Sugar transport across the plasma membrane of leaf cells: from physiology to transgenesis

Abstract

Sugars made by photosynthesis must cross several cells before being loaded into the phloem. The first part of the paper will review the data available on the physiology, biochemistry and molecular biology of sucrose and hexose transport across the plasma membrane of the different leaf cells. The second part will describe data involving transgenesis with sugar transporters cDNAs or promoters. Among these, a detailed study on transgenic tobacco plants modified on hexose transport will be reported. Tobacco plants were transformed with a chimeric construct containing the VvHT1 cDNA (putative hexose transporter expressed in young leaves and berries of grapevine) under the CaMV 35S promoter. About 20 sense plants and 10 anti-sense plants were obtained and cultured in vitro. Two sense plants present a very marked phenotype, due to a smaller leaf size and a poor development of the roots. This is likely due to the strong homology existing between VvHT1, and NtMST1, the endogenous hexose transporter from tobacco, which may indirectly induce a general repression of hexose transport activity. Glucose uptake was reduced in leaf discs from these plants. The phenotype can be suppressed by the addition of high glucose concentrations into the culture media, suggesting a relationship between hexose availability and plant size. The activity of the VvHT1 promoter was also studied with reporter genes. In transgenic tobacco plants, GUS activity was much higher in young leaves than in source leaves. In tobacco BY2 cells, the promoter activity was stimulated by sucrose, glucose, and palatinose, but not by fructose.