Efflux and active re-absorption of glucose in roots of cotton plants grown under saline conditions

Abstract

The effect of growth under saline condition (100 mol m-3 NaCl in the nutrient solution) on the influx and the efflux of glucose from roots of cotton plants was analysed utilising the non metabolisable glucose analogue [14C]-3-O-methylglucose ([14C]3-OMG). Apical segments (1 cm long) excised from cotton roots took up [14C]3-OMG. At each tested concentration (5–500 mmol m-3), the influx was completely inhibited by the presence of the protonophore carbonylcyanide-m-chlorophenyl hydrazone (CCCP) indicating that it is mediated by a H+-coupled co-transport mechanism. The CCCP-sensitive [14C]3-OMG influx was lower in the root segments excised from plants grown on saline solution than in the controls, and this was particularly evident at lower external concentrations. This difference was not due to a lower H+ apoplastic availability. In fact, the saline condition did not affect the pH of the rhizosphere and indeed the H+–ATPase activity, evaluated in plasma membrane vesicles purified from saline-treated plants, was higher (+23%) than in the controls. The lower uptake of [14C]3-OMG into saline treated root segments was related to an enhanced value of the apparent Km of the carrier for the glucose analogue. This effect is discussed in relation to either the more positive value of the transmembrane electric potential difference (ΔΨ) measured in these root segments, or a competitive inhibition of Na+ on the H+ binding site of the carrier. Growth in saline solution slightly affected the efflux of the [14C]3-OMG preloaded in root segments, changing the membrane permeability to the molecule. The results strongly suggest that the higher (2.5 fold) net exudation of glucose, observed in short-term (4 h) collection experiments, from roots of cotton plants grown in saline condition, is mainly due to an effect of the saline growth condition on the system involved in the reabsorption of the hexose rather than on its efflux.