Molecular Basis Of The Tomato Green Shoulder Phenotype, Its Role In Fruit Nutritional Quality And Characterization Of Molecular Gradients In Fruit Development

Abstract

The tomato uniform ripening (u) mutation confers a light green fruit phenotype absent the wild-type dark green top or "green shoulder" and is widely used in breeding for tomatoes producing evenly ripened fruit, facilitating selection of harvest time and sale of visually appealing fresh fruit. Positional cloning of u revealed that U encodes a Golden 2-like (GLK) transcription factor, SlGLK2, influencing chloroplast and chlorophyll accumulation and distribution in developing fruit. Transgene-mediated ectopic expression of two GLK genes, SlGLK1 and SlGLK2, affects chlorophyll content, chloroplast number and thylakoid grana stacks. Ectopic GLK expression also elevated levels of starch in green fruit and sugars, carotenoids and ascorbic acid in ripe fruit indicating that manipulating GLK expression is a means to enhance fruit nutritional quality. Use of the u allele thus likely confers an unintended consequence of reduced fruit nutritional value. To test this hypothesis, nearly isogenic lines for the U locus in an elite background (cv. M82) were developed to assess the effects of the green shoulder phenotype on fruit quality. Although plants with green shoulder fruit have 5-10% more fruit with a yellow shoulder phenotype than u plants, U fruit displayed 10-20% increases in soluble solids and carotenoids compared to u fruit. The isogenic lines also present tightly linked and novel genetic polymorphisms with potential as markers for molecular assisted selection. A latitudinal gradient of GLK2 expression influences the typical uneven coloration of green and ripe wild-type fruit. Transcriptome profiling highlighted a broader gene expression gradient through fruit development revealing additional molecular complexity during ripening. As with the latitudinal gradient, spatial and temporal gene expression differences in different fruit tissues are still poorly understood. Comparing transcriptomes from locular and pericarp tissues during fruit development suggests that climacteric ethylene and overall ripening may initiate in the locules.