Monosaccharide transporters mediate the membrane transport of a variable range of monosaccharides, which plays a crucial role in sugar distribution throughout the plant. To investigate the significance of monosaccharide transporters for rice (Oryza sativa L.) seed development, cDNA of a new putative monosaccharide transporter gene OsMST4 was isolated. The deduced OsMST4 protein shows typical features of monosaccharide transporters, and shares high homology with other plant homologues. Heterologous expression in yeast (Saccharomyces cerevisiae) showed that OsMST4 is a functional monosaccharide transporter capable of transporting glucose, fructose, mannose and galactose. Transcriptional analysis revealed that OsMST4 is expressed in all tested organs/tissues. In developing caryopses, its expression is high at the early and middle grain filling stages, and declines gradually to low levels after that. Further analysis revealed that it is expressed in both the maternal tissue and the filial tissue, with its highest expression in embryo. Cellular location in young caryopses through RNA in situ hybridization showed that OsMST4 mRNA mainly accumulates in the vascular parenchyma of the chalazal vein, cross-cells, nucellar tissue and endosperm. The expression pattern of OsMST4 was further confirmed by histochemical analysis of the OsMST4-promoter-β-glucuronidase (GUS) transgenic rice plants. These data indicate that OsMST4 is actively involved in monosaccharides supply for seed development during the course of grain filling. In addition, the cell type-specific expression patterns of OsMST4 in other sink and source tissues were also investigated, and its corresponding physiological roles were discussed.
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Acknowledgements We thank Dr. Eckhard Boles (Heinrich-Heine-Universität, Düsseldorf, Germany) for providing the S. cerevisiae strain EBY.VW4000 and Dr. Michael Büttner (Universität Erlangen-Nürnberg, Erlangen, Germany) for providing the S. cerevisiae/E. coli shuttle vector pEX-Tag. We are grateful to Dr. Yule Liu of the Tsinghua University for valuable advice on revision of the manuscript. This work is supported by grants from National Program on Key Basic Research Projects (2004CB720406), Innovation Foundation of Chinese Academy of Science (KSCX2-SW-306 and KSCX1-SW-03), the Programme for Strategic Scientific Alliances (04-PSA-BD-04) between China and the Netherlands of the Royal Netherlands Academy of Arts and Sciences (KNAW) for P.B.F.O. and the KNAW China Exchange Programme (04CDP022) for Y.X.
Copyright 2008 Elsevier B.V., All rights reserved.
- Gene expression
- Monosaccharide transporter
- Oryza sativa L.
- Seed development
- cDNA cloning