Cell-Free Extracts of the Ginseng Soil Bacterium Pseudomonas plecoglossicida Promote Suppression of Resistance of American Ginseng (Panax quinquefolius) to Root Rot Caused by Ilyonectria mors-panacis

A prior report showed that soil previously planted with American ginseng (<i>Panax quinquefolius</i>) contained compound(s) which could reduce ginseng resistance to root infection by <i>Ilyonectria mors-panacis</i>, and this was not found in extracts from ginseng roots or soils not previously planted with ginseng. However, the origin of this ginseng-related factor in ginseng soils is unknown. An isolate of <i>Pseudomonas plecoglossicida</i> obtained from soil where <i>P. quinquefolius</i> had been harvested grew more in culture media when ginseng root extract was included, indicating the use of compounds in the extract as nutrients. Treatment with cell-free extracts from media containing ginseng root extracts where <i>P. plecoglossicida</i> had been cultured resulted in root lesions caused by <i>I. mors-panacis</i> being significantly larger than roots treated with fresh media containing root extract or with cell-free media inoculated with the same bacterial isolate without root extract. Levels of ginsenosides in the media decreased over time with incubation. Genome sequencing revealed that the bacterium had genes homologous to those reported for ginsenoside metabolism, which can release sugars for microbial growth. Thus, a ginseng soil bacterium, <i>P. plecoglossicida</i>, can create compound(s) suppressive to root rot resistance, similar to that found in soils previously planted with ginseng, indicating that the activity suppressing root rot resistance in soil previously planted with ginseng may be of microbial origin, utilizing compounds from ginseng roots.