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Plant Physiology Preview Published on June 15, 2006; 10.1104/pp.106.082537
Received May 2, 2006 Salicylic acid, an ambimobile molecule exhibiting a high ability to accumulate in the phloem
Laboratoire "Synthèse et Réactivité des Substances Naturelles" (UMR 6514, CNRS), Université de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers cedex, France * Corresponding author; email: jl.bonnemain{at}voila.fr.
The ability of exogenous salicylic acid (SA) to accumulate in the 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 upwards in small amounts via the xylem. This contributes 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). Like 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 analogue 3,5-dichlorosalicylic acid, which is predicted to be non-mobile 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.
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