Expression And Physiology Functional Analysis Of Hexose Transporter CsHT1in Cucumber

Carbohydrate synthesis and transport is the basis of plant growth and development, the supply of pollination directly affect pollen development and pollen tube growth, thus affecting the formation of seeds. Although sucrose transporters have been shown to play a role in pollen tube development, the role of hexoses and hexose transporters is not as well established. The pollens of model plants such as Arabidopsis and tobacco rely on the sucrose but not hexose when germination and tube growth in vitro, and knockouts of individual hexose transporters has not been shown to impair fertilization, so it was considered a functional redundancy. Given the extreme length of the cucumber ovary on the day of flowering and Glucose and Frutose are the most abundant sugars in the cucumber ovary/fruit, hexose and hexose transporters may play more important role in cucumber pollen. Here, the functions of CsHT1, a hexose transporter from cucumber (Cucumis sativus L.) are studied using a combination of heterologous expression in yeast, histochemical and immunohistochemical localization and reverse genetics. The mainly results are as follows:CsHTl mRNA is transcribed only in pollen and pollen tube, the transcription increases gradually during the pollen development and maturation. However, no CsHT1protein was detectable in ungerminated pollen. CsHT1protein expression was only detected in germinating pollen and in the growing pollen tube. Otherwise, transient expression of CsHT1-GFP demonstrated that the fusion protein was targeted to the plasma membrane of epidermal cells of onion and cucumber protoplasts. These results indicate that pollen grains are preloaded with CsHT1mRNA during maturation, but translation does not start until the pollen germinates.CsHT1can restore the growth of the hexosetransporter-deficient yeast (Saccharomyces cerevisiae) strain EBY.VW4000on Glucose madium.[14C]Glucose uptake analysis demonstrated that CsHTl is an energy-dependent hexose/H+symporter with high affinity for Glc and may also transport Gal but apparently with a lower affinity than for Glc.Overexpression of CsHT1in cucumber pollen results in higher pollen germination ratio and longer pollen tube growth than wild type pollen in glucose-or galactose-containing medium. In contrast, antisense suppression of CsHT1leads to inhibition of pollen germination and pollen tube elongation in the same medium, and results in a decrease of seed number per fruit and seed size when antisense transgenic pollen is used to fertilize wild type or transgenic cucumber plants. These results demonstrate that antisense expression of CsHT1impedes the function of pollen and affects seed development but does not impede the function of female tissues and organs.In summary, CsHT1is a plasma membrane hexose transporter expressed only in pollen and translated only during pollen germination and tube growth. CsHTl specifically transports Glc and Gal and affects seed formation by controlling pollen tube growth in cucumber ovary/fruit (the most abundant sugars are glucose and fructose). This is the first report regarding the biology function of a hexose transporter in pollen tube growth.