Regulation of phytoene desaturase expression is independent of leaf pigment content in Arabidopsis thaliana

Synthesis of carotenoids in higher plants occurs in the plastids, but all of the required enzymes are coded for in the nuclear genome and are post-transcriptionally imported into the plastid compartment. Regulation of the synthesis of the enzymes is poorly understood. The two-step desaturation of phytoene to zeta-carotene, carried out by the enzyme phytoene desaturase (PDS), is one of the earliest steps in the pathway and has been studied in several systems. Previous analyses of phytoene-accumulating tissue suggested that there may be feedback regulation of PDS gene transcription, with higher expression in white tissue. To investigate this regulation further, we examined phytoene-accumulating tissue in Arabidopsis thaliana (L.) Heynh. Two types of phytoene-accumulating tissue were studied: Norflurazon-bleached plants and white sectors from the immutans variegation mutant. Based on competitive RT-PCR measurements of PDS mRNA and immunochemical detection of PDS protein, we determined that there is no significant induction of PDS gene expression specific to white tissue, indicating that PDS expression is independent of the pigment status of the cells. Reasons why our results differ from those in other systems are discussed.