AGE-DEPENDENT INDUCTION OF PYRUVATE, ORTHOPHOSPHATE DIKINASE IN MESEMBRYANTHEMUM-CRYSTALLINUM L

A full-length transcript for pyruvate, orthophosphate dikinase (PPDK; EC 2.7.9.1), has been characterized from Mesembryanthemum crystallinum . Under salt stress or with increasing age, this plant shows a transit ion from C-3 to Crassulacean acid metabolism (CAM). The PPDK plays a c entral role in gluconeogenesis during the light phase of CAM. The tran script is 3165 bases in length with a single open reading frame of 273 9 nucleotides specifying a protein of molecular mass 103098, including a transit peptide of mass 7902 for chloroplast import. The protein sh ares 44-77% sequence identity with PPDK from C-4-plants and microorgan isms. Known functional and regulatory amino acids are conserved. South ern-type hybridizations indicated one copy or very few closely related copies of the gene per haploid genome. We investigated the induction of PPDK at the mRNA and protein revels, using the well characterized i nduction of a CAM-form of phosphoenol pyruvate carboxylase (PEPCase) a s internal standard. During wilting of excised leaves PEPCase mRNA amo unts increased strongly within 8 h. Under these conditions amounts of PPDK mRNA remained constant. Re-hydrating leaves from previously stres sed plants led to a decrease in PEPCase and PPDK mRNA amounts. During salt stress, no correlation between PEPCase and PPDK was observed. Ana lysis of plants of different ages indicated that, even in well-watered plants, PPDK-specific protein and mRNA increased when the plants reac hed a certain age. In old plants, salt stress failed to further increa se PPDK mRNA. or protein levels. We conclude that PPDK and PEPCase, wh ich are both required for CAM to be functional, are controlled by diff erent regulatory mechanisms in the intact plant.