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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Regulation of Phosphoenolpyruvate Carboxylase Phosphorylation by Metabolites and Abscisic Acid during the Development and Germination of Barley Seeds^1,[C],[W]

Departamento de Biología Vegetal, Facultad de Biología, Universidad de Sevilla, 41012 Seville, Spain (A.-B.F., R.A., L.C., S.G.-M., C.E.); and Institut de Biotechnologie des Plantes, UMR 8618, Université de Paris-Sud 11, 91405 Orsay cedex, France (J.V.)

During barley (Hordeum vulgare) seed development, phosphoenolpyruvate carboxylase (PEPC) activity increased and PEPC-specific antibodies revealed housekeeping (103-kD) and inducible (108-kD) subunits. Bacterial-type PEPC fragments were immunologically detected in denatured protein extracts from dry and imbibed conditions; however, on nondenaturing gels, the activity of the recently reported octameric PEPC (in castor [Ricinus communis] oil seeds) was not detected. The phosphorylation state of the PEPC, as judged by L-malate 50% inhibition of initial activity values, phosphoprotein chromatography, and immunodetection of the phosphorylated N terminus, was found to be high between 8 and 18 d postanthesis (DPA) and during imbibition. In contrast, the enzyme appeared to be in a low phosphorylation state from 20 DPA up to dry seed. The time course of 32/36-kD, Ca²⁺-independent PEPC kinase activity exhibited a substantial increase after 30 DPA that did not coincide with the PEPC phosphorylation profile. This kinase was found to be inhibited by L-malate and not by putative protein inhibitors, and the PEPC phosphorylation status correlated with high glucose-6-phosphate to malate ratios, thereby suggesting an in vivo metabolic control of the kinase. PEPC phosphorylation was also regulated by photosynthate supply at 11 DPA. In addition, when fed exogenously to imbibing seeds, abscisic acid significantly increased PEPC kinase activity. This was further enhanced by the cytosolic protein synthesis inhibitor cycloheximide but blocked by protease inhibitors, thereby suggesting that the phytohormone acts on the stability of the kinase. We propose that a similar abscisic acid-dependent effect may contribute to produce the increase in PEPC kinase activity during desiccation stages.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Cristina Echevarría (echeva{at}us.es).

^[C] Some figures in this article are displayed in color online but in black and white in print.

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.124982

^* Corresponding author; e-mail echeva{at}us.es.

Received June 19, 2008; accepted August 14, 2008; published August 27, 2008.