Ion homeostasis is a crucial process in plants that is closely linked to the efficiency of nutrient uptake, stress tolerance and overall plant growth and development. Nevertheless, our understanding of the fundamental processes of ion homeostasis is still incomplete and highly fragmented. Especially at the mechanistic level, we are still in the process of dissecting physiological systems to analyse the different parts in isolation. However, modelling approaches have shown that it is not individual transporters but rather transporter networks (homeostats) that control membrane transport and associated homeostatic processes in plant cells. To facilitate access to such theoretical approaches, the modelling of the potassium homeostat is explained here in detail to serve as a blueprint for other homeostats. The unbiased approach provided strong arguments for the abundant existence of electroneutral H+/K+ antiporters in plants.

Evidence in the literature has generally supported either of two paraquat resistance mechanisms: an increase in activity of oxygen radical-scavenging enzymes in resistant plants which affords protection from active oxygen species formed by paraquat; and sequestration of paraquat away from its site of action in the chloroplast. Evidence for the first model relies primarily on measurement of increased enzyme activity and cross-resistance to other oxygen radical-generating stresses in resistant plants. The sequestration model is supported by data showing decreased translocation of paraquat and absence of paraquat injury in plant systems that do not have increased levels of protective enzymes. An alteration in paraquat transport at one of several plant cell membranes could confer resistance by modifying movement of paraquat into the compartment bounded by that membrane. Properties of the plasmalemma, chloroplast envelope, and tonoplast that may be important to paraquat transport are discussed and data supporting or discounting specific membrane alterations in resistant plants are presented. Finally, the possibility that both mechanisms may work in concert is addressed.

Maillard products arise from condensation reactions between amino acids or proteins with reducing sugars during food processing. As ubiquitous components of human food, these early or advanced glycation products may be subject to intestinal absorption. The present study was performed to investigate the intestinal uptake of Maillard products and to determine whether they are substrates for peptide and amino acid transporters expressed at the apical membrane of Caco-2 cells. At a concentration of 10mm, Nɛ-(carboxymethyl)-L-lysine, Nα-hippuryl-Nɛ-(1-deoxy-d-fructosyl)-l-lysine,Nα-hippuryl-Nɛ-(carboxymethyl)-L-lysine and Nɛ-(1-deoxy-d-fructosyl)-l-lysine inhibited the [14C]glycylsarcosine uptake mediated by the H+–peptide co-transporter PEPT1 by 13 to 45%.For Nɛ-(1-deoxy-d-fructosyl)-l-lysine, an inhibitory constant of 8·7mm was determined, reflecting a low affinity to PEPT1 in comparison with natural dipeptides. Uptake of l-[3H]lysine was weakly affected by Nɛ-(carboxymethyl)-L-lysine, Nα-hippuryl-l-lysine and Nα-hippuryl-Nɛ-(carboxymethyl)-L-lysine but strongly inhibited by Nɛ-(1-deoxy-d-fructosyl)-l-lysine (81%). None of the Maillard products was able to inhibit the uptake of l-[3H]leucine by more than 15%. We also studied the transepithelial flux of Maillard productsacross Caco-2 cell monolayers cultured on permeable filters. The flux rates of Maillard products ranged from 0·01 to 0·3%/cm2 per h and were shown to be muchlower than those of carrier substrates such as glycylsarcosine, l-proline and the space marker 14C]mannitol. We conclude that the Maillard products investigated in the present study are neither transported by PEPT1 nor by carriers for neutral amino acids. The low transepithelial flux measured for these compounds most probably occurs by simple diffusion.