Transcript Abundance of Phytoene Synthase 1 and Phytoene Synthase 2 Is Associated with Natural Variation of Storage Root Carotenoid Pigmentation in Carrot

Abstract

Carotenoids are isoprenoid compounds synthesized in plants that serve as photoprotectants essential for photosynthesis and provide plant tissues with red, orange, and yellow pigmentation. These compounds are important in human health, because they serve as both vitamin A precursors as well as having antioxidant properties. Carrot (Daucus carota ssp. sativus) provides an important source of carotenoids in the human diet, providing up to 30% of provitamin A in the United States. Although essential to human health, very little is currently understood about the accumulation of carotenoids in carrot. To better understand the molecular mechanism for carotenoid accumulation in carrot, we used reverse-transcription quantitative polymerase chain reaction (PCR) to evaluate the expression of nine genes in the carotenoid biosynthetic pathway in storage root tissue. No significant difference was found among white, yellow, orange, and dark orange carrot roots in seven of the nine genes evaluated. However, increased phytoene synthase 1 (PSY1) and phytoene synthase 2 (PSY2) expression was observed in orange and dark orange carrot roots compared with yellow and white carrots. Increased PSY1 and PSY2 expression was not observed in the leaf tissue of these genotypes, indicating a different mechanism for carotenoid accumulation in the leaf tissue of carrot. This study is the first to demonstrate that naturally occurring mutations that dramatically increase carotenoid accumulation in orange carrot are associated with increased PSY1 and PSY2 expression and it provides insights into the mechanism underlying the biosynthesis of these important photoprotectants and nutrients.