title: Light-dependent activation of phosphoenolpyruvate carboxylase by reversible phosphorylation in cluster roots of white lupin plants: diurnal control in response to photosynthate supply.
abstract: Background and Aims Phosphoenolpyruvate carboxylase (PEPC) is a tightly regulated enzyme that controls carbohydrate partitioning to organic acid anions (malate, citrate) excreted in copious amounts by cluster roots of inorganic phosphate (Pi)-deprived white lupin plants. Excreted malate and citrate solubilize otherwise inaccessible sources of mineralized soil Pi for plant uptake. The aim of this study was to test the hypotheses that (1) PEPC is post-translationally activated by reversible phosphorylation in cluster roots of illuminated white lupin plants, and (2) light-dependent phosphorylation of cluster root PEPC is associated with elevated intracellular levels of sucrose and its signalling metabolite, trehalose-6-phosphate. Methods White lupin plants were cultivated hydroponically at low Pi levels (≤1 µm) and subjected to various light/dark pretreatments. Cluster root PEPC activity and in vivo phosphorylation status were analysed to assess the enzyme's diurnal, post-translational control in response to light and dark. Levels of various metabolites, including sucrose and trehalose-6-phosphate, were also quantified in cluster root extracts using enzymatic and spectrometric methods. Key Results During the daytime the cluster root PEPC was activated by phosphorylation at its conserved N-terminal seryl residue. Darkness triggered a progressive reduction in PEPC phosphorylation to undetectable levels, and this was correlated with 75-80 % decreases in concentrations of sucrose and trehalose-6- phosphate. Conclusions Reversible, light-dependent regulatory PEPC phosphorylation occurs in cluster roots of Pi-deprived white lupin plants. This likely facilitates the well-documented light- and sucrose-dependent exudation of Pi-solubilizing organic acid anions by the cluster roots. PEPC's in vivo phosphorylation status appears to be modulated by sucrose translocated from CO2-fixing leaves into the non-photosynthetic cluster roots.
authors.affiliation: School of Plant Biology (M084), Faculty of Natural and Agricultural Sciences, The University of Western Australia, Crawley 6009, Australia.|Plant Ecophysiology and Evolution Group, State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources, and College of Forestry, Guangxi University, Daxuedonglu 100, Nanning 530004, Guangxi, China.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Shane
authors.affiliation: Department of Metabolic Networks, Max Planck Institute of Molecular Plant Physiology, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany.
authors.forename: Regina
authors.initials: R
authors.lastname: Feil
authors.affiliation: Department of Metabolic Networks, Max Planck Institute of Molecular Plant Physiology, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany.
authors.forename: John E
authors.initials: JE
authors.lastname: Lunn
authors.affiliation: Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada.|Department of Biomedical and Molecular Biosciences Queen's University, Kingston, Ontario K7L 3N6, Canada.
authors.forename: William C
authors.initials: WC
authors.lastname: Plaxton
keywords: Cluster (proteoid) roots; Lupinus albus L. (white lupin); phosphate limitation; phosphoenolpyruvate carboxylase; protein phosphorylation; sucrose signalling; trehalose-6-phosphate