Identifying target enzymes for the regulation of carbon partitioning associated to starch and sugar-alcohols metabolism

IGLESIAS, ALBERTO A.; FIGUEROA, CARLOS M.; PIATTONI, CLAUDIA V.; HARTMAN, MATÍAS D.

Holderness, New Hamphsire

Conferencia; Plant Molecular Biology, Genomic Approaches to Plant Signaling Systems; 2012

Gordon Research Conferences

Photoassimilates partitioning is a complex process in higher plants that takes place at the intracellular level as well as between photosynthetic (source) and heterotrophic (sink) tissues. In most plants, starch and sucrose are the main final products of photosynthesis; but many species pertaining to the Rosaceae and Plantaginaceae families, also produce important amounts of sugar-alcohols. To gain information about regulation of carbohydrate metabolism in the cytosol of plant cells we studied posttranslational phosphorylation of enzymes involved in key steps of carbon partitioning. Specifically, we characterized modification of wheat glyceraldehyde-3-phosphate dehydrogenase [both, the phosphorylating (Ga3PDHase, EC 1.2.1.12) and the non-phosphorylating (np-Ga3PDHase, EC 1.2.1.9) enzymes], and of peach aldose-6-phosphate reductase (Ald-6PRase, EC 1.1.1.200). The pair Ga3PDHase/np-Ga3PDHase conform a branch point in glycolysis by which triose-P are utilized to alternatively produce ATP and NADH or NADPH. It is expected that such an alternative, critical from a bioenergetic view, should be regulated. In such a context, we found that both enzymes are target of phosphorylation by a SnRK1-like protein kinase. The latter was found to be regulated by different metabolites, with ribose-5P behaving as a main inhibitor. We identified that np-Ga3PDHase is phosphorylated at serine-404 only in heterotrophic tissues; whereas serine-205 is the phosphorylation site in Ga3PDHase. On the other hand, Ald-6PRase is involved in the production of the sugar-alcohol glucitol in leaves. We found that purified Ald-6PRase was phosphorylated after incubation with extracts from mature peach leaves, mainly under conditions favoring the activity of a protein kinase from the SOS2 family. The kinase was partially purified and characterized as being sensitive to inhibition mainly by ribose-5P or pyrophosphate, and also by glucose-6P, glucitol-6P, fructose-1,6-bisP or phosphoenolpyruvate. The overall picture supports that triose-P oxidation and glucose-6P reduction are key steps for photoassimilates partitioning that could be under a fine regulation. Signaling for such a control could be levels of specific metabolites acting in combination with certain protein kinases to exert phosphorylation of the target enzymes. It is of value to discuss results in the framework of the different metabolic scenarios taking place in plant cells exhibiting autotrophic or heterotrophic characteristics for carbon metabolism.