Properties and formation of the squash high-affinity glucose transport system

Abstract

Uptake of 3-o-methylglucose (MeG) by excised squash (Cucurbita maxima Dcne) hypocotyl sections and other plants increased greatly with age. A high-affinity (HA) transport system, with an apparent Michaelis constant (Km) of 0.6–0.8 mM, was formed during ageing in squash sections and was probably responsible for the enhanced MeG uptake. Net transport occurred against a concentration gradient in aged tissues. Accumulation of MeG by freshly excised sections, which possessed a low-affinity (LA) transport system (Km, 25–45 mM), was negligible. Glucose competitively inhibited MeG uptake by both the LA and HA systems and appeared to be the preferred substrate in vivo. When sections were bathed in glucose or galactose, an efflux of preabsorbed MeG occurred. Though inhibited by UO22+, uptake of MeG by the HA system was not affected by Ca2+, Mg2+, or K+.Glucose (10 mM) and indoleacetic acid (> 0.1 mM) repressed the formation of the HA system, while kinetin, gibberellin, and indoleacetic acid at lower concentrations (< 0.01 mM) had no effect.