Influence of exogenously applied methyl jasmonate on soilborne pathogen, Cylindrocarpon destructans in vitro

Ginseng (Panax ginseng) is a traditional medicinal plant in China, and the dried root is highly valued for its medicinal properties and is widely used in Chinese traditional medicine. Cylindrocarpon root rot of ginseng caused by Cylindrocarpon destructans is the main disease, which is difficult to be eliminated from soil, leading to serious crop yield losses and limited the re-use of fields for successive ginseng crops. Methyl jasmonate (MeJA), which is one of the plant lipid derivatives, considered as a signaling substance during plant-microbe interactions. It is related to plant resistance to biotic and abiotic stress, can be involved in plant response to pathogens and other adversity stress and signal transmission, and can be used to induce plant disease resistance reaction. The objective of this study was to test the effects of MeJA on C. destructans and to find the possible relationships between MeJA and pathogenic factors of C. destructans. To assess the influence of MeJA on soil pathogens, the direct effects of artificially applied MeJA on C. destructans were evaluated, including the effects on colony growth, spore germination, biomass and phytopathogenic enzyme activities of this pathogen. The results showed that MeJA strongly inhibited its growth and spore germination, whereas it stimulated phytopathogenic enzyme activities. The colony diameter of C. destructans on PDA decreased from (8.23±0.15) cm (control) to (0.71±0.00) cm (800 μg·mL~(-1)). At the highest concentration (800 μg·mL~(-1)) of MeJA, the colony diameter of C. destructans had almost totally inhibited, but the diameter had no difference compared with the untreated control at lower concentrations (1-50 μg·mL~(-1) MeJA). The biomass reduced by 65.3%-100% and the percent of spore germination decreased by 100% at concentrations higher than 400 μg·mL~(-1). Potent suppression of the growth of germ-tubes was observed at different concentrations, especially at (400-800) μg·mL~(-1), where the growth of germ-tubes was inhibited completely. However, activities of phytopathogenic enzymes (pectinase, cellulase, amylase, and protease) were greatly stimulated by MeJA at higher concentrations (MeJA>200 μg·mL~(-1)), while the activity of protease was little changed. The activity of pectinase was high up to (0.61±0.05) U·mL~(-1) · min~(-1) at the concentration of 800 μg·mL~(-1). The activity of cellulase was stimulated at high concentrations of MeJA in liquid culture, while it was suppressed at low concentrations (1-50 μg·mL~(-1)). The activity of cellulase was (0.31± 0.02) μmol·min~(-1) at 800 μg·mL~(-1) of MeJA. Amylase activity substantial increased at concentrations of (200-800) μg·mL~(-1), which was (0.45±0.02) μmol·min~(-1) at the concentration of 800 μg·mL~(-1). It was concluded that MeJA greatly inhibited C. destructans growth and spore germination, but stimulated activities of hydrolytic enzymes of C. destructans at higher doses. There might be different mechanisms presented for the effects of MeJA on hyphal growth and virulence factors of C. destructans. The critical concentration of MeJA inhibitory effects on C. destructans was 200 μg· mL~(-1). The research lays a foundation for the subsequent experiment using MeJA to induct disease resistance. This is of help to understand the mechanism of Cylindrocarpon root rot of ginseng and to control pathogen in practice.