A Lycopene ε-Cyclase TILLING Allele Enhances Lycopene and Carotenoid Content in Fruit and Improves Drought Stress Tolerance in Tomato Plants.

In the scenario of climate change, the availability of genetic resources for tomato cultivation that combine improved nutritional properties and more tolerance to water deficiency is highly desirable. Within this context, the molecular screenings of the Red Setter cultivar-based TILLING platform led to the isolation of a novel lycopene ε-cyclase gene (SlLCY-E) variant (G/3378/T) that produces modifications in the carotenoid content of tomato leaves and fruits. In leaf tissue, the novel G/3378/T SlLCY-E allele enhances ß,ß-xanthophyll content at the expense of lutein, which decreases, while in ripe tomato fruit the TILLING mutation induces a significant increase in lycopene and total carotenoid content. Under drought stress conditions, the G/3378/T SlLCY-E plants produce more abscisic acid (ABA) and still conserve their leaf carotenoid profile (reduction of lutein and increase in ß,ß-xanthophyll content). Furthermore, under said conditions, the mutant plants grow much better and are more tolerant to drought stress, as revealed by digital-based image analysis and in vivo monitoring of the OECT (Organic Electrochemical Transistor) sensor. Altogether, our data indicate that the novel TILLING SlLCY-E allelic variant is a valuable genetic resource that can be used for developing new tomato varieties, improved in drought stress tolerance and enriched in fruit lycopene and carotenoid content.

Induced mutations in SlE8 and SlACO1 control tomato fruit maturation and shelf-life.

Fruit maturation and softening are critical traits that control fruit shelf-life. In the climacteric tomato (Solanum lycopersicum L.) fruit, ethylene plays a key role in fruit ripening and softening. We characterized two related proteins with contrasting impact on ethylene production, ACC oxidase 1 (SlACO1) and SlE8. We found SlACO1 and SlE8 to be highly expressed during fruit ripening. To identify loss-of-function alleles, we analysed the tomato genetic diversity but we did not find any natural mutations impairing the function of these proteins. We also found the two loci evolving under purifying selection. To engineer hypomorphic alleles, we used TILLING (target-induced local lesions in genomes) to screen a tomato ethylmethane sulfonate-mutagenized population. We found 13 mutants that we phenotyped for ethylene production, shelf-life, firmness, conductivity, and soluble solid content in tomato fruits. The data demonstrated that slaco1-1 and slaco1-2 alleles could be used to improve fruit shelf-life, and that sle8-1 and sle8-2 alleles could be used to accelerate ripening. This study highlights further the importance of SlACO1 and SlE8 in ethylene production in tomato fruit and how they might be used for post-harvest fruit preservation or speeding up fruit maturation.

Identification and Characterization of a Thermotolerant TILLING Allele of Heat Shock Binding Protein 1 in Tomato.

The identification of heat stress (HS)-resilient germplasm is important to ensure food security under less favorable environmental conditions. For that, germplasm with an altered activity of factors regulating the HS response is an important genetic tool for crop improvement. Heat shock binding protein (HSBP) is one of the main negative regulators of HS response, acting as a repressor of the activity of HS transcription factors. We identified a TILLING allele of Solanum lycopersicum (tomato) HSBP1. We examined the effects of the mutation on the functionality of the protein in tomato protoplasts, and compared the thermotolerance capacity of lines carrying the wild-type and mutant alleles of HSBP1. The methionine-to-isoleucine mutation in the central heptad repeats of HSBP1 leads to a partial loss of protein function, thereby reducing the inhibitory effect on Hsf activity. Mutant seedlings show enhanced basal thermotolerance, while mature plants exhibit increased resilience in repeated HS treatments, as shown by several physiological parameters. Importantly, plants that are homozygous for the wild-type or mutant HSBP1 alleles showed no significant differences under non-stressed conditions. Altogether, these results indicate that the identified mutant HSBP1 allele can be used as a genetic tool in breeding, aiming to improve the thermotolerance of tomato varieties.

Induced mutations in tomato SlExp1 alter cell wall metabolism and delay fruit softening.

Fruit ripening and softening are key traits for many fleshy fruit. Since cell walls play a key role in the softening process, expansins have been investigated to control fruit over ripening and deterioration. In tomato, expression of Expansin 1 gene, SlExp1, during fruit ripening was associated with fruit softening. To engineer tomato plants with long shelf life, we screened for mutant plants impaired in SlExp1 function. Characterization of two induced mutations, Slexp1-6_W211S, and Slexp1-7_Q213Stop, showed that SlExp1 loss of function leads to enhanced fruit firmness and delayed fruit ripening. Analysis of cell wall polysaccharide composition of Slexp1-7_Q213Stop mutant pointed out significant differences for uronic acid, neutral sugar and total sugar contents. Hemicelluloses chemistry analysis by endo-ß-1,4-d-glucanase hydrolysis and MALDI-TOF spectrometry revealed that xyloglucan structures were affected in the fruit pericarp of Slexp1-7_Q213Stop mutant. Altogether, these results demonstrated that SlExp1 loss of function mutants yield firmer and late ripening fruits through modification of hemicellulose structure. These SlExp1 mutants represent good tools for breeding long shelf life tomato lines with contrasted fruit texture as well as for the understanding of the cell wall polysaccharide assembly dynamics in fleshy fruits.

Identification of candidate genes for phenolics accumulation in tomato fruit.

Phenolics are antioxidants present in tomato fruit that confer healthy benefits and exhibit crucial roles for plant metabolism and response to environmental stimuli. An approach based on two genomics platforms was undertaken to identify candidate genes associated to higher phenolics content in tomato fruit. A comparative transcriptomic analysis between the S. pennellii Introgression Line 7-3, which produced an average higher level of fruit phenolics, and the cultivated variety M82, revealed that their differences are attributed to genes involved in phenolics accumulation into the vacuole. The up-regulation of genes coding for one MATE-transporter, one vacuolar sorting protein and three GSTs supported this hypothesis. The observed balancing effect between two ethylene responsive factors (ERF1 and ERF4) was also hypothesized to drive the transcriptional regulation of these transport genes. In order to confirm such model a TILLING platform was explored. A mutant was isolated harbouring a point mutation in the ERF1 cds that affects the protein sequence and its expected function. Fruits of the mutant exhibited a significant reduced level of phenolics than the control variety. Changes in the expression of genes involved in sequestration of phenolics in vacuole also supported the hypothesized key-role of ERF1 in orchestrating these genes.

A new mutant genetic resource for tomato crop improvement by TILLING technology.

BACKGROUND: In the last decade, the availability of gene sequences of many plant species, including tomato, has encouraged the development of strategies that do not rely on genetic transformation techniques (GMOs) for imparting desired traits in crops. One of these new emerging technology is TILLING (Targeting Induced Local Lesions In Genomes), a reverse genetics tool, which is proving to be very valuable in creating new traits in different crop species. RESULTS: To apply TILLING to tomato, a new mutant collection was generated in the genetic background of the processing tomato cultivar Red Setter by treating seeds with two different ethylemethane sulfonate doses (0.7% and 1%). An associated phenotype database, LycoTILL, was developed and a TILLING platform was also established. The interactive and evolving database is available online to the community for phenotypic alteration inquiries. To validate the Red Setter TILLING platform, induced point mutations were searched in 7 tomato genes with the mismatch-specific ENDO1 nuclease. In total 9.5 kb of tomato genome were screened and 66 nucleotide substitutions were identified. The overall mutation density was estimated and it resulted to be 1/322 kb and 1/574 kb for the 1% EMS and 0.7% EMS treatment respectively. CONCLUSIONS: The mutation density estimated in our collection and its comparison with other TILLING populations demonstrate that the Red Setter genetic resource is suitable for use in high-throughput mutation discovery. The Red Setter TILLING platform is open to the research community and is publicly available via web for requesting mutation screening services.

The tomato Blind gene encodes a MYB transcription factor that controls the formation of lateral meristems.

The multitude of forms observed in flowering plants is largely because of their ability to establish new axes of growth during postembryonic development. This process is initiated by the formation of secondary meristems that develop into vegetative or reproductive branches. In the blind and torosa mutants of tomato, initiation of lateral meristems is blocked during shoot and inflorescence development, leading to a strong reduction in the number of lateral axes. In this study, it is shown that blind and torosa are allelic. The Blind gene has been isolated by positional cloning, and it was found that the mutant phenotype is caused by a loss of function of an R2R3 class Myb gene. RNA interference-induced blind phenocopies confirmed the identity of the isolated gene. Double mutant analysis shows that Blind acts in a novel pathway different from the one to which the previously identified Lateral suppressor gene belongs. The findings reported add a new class of transcription factors to the group of genes controlling lateral meristem initiation and reveal a previously uncharacterized function of R2R3 Myb genes.