GDP-mannose pyrophosphorylase (GMP) catalyzed the formation of GDP-mannose, which serves as a donor for the biosynthesis of mannose-containing polysaccharides. In this study, three GMP genes from Dendrobium officinale (i.e., DoGMPs) were cloned and analyzed. The putative 1000 bp upstream regulatory region of these DoGMPs was isolated and cis-elements were identified, which indicates their possible role in responses to abiotic stresses. The DoGMP1 protein was shown to be localized in the cytoplasm. To further study the function of the DoGMP1 gene, 35S:DoGMP1 transgenic A. thaliana plants with an enhanced expression level of DoGMP1 were generated. Transgenic plants were indistinguishable from wild-type (WT) plants in tissue culture or in soil. However, the mannose content of the extracted water-soluble polysaccharides increased 67%, 96% and 92% in transgenic lines #1, #2 and #3, respectively more than WT levels. Germination percentage of seeds from transgenic lines was higher than WT seeds and the growth of seedlings from transgenic lines was better than WT seedlings under salinity stress (150 mM NaCl). Our results provide genetic evidence for the involvement of GMP genes in the biosynthesis of mannose-containing polysaccharides and the mediation of GMP genes in the response to salt stress during seed germination and seedling growth.