| The fixation of carbon by the photosynthetic activity of plants, particularly the leaves, is a fundamental process in functioning organs and structures. In most plants, this process has been broadly studied in leaves, leaving the regulation and importance of the green fruit aside. Green fruit contain functional chloroplasts, which can capture sunlight energy and fix carbon and also function as precursors of others metabolites that accumulate in fruit with the ripening process. In tomato (S. lycopersicum L.), we identified that two Golden 2-like (GLK) transcription factors (SlGLK1 and SlGLK2) expressed in leaves, but only SlGLK2 is expressed in fruit. The organ-specific pattern of GLK s expression is similar in fruit from other plants. A single base mutation in SlGLK2 results in light green tomato fruit early in development; and this mutation corresponds to the uniform ripening (u) trait that was identified over 70 years ago and introduced by breeding into most modern commercial tomato varieties. In the absence of this SlGLK2 mutation, unripe fruit exhibit the (U) phenotype, evident as dark green shoulders on the stem end. Constitutive overexpression of GLKs resulted in uniformly dark-green unripe fruit with larger and more chloroplasts, which contain twice as much developed thylakoid grana and four-fold more chlorophyll. The fruit that ripen from these dark green fruit have increased soluble sugars and carotenoids. The SlGLK2 transcription factor determines the extent of chloroplast formation and chlorophyll biosynthesis in unripe fruit and influences the fruit's capacity for photosynthesis, making it an attractive trait to be incorporated into breeding selections for improving the quality and nutritional contents of fruit. |
