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WHOLE PLANT AND ECOPHYSIOLOGY

Salicylic Acid Transport in Ricinus communis Involves a pH-Dependent Carrier System in Addition to Diffusion^1,[OA]

Laboratoire Synthèse et Réactivité des Substances Naturelles, Université de Poitiers, UMR CNRS 6514, F–86022 Poitiers cedex, France (F.R., J.-F.C., S.L., C.J.); Laboratoire Physiologie Moléculaire du Transport des Sucres chez les Végétaux, Université de Poitiers, FRE CNRS 3091, F–86022 Poitiers cedex, France (C.J., R.L., M.F., J.-L.B.); and Department of Biology, Program in Molecular Plant Biology, Colorado State University, Fort Collins, Colorado 80523 (D.R.B.)

Despite its important functions in plant physiology and defense, the membrane transport mechanism of salicylic acid (SA) is poorly documented due to the general assumption that SA is taken up by plant cells via the ion trap mechanism. Using Ricinus communis seedlings and modeling tools (ACD LogD and Vega ZZ softwares), we show that phloem accumulation of SA and hydroxylated analogs is completely uncorrelated with the physicochemical parameters suitable for diffusion (number of hydrogen bond donors, polar surface area, and, especially, LogD values at apoplastic pHs and LogD between apoplast and phloem sap pH values). These and other data (such as accumulation in phloem sap of the poorly permeant dissociated form of monohalogen derivatives from apoplast and inhibition of SA transport by the thiol reagent p-chloromercuribenzenesulfonic acid [pCMBS]) lead to the following conclusions. As in intestinal cells, SA transport in Ricinus involves a pH-dependent carrier system sensitive to pCMBS; this carrier can translocate monohalogen analogs in the anionic form; the efficiency of phloem transport of hydroxylated benzoic acid derivatives is tightly dependent on the position of the hydroxyl group on the aromatic ring (SA corresponds to the optimal position) but moderately affected by halogen addition in position 5, which is known to increase plant defense. Furthermore, combining time-course experiments and pCMBS used as a tool, we give information about the localization of the SA carrier. SA uptake by epidermal cells (i.e. the step preceding the symplastic transport to veins) insensitive to pCMBS occurs via the ion-trap mechanism, whereas apoplastic vein loading involves a carrier-mediated mechanism (which is targeted by pCMBS) in addition to diffusion.

² These authors contributed equally to the article.

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).

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www.plantphysiol.org/cgi/doi/10.1104/pp.109.140095

^* Corresponding author; e-mail jl.bonnemain{at}voila.fr.

Received April 17, 2009; accepted June 1, 2009; published June 3, 2009.

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On the Inside

Peter V. Minorsky
Plant Physiol. 2009 150: 1762-1763. [Full Text]