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

Abstract

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 ϩ accumulated in the seed coat tissue 5.1 Ϯ 0.8-fold, His ϩ 2.4 Ϯ 0.3-fold, and His 0 1.3 Ϯ 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 Ϫ55 Ϯ 3 mV. At a concentration of 100 mm, choline ϩ and His ϩ , 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 mol g Ϫ1 fresh weight min Ϫ1 m Ϫ1 , similar to that found for neutral amino acids, sucrose, glucose, and mannitol. At lower concentrations, the relative influx of choline ϩ and His ϩ 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.During the development of the pea (Pisum sativum) seed, the embryo receives its nutrients from the surrounding maternal seed coat. These nutrients arrive in the seed coat mainly through the phloem of three vascular bundles and are then distributed by cell-tocell transport over the seed coat parenchyma (Grusak and Minchin, 1988; Patrick et al., 1995; Tegeder et al., 1999). Because embryo and seed coat are symplasmically isolated from each other, the nutrients have to be unloaded into the apoplast. This implies the transport of the nutrients across the plasma membrane of the seed coat parenchyma cells. The mechanism by which the various nutrients (Suc, amino acids, and inorganic ions) are released from the parenchyma cells is still incompletely understood.The release of sugars and amino acids from the pea seed coat is biphasic, consisting of a fast and a slow component (De Jong and Wolswinkel, 1995). It is most likely that the fast component represents the efflux across the plasma membrane of seed coat parenchyma cells. The rate constant for the fast component in the release of Suc and amino acids amounts to approximately 1.4 h Ϫ1 , corresponding to a half-life time of approximately 0.5 h. For a spherical cell, the permeability coefficient (P) is related to the half-life time (t 1/2 ) and the cell radius (r) by P ϭ ln2 ϫ r/3t 1/2

Phloem Import and Storage Metabolism Are Highly Coordinated by the Low Oxygen Concentrations within Developing Wheat Seeds

“…This confirms the supply follows demand model of assimilate unloading proposed by van Dongen et al (2001). According to this model, which is originally based on experiments with seed coats of legume seeds, the supply of nutrients by the maternal seed coat is determined by the demand of the developing embryo.…”

“…The concentration gradient acts as the driving force for unloading of fresh nutrients into the apoplast. For pea, evidence exists that poorly selective pores are present in the plasma membrane of seed coat parenchyma cells to facilitate the unloading of assimilates (de Jong et al, 1996(de Jong et al, , 1997van Dongen et al, 2001). Also in wheat, nucellus unloading appears to be a passive process, thus driven by the concentration gradient (Wang and Fisher, 1995).…”

A Cascade of Sequentially Expressed Sucrose Transporters in the Seed Coat and Endosperm Provides Nutrition for the Arabidopsis Embryo

“…In legume seeds, results from inhibitor studies led to the proposal that nonselective passive pores are responsible for sucrose efflux (van Dongen et al, 2001;Ritchie et al, 2003). Neither the predicted antiporters nor the nonselective pores have been identified at the molecular level to date.…”

Members of the aquaporin family in the developing pea seed coat include representatives of the PIP, TIP, and NIP subfamilies

“…The molecular basis for the efflux of these solutes is still unknown. It has been suggested that in seed coats of common bean (Phaseolus vulgaris) and broad bean (Vicia faba) sucrose is released by an H + -antiport mechanism , whereas work in our group on pea has provided evidence for poorly selective pores in the plasma membrane of seed coat parenchyma cells that allow the passage of sugars, amino acids and possibly inorganic ions as well (de Jong et al, 1996(de Jong et al, , 1997van Dongen et al, 2001).…”