Identification and Characterization of AtSTP14, a Novel Galactose Transporter from Arabidopsis

AtSTP14, a new Arabidopsis sugar transporter, was identified and characterized on the molecular and physiological level. Reverse transcriptase–PCR analyses and reporter plants demonstrate high AtSTP14 expression levels in the seed endosperm and in cotyledons, as well as in green leaves. Thus, unlike previously characterized monosaccharide transporters, AtSTP14 is expressed in both source and sink tissues and represents the first monosaccharide transporter in the female gametophyte. Heterologous expression in yeast revealed that AtSTP14 does not transport glucose or fructose, but is the first plant transporter specific for galactose. Interestingly, AtSTP14 expression is regulated by factors which also induce cell wall degradation such as extended dark periods or changes in the sugar level, i.e. AtSTP14 is induced 3-fold by 24 h darkness and repressed 3-fold by 2% glucose and 2% sucrose. Two independent Atstp14 mutant lines were identified, but no effect on seed development or other differences during growth under normal conditions could be observed. A putative role for AtSTP14 in the recycling of cell wall-derived galactose during different developmental processes is discussed.