Genetic and molecular mechanisms underlying the parthenocarpic fruit mutation in tomato.

Parthenocarpy allows fruit set independently of fertilization. In parthenocarpic-prone tomato genotypes, fruit set can be achieved under pollen-limiting environmental conditions and in sterile mutants. Parthenocarpy is also regarded as a quality-related trait, when seedlessness is associated with positive fruit quality aspects. Among the different sources of genetic parthenocarpy described in tomato, the parthenocarpic fruit (pat) mutation is of particular interest because of its strong expressivity, high fruit set, and enhanced fruit quality. The complexity of the pat "syndrome" associates a strong competence for parthenocarpy with a complex floral phenotype involving stamen and ovule developmental aberrations. To understand the genetic basis of the phenotype, we mapped the pat locus within a 0.19-cM window of Chr3, comprising nine coding loci. A non-tolerated missense mutation found in the 14th exon of Solyc03g120910, the tomato ortholog of the Arabidopsis HD-Zip III transcription factor HB15 (SlHB15), cosegregated with the pat phenotype. The role of SlHB15 in tomato reproductive development was supported by its expression in developing ovules. The link between pat and SlHB15 was validated by complementation and knock out experiments by co-suppression and CRISPR/Cas9 approaches. Comparing the phenotypes of pat and those of Arabidopsis HB15 mutants, we argued that the gene plays similar functions in species with fleshy and dry fruits, supporting a conserved mechanism of fruit set regulation in plants.

A transcriptomic approach to identify regulatory genes involved in fruit set of wild-type and parthenocarpic tomato genotypes.

The tomato parthenocarpic fruit (pat) mutation associates a strong competence for parthenocarpy with homeotic transformation of anthers and aberrancy of ovules. To dissect this complex floral phenotype, genes involved in the pollination-independent fruit set of the pat mutant were investigated by microarray analysis using wild-type and mutant ovaries. Normalized expression data were subjected to one-way ANOVA and 2499 differentially expressed genes (DEGs) displaying a >1.5 log-fold change in at least one of the pairwise comparisons analyzed were detected. DEGs were categorized into 20 clusters and clusters classified into five groups representing transcripts with similar expression dynamics. The "regulatory function" group (685 DEGs) contained putative negative or positive fruit set regulators, "pollination-dependent" (411 DEGs) included genes activated by pollination, "fruit growth-related" (815 DEGs) genes activated at early fruit growth. The last groups listed genes with different or similar expression pattern at all stages in the two genotypes. qRT-PCR validation of 20 DEGs plus other four selected genes assessed the high reliability of microarray expression data; the average correlation coefficient for the 20 DEGs was 0.90. In all the groups were evidenced relevant transcription factors encoding proteins regulating meristem differentiation and floral organ development, genes involved in metabolism, transport and response of hormones, genes involved in cell division and in primary and secondary metabolism. Among pathways related to secondary metabolites emerged genes related to the synthesis of flavonoids, supporting the recent evidence that these compounds are important at the fruit set phase. Selected genes showing a de-regulated expression pattern in pat were studied in other four parthenocarpic genotypes either genetically anonymous or carrying lesions in known gene sequences. This comparative approach offered novel insights for improving the present molecular understanding of fruit set and parthenocarpy in tomato.

Novel phenotypes related to the breeding of purple-fruited tomatoes and effect of peel extracts on human cancer cell proliferation.

The production of anthocyanins in the tomato (Solanum lycopersicum L.) fruit is normally absent or poor, but a number of mutants or introgression lines are known to increase anthocyanin levels in vegetative and reproductive tissues. Through conventional breeding, a genetic combination was obtained with the remarkable phenotype of a deep purple fruit pigmentation, due to an accumulation of anthocyanins on the peel. Such a genotype was named Sun Black (SB) as a consequence of its sensitivity to light induction. When characterized for morpho-agronomic traits, SB plants showed increased fertility. Purple fruits displayed an arrangement of the epicarp cells different from normal tomatoes, a feature that could account for different mechanical properties and shelf-life potential. The SB genotype and, to a lesser extent, its single mutant parents showed the capacity to accumulate anthocyanins in the seedling root when grown under light. This phenotype, which was greatly improved by the addition of sucrose to the germination medium, proved to be useful as selection index and gave new insights for in vitro production of anthocyanin extracts. To assess the nutraceutical potential of purple tomatoes, we tested the activity of SB skin extracts on the proliferation of two human cancer cells lines. Cell proliferation was significantly inhibited by SB extract in a dose-dependent manner. When the bioactivity of SB extracts was compared with that of other anthocyanin-containing fruits or vegetables, a significant "Extract*Line" interaction was evidenced, suggesting a crucial role for the extract composition in terms of anthocyanidins and other eventual cell growth-inhibiting compounds.