A STAY-GREEN protein SlSGR1 regulates lycopene and β-carotene accumulation by interacting directly with SlPSY1 during ripening processes in tomato.

As a primary source of lycopene in the human diet, fleshy fruits synthesize this compound both de novo and via chlorophyll metabolism during ripening. SlSGR1 encodes a STAY-GREEN protein that plays a critical role in the regulation of chlorophyll degradation in tomato leaves and fruits. We report that SlSGR1 can regulate tomato (Solanum lycopersicum) lycopene accumulation through direct interaction with a key carotenoid synthetic enzyme SlPSY1, and can inhibit its activity. This interaction with SlSGR1 mediates lycopene accumulation during tomato fruit maturation. We confirmed this inhibitory activity in bacteria engineered to produce lycopene, where the introduction of SlSGR1 reduced dramatically lycopene biosynthesis. The repression of SlSGR1 in transgenic tomato fruits resulted in altered accumulation patterns of phytoene and lycopene, whilst simultaneously elevating SlPSY1 mRNA accumulation and plastid conversion at the early stages of fruit ripening, resulting in increased lycopene and β-carotene (four- and nine-fold, respectively) in red ripe fruits. SlSGR1 influences ethylene signal transduction via the altered expression of ethylene receptor genes and ethylene-induced genes. Fruit shelf-life is extended significantly in SlSGR1-repressed tomatoes. Our results indicate that SlSGR1 plays a pivotal regulatory role in color formation and fruit ripening regulation in tomato, and further suggest that SlSGR1 activity is mediated through direct interaction with PSY1.