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PLANT PHYSIOLOGY , Vol 114, Issue 2 731-736, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
The Mechanism of Amino Acid Efflux from Seed Coats of Developing Pea Seeds as Revealed by Uptake Experiments
A. de Jong, J. W. Koerselman-Kooij, JAMJ. Schuurmans and A. C. Borstlap
Transport Physiology Research Group, Department of Plant Ecology and Evolutionary Biology, Utrecht University, Sorbonnelaan 16, NL-3584 CA Utrecht, The Netherlands
The uptake of amino acids by excised seed coat halves of developing seeds
of pea (Pisum sativum L.) was characterized. The influx of L-valine and
L-glutamic acid was proportional to their external concentration, with
coefficients of proportionality (k) of 11.0 and 7.1 [mu]mol g-1 fresh
weight min-1 M-1, respectively. The influx of L-lysine could be analyzed
into a component with linear kinetics (k = 8.1 [mu]mol g-1 fresh weight
min-1 M-1) and one with saturation kinetics (Michaelis constant = 6.5 mM),
but the latter may have resulted from the mutual interaction between the
influx of the cationic lysine and the membrane potential. The influx of the
amino acids was not affected by 10 [mu]M carbonylcyanide
m-chlorophenylhydrazone, but was inhibited by about 50% in the presence of
2.5 mM p-chloromercuribenzene sulfonic acid. Conservative estimates of the
permeability coefficients of the plasma membrane of seed coat parenchyma
cells for lysine, glutamic acid, and several neutral amino acids were all
in the range of 4 x 10-7 cm s-1 to 9 x 10-7 cm s-1, which is 4 to 5 orders
of magnitude greater than those reported for artificial lipid bilayers. It
is concluded that nonselective pores constitute a pathway in the plasma
membrane for passive transport of amino acids. It is argued that this
pathway is also used for the efflux of endogenous amino acids, the process
by which nitrogen becomes available for the embryo.
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