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WHOLE PLANT AND ECOPHYSIOLOGY

Salicylic Acid, an Ambimobile Molecule Exhibiting a High Ability to Accumulate in the Phloem¹

Laboratoire Synthèse et Réactivité des Substances Naturelles, Unité Mixte de Recherche 6514 (F.R., J.-F.C., C.J.), and Laboratoire Transport des Assimilats, Unité Mixte de Recherche 6161 (C.J., J.-L.B.), Centre National de la Recherche Scientifique, Université de Poitiers, 86022 Poitiers cedex, France

The ability of exogenous salicylic acid (SA) to accumulate in castor bean (Ricinus communis) phloem was evaluated by HPLC and liquid scintillation spectrometry analyses of phloem sap collected from the severed apical part of seedlings. Time-course experiments indicated that SA was transported to the root system via the phloem and redistributed upward in small amounts via the xylem. This helps to explain the peculiarities of SA distribution within the plant in response to biotic stress and exogenous SA application. Phloem loading of SA at 1, 10, or 100 µM was dependent on the pH of the cotyledon incubating solution, and accumulation in the phloem sap was the highest (about 10-fold) at the most acidic pH values tested (pH 4.6 and 5.0). As in animal cells, SA uptake still occurred at pH values close to neutrality (i.e. when SA is only in its dissociated form according to the calculations made by ACD LogD suite software). The analog 3,5-dichlorosalicylic acid, which is predicted to be nonmobile according to the models of Bromilow and Kleier, also moved in the sieve tubes. These discrepancies and other data may give rise to the hypothesis of a possible involvement of a pH-dependent carrier system translocating aromatic monocarboxylic acids in addition to the ion-trap mechanism.

² These authors contributed equally to the paper.

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: Jean-Louis Bonnemain (jl.bonnemain{at}voila.fr).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.082537.

^* Corresponding author; e-mail jl.bonnemain{at}voila.fr; fax 33–5–49–45–39–65.

Received May 2, 2006; returned for revision May 2, 2006; accepted June 9, 2006.

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