The plant growth-promoting endophytic Fusarium oxysporum GG22 enhances Rehmannia glutinosa secondary metabolites accumulation

Fusarium oxysporum GG22 is an endophytic fungus isolated from the leaves of Rehmannia glutinosa and shown to promote the growth of R. glutinosa. There is currently no in-depth study on the effect of plant growth-promoting endophytic fungi on the defensive metabolic response of medicinal herbs.A symbiotic system of GG22 with R. glutinosa tissue culture seedlings was established to analyze the effect of GG22 infection on the biomass of R. glutinosa tissue culture seedlings and on the accumulation of secondary metabolites. Antioxidant enzymes (SOD, POD and CAT) and endogenous signaling molecules (NO and H2O2) were examined. Quantitative real-time PCR was used to analyze the expression of enzyme genes related to secondarymetabolites. GG22 promoted the growth of the plant height, leaf area, fresh weight, and dry weight of R. glutinosa tissue culture seedlings and significantly increased catalpol and verbascoside. After GG22 infection, the activities of the antioxidant enzyme system (SOD, POD, CAT) of R. glutinosa increased differentially. In addition, significant accumulation of NO and H2O2 signaling molecules occurred. According to the analysis of the expression of genes encoding key biosynthesis enzymes for iridoid glycoside and verbascoside components in R. glutinosa, the levels of AACT, HMGR, DXR, GS, 4CL, C4H and C3H were all significantly increased after GG22 infection. Therefore, it is speculated that infection with the endophytic fungus GG22 induces a burst of reactive oxygen species and NO in plants to activate or enhance the expression of related enzymes in primary metabolic pathways, thereby increasing secondary metabolite contents in R. glutinosa. Our results confirmed that GG22 not only contributes to the stimulation of R. glutinosa growth but also enhances secondary metabolism, thus demonstrating its application potential as a bio-fertilizer for R. glutinosa cultivation.