Regulatory control of carotenoid accumulation in winter squash during storage

Storage promotes carotenoid accumulation and converts amylochromoplasts into chromoplasts in winter squash. Such carotenoid enhancement is likely due to continuous biosynthesis along with reduced turnover and/or enhanced sequestration. Postharvest storage of fruits and vegetables is often required and frequently results in nutritional quality change. In this study, we investigated carotenoid storage plastids, carotenoid content, and its regulation during 3-month storage of winter squash butternut fruits. We showed that storage improved visual appearance of fruit flesh color from light to dark orange, and promoted continuous accumulation of carotenoids during the first 2-month storage. Such an increased carotenoid accumulation was found to be concomitant with starch breakdown, resulting in the conversion of amylochromoplasts into chromoplasts. The butternut fruits contained predominantly β-carotene, lutein, and violaxanthin. Increased ratios of β-carotene and violaxanthin to total carotenoids were noticed during the storage. Analysis of carotenoid metabolic gene expression and PSY protein level revealed a decreased expression of carotenogenic genes and PSY protein following the storage, indicating that the increased carotenoid level might not be due to increased biosynthesis. Instead, the increase likely resulted from a continuous biosynthesis with a possibly reduced turnover and/or enhanced sequestration, suggesting a complex regulation of carotenoid accumulation during fruit storage. This study provides important information to our understanding of carotenogenesis and its regulation during postharvest storage of fruits.