Heterologous expression of kiwifruit (Actinidia chinensis) GOLDEN2-LIKE homolog elevates chloroplast level and nutritional quality in tomato (Solanum lycopersicum).

MAIN CONCLUSION: Constitutive expression of AchGLK from A. chinensis in transgenic tomato (S. lycopersicum) confers fruits an elevated plastid biogenesis with intensified granule thylakoid stacks and increased content of nutritional compounds. Plastid development is regulated by multiple environmental and genetic factors. Golden2-like (GLK) genes, members from GARP subfamily of the MYB transcription factors, have been shown to regulate plastid biogenesis and development in several species. In tomato (S. lycopersicum), SlGLK2 gene is expressed in the fruit in a manner of latitudinal gradient and determines the green shoulder phenotype. Here, we report the isolation and characterization of a kiwifruit (A. chinensis) GLK homolog (AchGLK). Transcription analysis showed that AchGLK is highly expressed in mature leaves and in fruits 60-day post-anthesis. Overexpression of AchGLK in transgenic tomato resulted in dark green immature fruit with higher concentration of chlorophyll and overall increased chloroplast compartment, both number and size. The ripened fruit had higher levels of carotenoids and sugars as well. Nevertheless, no phenotypic changes were observed in the transgenic leaves, similar to the previously described overexpression of SlGLK2 in tomato. Our study suggests that AchGLK is functionally homologous to tomato GLK2 with a potential in regulating plastid level in fruit that could contribute to improvement of fruit quality.