HEXOSE-TRANSPORT IN GROWING PETUNIA POLLEN TUBES AND CHARACTERIZATIONOF A POLLEN-SPECIFIC, PUTATIVE MONOSACCHARIDE TRANSPORTER

We investigated the molecular and physiological processes of sugar upt ake and metabolism during pollen tube growth and plant fertilization. In vitro germination assays showed that petunia (Petunia hybrida) poll en can germinate and grow not only in medium containing sucrose (Suc) as a carbon source, but also in medium containing the monosaccharides glucose (Glc) or fructose (Fru). Furthermore, high-performance liquid chromatography analysis demonstrated a rapid and complete conversion o f Suc into equimolar amounts of Glc and Fru when pollen was cultured i n a medium containing 2% Suc. This indicates the presence of wall-boun d invertase activity and uptake of sugars in the form of monosaccharid es by the growing pollen tube. A cDNA designated pmt1 (petunia monosac charide transporter 1), which is highly homologous to plant monosaccha ride transporters, was isolated from petunia. Pmt1 belongs to a small gene family and is expressed specifically in the male gametophyte, but not in any other vegetative or floral tissues. Pmt1 is activated afte r the first pollen mitosis, and high levels of mRNA accumulate in matu re and germinating pollen. A model describing the transport of sugars to the style, the conversion of Suc into Glc and Fru, and the active u ptake by a monosaccharide transporter into the pollen tube is presente d.