PLANT PHYSIOLOGY , Vol 106, Issue 3 1023-1031, Copyright © 1994 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Proton-Peptide Co-Transport in Broad Bean Leaf Tissues
A. Jamai, J. F. Chollet and S. Delrot
Laboratoire de Physiologie et Biochimie Vegetales (A.J., S.D.), and Laboratoire de Synthese Organique et Organometallique (J.-F.C.), Unite Associee Centre National de la Recherche Scientifique 574, Universite de Poitiers, 25 Rue du Faubourg Saint-Cyprien, 86000 Poitiers, France
The transport of [14C]glycyl-glycine (Gly-Gly) has been characterized in
leaf discs from mature exporting leaves of broad bean (Vicia faba L.). In
terms of glycine (Gly) equivalents, the rate of transport of Gly-Gly was
similar to that of Gly uptake. Uptake of Gly-Gly was localized mainly in
the mesophyll cells, with little accumulation in the veins. It was optimal
at pH 6.0, sensitive to thiol reagents and metabolic inhibitors, and
exhibited a single saturable phase with an apparent Michaelis constant of
16 mM. Gly-Gly did not inhibit the uptake of labeled Gly. Addition of
Gly-Gly induced a concentration-dependent pH rise in the medium, showing
that peptide uptake is mediated with proton co-transport. Gly-Gly also
induced a concentration-dependent transmembrane depolarization of mesophyll
cells with an apparent Michaelis constant of 15 mM. This depolarization was
followed by a transient hyperpolarization. When present at a 10-fold
excess, various peptides and tripeptides were able to inhibit Gly-Gly
uptake with the following decreasing order of efficiency: Gly-Gly-Gly =
leucine-Gly > Gly-tyrosine > Gly-glutamine = Gly-glutamic acid >
Gly-phenylalanine > Gly-threonine > Gly-aspartic acid =
Gly-asparagine = aspartic acid-Gly. Gly inhibited the uptake of Gly-Gly
only slightly, whereas tetraGly and the tripeptide glutathione were not
inhibitory. The dipeptides inhibiting Gly-Gly uptake also induced changes
in the transmembrane potential difference of mesophyll cells and were able
to affect in a complex way the response normally induced by Gly-Gly.
Altogether, the data demonstrate the existence of a low-affinity,
broad-specificity H+/peptide co-transporter at the plasma membrane of
mesophyll cells. The physiological importance of this transporter for the
exchange of nitrogenous compounds in mature leaves remains to be
determined, as do the details of the electrophysiological events induced by
the dipeptides.
