Auxin and Fusicoccin Enhancement of beta-Glucan Synthase in Peas : An Intracellular Enzyme Activity Apparently Modulated by Proton Extrusion

Abstract

Fusicoccin (FC), like indoleacetic acid (IAA), causes Golgi-localized beta-1,4-glucan synthase (GS) activity to increase when applied to pea third internode segments whose GS activity has declined after isolation from the plant. This suggests that GS activity is modulated by H(+) extrusion; in agreement, vanadate and nigericin inhibit the GS response. The GS response is not due to acidification of the cell wall. Treatment of tissue with heavy water, which in effect raises intracellular pH, mimics the IAA/FC GS response. However, various treatments that tend to raise cytoplasmic pH directly, other than IAA- or FC-induced H(+) extrusion, failed to increase GS activity, suggesting that cytoplasmic pH is not the link between H(+) extrusion and increased GS activity. Although FC stimulates H(+) extrusion more strongly than IAA does, FC enhances GS activity at most only as much as, and often somewhat less than, IAA does. This and other observations indicate that GS enhancement is probably not due to membrane hyperpolarization, stimulated sugar uptake, or changes in ATP level, but leave open the possibility that GS is controlled by H(+) transport-driven changes in intracellular concentrations of ions other than H(+).