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 mM) 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 Nterminal 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 CO 2 -fixing leaves into the non-photosynthetic cluster roots.

Leaf Phosphorus Concentration Regulates the Development of Cluster Roots and Exudation of Carboxylates in Macadamia integrifolia

“…Exudation of malate and citrate was strongly stimulated under 0 μM P, as reported for other species bearing cluster roots when growing under low-P conditions ( Shane et al, 2004c ; Delgado et al, 2014 ), and citrate was exuded relatively more than other carboxylates ( Supplementary Table S1 ). A possible explanation is that substantial exudation of citrate and malate, especially citrate, has been linked to increased biosynthesis and decreased metabolism of citrate in the tricarboxylic acid cycle ( Neumann et al, 1999 , 2000 ; Shane et al, 2016 ).…”

A Tale of Two Sugars: Trehalose 6-Phosphate and Sucrose

“…As an aside, a correlative relationship between Suc, Tre6P, and the posttranslational activation status of PEPC has also recently been reported in cluster roots of white lupin (Lupinus albus), which synthesizes organic acids in the roots and exudes them into the rhizosphere to mobilize Pi, iron, and other nutrients for uptake by the roots (Shane et al, 2016). White lupin root exudates can represent up to 25% of the plant's overall C budget, and it was suggested that Tre6P links the activation status of PEPC and anaplerotic synthesis of organic acids in the roots to the supply of Suc from the source leaves.…”

Coimmunoprecipitation of reversibly glycosylated polypeptide with sucrose synthase from developing castor oilseeds

“…A–D). Cluster roots are strong photosynthate sinks that excrete copious amounts of sucrose‐derived organic anions into the surrounding rhizosphere to solubilize otherwise inaccessible sources of mineralized soil P i for plant uptake . We also detected p41 protein‐staining polypeptides of varying intensities following anti‐RcSUS1 co‐IPs of developing sunflower, pine, and maize seed extracts (Fig.…”

“…Castor ( R. communis ; cv Baker 296), white lupin ( Lupinus albus ), and Harsh Hakea ( Hakea prostrata ) plants were cultivated in a greenhouse as previously described . Developing sweet corn ( Zea mays ) cobs and sunflower ( Helianthus annuus ) heads were collected in mid‐August from intact plants cultivated outdoors at Forman Farms near Seeley's Bay, Ontario, Canada.…”