Electrodiffusional Uptake of Organic Cations by Pea Seed Coats. Further Evidence for Poorly Selective Pores in the Plasma Membrane of Seed Coat Parenchyma Cells

In developing seeds, the permeability of the plasma membrane of seed coat parenchyma cells is crucial for the supply of nutrients to the embryo. Here, we report characteristics of the transport of the organic cation choline and the basic amino acid L- histidine (His; cation at pH 5, electroneutral at pH 7) into isolated seed coats of pea (Pisum sativum). Supplied at sub-micromolar concentrations, choline(+) accumulated in the seed coat tissue 5.1 +/- 0.8-fold, His(+) 2.4 +/- 0.3-fold, and His(0) 1.3 +/- 0.2-fold. Taking into consideration that at pH 5 His influxes as a cation but effluxes as a neutral molecule, these accumulations are in reasonable agreement with (electro) diffusional uptake at the prevailing membrane potential of -55 +/- 3 mV. At a concentration of 100 mM, choline(+) and His(+), but not His(0), depolarized the membrane of the parenchyma cells and neither of the substrates was accumulated. At this concentration, the relative influx (the ratio of influx and external concentration, a measure for membrane permeability) of choline and His was approximately 10 micromol g(-1) fresh weight min(-1) M(-1), similar to that found for neutral amino acids, sucrose, glucose, and mannitol. At lower concentrations, the relative influx of choline(+) and His(+) increased because of increasingly more negative membrane potentials, giving rise to apparent saturation kinetics. It is suggested that transport of organic cations can proceed by a general, poorly selective pore in the plasma membrane of seed coat parenchyma cells. This pore is thought to be responsible for the unloading of a range of solutes that serve as nutrients for the embryo.