Glucose Induced H+Influx and Transient Currents in Excised Roots: particularly those ofZea mays

Abstract

The application of D-glucose to solutions bathing excised maize, wheat, pea and bean roots causes a rapid depolarization of the electrical potentials between the cut tops of the roots and the bathing solutions. Similar effects are observed for the plasma membrane potentials of maize lateral roots. A flow cell apparatus was used to demonstrate qualitative and quantitative relations between glucose induced H⁺ influx and the transient decrease in current through the root. The current changes appear to be due entirely to H⁺ fluxes. Current and H⁺ fluxes are strongly influenced by external pH, the optimum pH for glucose induced current change being about 4.0. A similar pH optimum was found for 3-O-methyl-D-glucopyranoside but 1-O-methyl-α-D-glucopyranoside did not significantly affect the trans-root potential at any pH, suggesting a significant role for the anomeric hydroxyl group of glucose. Compounds which depolarize the trans-root potential also inhibit the glucose induced depolarization. Surface -SH groups are probably not involved in the glucose/H⁺ cotransport. Eadie-Hofstee plots relating the depolarization of trans-root potential to the concentrations of D-glucose or 3-O-methyl-D-glucopyranoside have shown that K_m values increase with increasing monosaccharide concentration and are very similar to reported values of 3-O-methyl-D-glucopyranoside uptake in maize root segments. K_m values for a similar range of D-glucose concentrations do not vary significantly with pH or with membrane depolarization due to a 10-fold increase of KCl concentration. However, δV_max is lowered by an increase in external pH or a decrease in trans-root potential. It appears that both proton and electrical gradients can affect glucose induced H⁺ influx. The auxin herbicide, 2, 4-dichlorophenoxyethanoic acid (0.01 mM) stimulates the glucose induced depolarizations in a manner consistent with an increase in cytoplasmic pH. This is discussed in relation to the reported action of indole-3-acetic acid and fusicoccin on maize root tissue.