The β-1,3-galactosetransferase gene DoGALT2 is essential for stigmatic mucilage production in Dendrobium officinale

Stigmatic mucilage plays a crucial role in pollen-grain adhesion on the stigma in flowering plants. Little information is available regarding mucilage biosynthesis in orchid plants. In the present study, stigmatic mucilage is rich in galactose-containing polysaccharides, mainly consisted of galactose and arabinose in Dendrobium officinale. Thirteen galactosyltransferases involved in biosynthesis of the beta-1,3-galactose linkage polysaccharides, belonging to the CAZY GT31 family, were identified from D. officinale genome. A positive correlation between the mucilage content and the DoGALT2 expression at different stages was observed. DoGALT2 expressed overall sampled tissues with the highest in D. officinale stigmatic mucilage that contributes to pollen adhesion and elongation. DoGALT2 was targeted to Golgi, and had a GALT domain (PF01762) that was homologous to the characterized GALT2 in Arabidopsis. Compared to wild-type Arabidopsis, DoGALT2 over-expressing plants showed a higher content of galactose and galactose-containing alcohol-insoluble residues, and enhanced tolerance to abiotic stress. DoGALT2 complemented Arabidopsis GALT2 mutant (galt2-1), with an equivalent galactose with wild-type Arabidopsis but significantly higher than galt2-1. These findings provide evidence that DoGALT2 might be involved in regulating the biosynthesis of galactose-containing polysaccharides during D. officinale pollen development.