A Golden 2-Like Transcription Factor Influencing Fruit Chloroplast Development and Fruit Quality at the Tomato Uniform Ripening (u) Locus

The tomato uniform ripening (u) mutation confers a light green fruit phenotype absent the wild-type green shoulder and is widely used in breeding for tomato varieties producing evenly ripened fruit to facilitate determination 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 chlorophyll accumulation and distribution in developing fruit. Two GLKs could be identified in the tomato genome sequence — SlGLK1 and SlGLK2.  Both genes are expressed in leaves, but only SlGLK2 is expressed in fruit. Transgene-mediated ectopic expression of either SlGLK1 or SlGLK2 increased fruit chlorophyll content, while suppressing SlGLK2 mimicked the u mutant fruit phenotype and SlGLK1 suppression yielded plants with pale leaves and no notable impact on fruit. These results reflect a degree of functional redundancy of SlGLK1 and SlGLK2 and the more important role of SlGLK1 in leaves and SlGLK2 in fruit. Further characterization of our transgenic lines indicated that GLK over-expression up-regulated fruit photosynthesis related genes increasing chlorophyll content, chloroplast numbers and thylakoid grana stacks.  Ectopic GLK expression also elevated starch in green fruit and sugars, carotenoids and ascorbate in ripe fruit. We crossed our SlGLK2 over-expression lines with the tomato high pigment 1 (hp1) mutant, which has elevated chlorophyll and carotenoid content in fruit due to a loss of tomato DDB1 (DAMAGED DNA BINDING PROTEIN 1) activity and resulting SlGLK2;hp1/hp1 lines indicate that SlGLK2 acts independently of DDB1. Combining SlGLK2 overexpression and hp1 elevated ripe fruit carotenoids. SlGLK2 influences photosynthesis in developing fruit, contributes to mature fruit quality characteristics and our results suggest that manipulating GLK expression is a means to enhance fruit quality and nutritional value.