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 transition from C-3 to Crassulacean acid metabolism (CAM). The PPDK plays a central role in gluconeogenesis during the light phase of CAM. The transcript is 3165 bases in length with a single open reading frame of 2739 nucleotides specifying a protein of molecular mass 103098, including a transit peptide of mass 7902 for chloroplast import. The protein shares 44-77% sequence identity with PPDK from C-4-plants and microorganisms. Known functional and regulatory amino acids are conserved. Southern-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 induction of a CAM-form of phosphoenol pyruvate carboxylase (PEPCase) as internal standard. During wilting of excised leaves PEPCase mRNA amounts increased strongly within 8 h. Under these conditions amounts of PPDK mRNA remained constant. Re-hydrating leaves from previously stressed plants led to a decrease in PEPCase and PPDK mRNA amounts. During salt stress, no correlation between PEPCase and PPDK was observed. Analysis of plants of different ages indicated that, even in well-watered plants, PPDK-specific protein and mRNA increased when the plants reached a certain age. In old plants, salt stress failed to further increase PPDK mRNA. or protein levels. We conclude that PPDK and PEPCase, which are both required for CAM to be functional, are controlled by different regulatory mechanisms in the intact plant.