Molecular analysis of polysaccharide accumulation in Dendrobium nobile infected with the mycorrhizal fungus Mycena sp.

Polysaccharides are among the most important active compounds of Dendrobium nobile Lindl., and they improve immunity, strengthen memory, and prevent and fight tumours. Evidence suggests that the mycorrhizal fungus Mycena sp. may increase the polysaccharide content of D. nobile. To understand the regulatory mechanism by which Mycena sp. promotes polysaccharide accumulation in D. nobile, a model including MF23 (Mycena sp.) and D. nobile was developed. Content measurements and electron microscopy showed that the change in polysaccharides was associated with the level of colonization. Large-scale transcriptome sequencing analyses of D. nobile infected with MF23 and of wild D. nobile revealed that 30 unigenes encoding key enzymes were potentially associated with polysaccharide accumulation. A qRT-PCR experiment involving 13 randomly chosen unigenes suggested that the combined effects of the carbohydrate biosynthesis and consumption pathways resulted in changes in polysaccharide levels. MF23 might increase polysaccharide level by inducing genes involved in photosynthesis (petF), starch and sucrose metabolism (SUS, bcsA, glgA) and fructose and mannose metabolism (PMM, GPMM) and by repressing genes involved in glycolysis (PGK, PFK) and the citrate cycle (PDHB, CS, SDHA). This study provides a good example of an endophyte promoting the synthesis of bioactive compounds in its host and paves the way for further investigations of the associated regulatory mechanism using molecular methods.