A Sugar Transporter Takes Up both Hexose and Sucrose for Sorbitol-Modulated in vitro Pollen Tube Growth in Apple

Rapid pollen tube growth requires uptake of sucrose or its hydrolytic products, hexoses, from the apoplast of surrounding tissues in the style. Due to species-specific sugar requirements, reliance of pollen germination and tube growth on cell wall invertase and sucrose or hexose transporters varies between species, but it is not known if there exists a sugar transporter that mediates the uptake of both hexose and sucrose for pollen tube growth. Here, we show that a sugar transporter protein in apple (Malus domestica), MdSTP13a, takes up both hexose and sucrose when expressed in yeast, and is essential for pollen tube growth on glucose and sucrose but not on maltose. MdSTP13a-mediated direct uptake of sucrose is primarily responsible for apple pollen tube growth on sucrose medium. Sorbitol, a major photosynthate and transport carbohydrate in apple, modulates pollen tube growth via a MYB transcription factor, MdMYB39L, which binds to the promoter of MdSTP13a to activate its expression. Antisense repression of MdSTP13a blocks the sorbitol-modulated pollen tube growth. These findings demonstrate that MdSTP13a takes up both hexose and sucrose for sorbitol-modulated pollen tube growth in apple, revealing a situation where acquisition of sugars for pollen tube growth is regulated by a sugar alcohol.