diploid and aneuploid progeny. A monosomic alien addition line for S. sitiens chromosome 8 was identified, and the extra chromosome was stably transmitted to approximately 13% of the backcross progeny. This study demonstrates the feasibility of gene transfer from S. sitiens to L. esculentum through chromosome addition, substitution, and recombination in the progeny of complex aneuploid hybrids.

Genealogy and fine mapping of obscuravenosa, a gene affecting the distribution of chloroplasts in leaf veins, and evidence of selection during breeding of tomatoes (Lycopersicon esculentum; Solanaceae)

In the processes of plant domestication and variety development, some traits are under direct selection, while others may be introduced by indirect selection or linkage. In the cultivated tomato (Lycopersicon esculentum = Solanum lycopersicum), and all other Solanaceae examined, chloroplasts are normally absent from subepidermal and mesophyll cells surrounding the leaf veins, and thus, veins appear clear upon subillumination. The tomato mutant obscuravenosa (obv), in contrast, contains chloroplasts in cells around the vein, and thus, veins appear as dark as the surrounding leaf tissue. Among tomato cultivars, the obv allele is common in processing varieties bred for mechanical harvest, but is otherwise rare. We traced the source of obv in processing tomatoes to the cultivar Earliana, released in the 1920s. The obv locus was mapped to chromosome 5, bin 5G, using introgression lines containing single chromosome segments from the wild species L. pennellii. This region also contains a quantitative trait locus (QTL) for plant height, pht5.4, which cosegregated with SP5G, a paralog of self-pruning (sp), the gene that controls the switch between determinate and indeterminate growth in tomato. The pht5.4 QTL was partially dominant and associated with a reduced percentage of red fruit at harvest. Our data suggest that the prevalence of obv in nearly all processing varieties may have resulted from its tight linkage to a QTL conferring a more compact, and horticulturally desirable, plant habit.

Mapping of loci from Solanum lycopersicoides conferring resistance or susceptibility to Botrytis cinerea in tomato.

Cultivated tomato (Solanum lycopersicum, syn. Lycopersicon esculentum) is susceptible to the necrotrophic ascomycete and causal agent of gray mold, Botrytis cinerea. Resistance to this fungal pathogen is elevated in wild relatives of tomato, including Solanum lycopersicoides. An introgression line population (IL) containing chromosomal segments of S. lycopersicoides within the background of tomato cv. VF36 was used to screen the genome for foliar resistance and susceptibility to B. cinerea. Based on this screen, putative quantitative trait loci (QTL) were identified, five for resistance and two for susceptibility. Four resistance QTL decreased infection frequency while the fifth reduced lesion diameter. One susceptibility QTL increased infection frequency whereas the other increased lesion diameter. Overlapping chromosomal segments provided strong evidence for partial resistance on chromosomes 1 and 9 and for elevated susceptibility on chromosome 11. Segregation analysis confirmed the major resistance QTL on the long arm of chromosome 1 and susceptibility on chromosome 11. Linkage of partial resistance to chromosome 9 could not be confirmed. The usefulness of these data for resistance breeding and for map-based cloning of foliar resistance to B. cinerea is discussed.