title: Characterization of salt stress-enhanced phosphoenolpyruvate carboxylase kinase activity in leaves of Sorghum vulgare: independence from osmotic stress, involvement of ion toxicity and significance of dark phosphorylation.
abstract: C(4) phosphoenolpyruvate carboxylase (PEPCase: EC 4.1.1.31) is subjected to in vivo regulatory phosphorylation by a light up-regulated, calcium-independent protein kinase. Salt stress greatly enhanced phosphoenolpyruvate carboxylase-kinase (PEPCase-k) activity in leaves of Sorghum. The increase in PEPCase-k anticipated the time course of proline accumulation thereby suggesting that water stress was not involved in the kinase response to salt. Moreover, osmotic stress seemed not to be the main factor implicated, as demonstrated by the lack of effect when water availability was restricted by mannitol. In contrast, LiCl (at a concentration of 10 mM in short-term treatment of both excised leaves and whole plants) mimicked the effects of 172 mM NaCl salt-acclimation, indicating that the rise in PEPCase-k activity resulted primarily from the ionic stress. Both NaCl and LiCl treatments increased the activity of a Ca(2+)-independent, 35 kDa kinase, as demonstrated by an in-gel phosphorylation experiment. Short-term treatment of excised leaves with NaCl or LiCl partially reproduces the effects of whole plant treatments. Finally, salinization also increased PEPCase-k activity and the phosphorylation state of PEPCase in darkened Sorghum leaves. This fact, together with increased malate production during the dark period, suggests a shift towards mixed C(4) and crassulacean acid metabolism types of photosynthesis in response to salt stress.
authors.affiliation: Departamento de Biología Vegetal, Facultad de Biología, Universidad de Sevilla, Avenida Reina Mercedes no. 6, 41012 Seville, Spain. sgarma@us.es
authors.forename: Sofía
authors.initials: S
authors.lastname: García-Mauriño
authors.forename: José Antonio
authors.initials: JA
authors.lastname: Monreal
authors.forename: Rosario
authors.initials: R
authors.lastname: Alvarez
authors.forename: Jean
authors.initials: J
authors.lastname: Vidal
authors.forename: Cristina
authors.initials: C
authors.lastname: Echevarría
mesh_terms: D000222:Adaptation, Physiological / Q000187:drug effects* / Q000528:radiation effects; D003624:Darkness; D008027:Light; D018021:Lithium Chloride / Q000494:pharmacology; D008293:Malates / Q000378:metabolism; D019764:Phosphoenolpyruvate Carboxykinase (ATP) / Q000378:metabolism*; D010730:Phosphoenolpyruvate Carboxylase / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects / Q000528:radiation effects; D010788:Photosynthesis / Q000187:drug effects / Q000528:radiation effects; D045322:Photosynthetic Reaction Center Complex Proteins / Q000145:classification / Q000187:drug effects / Q000528:radiation effects; D018515:Plant Leaves / Q000187:drug effects / Q000201:enzymology / Q000528:radiation effects; D018526:Plant Transpiration / Q000187:drug effects / Q000528:radiation effects; D006109:Poaceae / Q000187:drug effects / Q000201:enzymology* / Q000528:radiation effects; D012965:Sodium Chloride / Q000494:pharmacology*
chemical_list: D008293:Malates; D045322:Photosynthetic Reaction Center Complex Proteins; D012965:Sodium Chloride; C030298:malic acid; D010730:Phosphoenolpyruvate Carboxylase; D019764:Phosphoenolpyruvate Carboxykinase (ATP); D018021:Lithium Chloride