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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

A Reassessment of the Function of the So-Called Compatible Solutes in the Halophytic Plumbaginaceae Limonium latifolium^1,[W],[OA]

Unité Mixte de Recherche 6026 Interactions Cellulaires et Moléculaires (D.G., C.D., R.L., F.R.L., A.B.) and Unité Mixte de Recherche 6553 Ecosystèmes, Biodiversité, Evolution (A.A.), Université de Rennes 1, Centre National de la Recherche Scientifique, Campus de Beaulieu, 35042 Rennes cedex, France

The compatible solute hypothesis posits that maintaining osmotic equilibrium under conditions of high salinity requires synthesis of organic compounds, uptake of potassium ions, and partial exclusion of NaCl. To assess whether osmotic adaptation in Limonium latifolium proceeds according to this hypothesis, a comprehensive analysis of solute accumulation during NaCl treatments was conducted. Determination of prevailing inorganic ions and establishment of the metabolic profiles for low M_r organic substances revealed that contrary to the mentioned hypothesis the major contributors to osmolarity were constituted by inorganic solutes. Independent of salinity, only 25% of this osmolarity resulted from organic solutes such as Suc and hexoses. Proline (Pro), -alanine betaine, and choline-O-sulfate were minor contributors to osmolarity. Compatible inositols also occurred, especially chiro-inositol, characterized for the first time in this species, to our knowledge. Principal component analysis showed that only a limited number of metabolic reconfigurations occurred in response to dynamic changes in salinity. Under such conditions only sugars, chiro-inositol, and Pro behave as active osmobalancers. Analysis of metabolic profiles during acclimatization to either mild salinity or nonsaline conditions showed that organic solute accumulation is predominantly controlled by constitutive developmental programs, some of which might be slightly modulated by salinity. Osmolarity provided under such conditions can be sufficient to maintain turgor in salinized seedlings. Compartmental analysis of Pro and -alanine betaine in leaf tissues demonstrated that these solutes, mainly located in vacuoles under nonsaline conditions, could be partly directed to the cytosol in response to salinization. Thus they did not conform with the predictions of the compatible solute hypothesis.

² Present address: Department of Plant Biology, Michigan State University, East Lansing, MI 48824.

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: Alain Bouchereau (alain.bouchereau{at}univ-rennes1.fr).

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

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www.plantphysiol.org/cgi/doi/10.1104/pp.107.099820

^* Corresponding author; e-mail alain.bouchereau{at}univ-rennes1.fr; fax 33–02–23–23–69–15.

Received March 20, 2007; accepted April 6, 2007; published April 27, 2007.

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