Development and application of a bioassay for assessing the dissipation rate of the synthetic strigolactone GR24 in soil

Root parasitic weeds of the genera Striga, Orobanche, and Phelipanche cause enormous economic losses for farmers the world over. Germination of the seeds of these species requires a chemotropic signal in the form of strigolactones in the soil. Once a root parasitic weed germinates and becomes established, it draws nutrition from the host plants, leading to plant death and hence yield reduction. Despite the obvious importance of strigolactones, there is currently almost no information about the fate of strigolactones in the soil, although microbial involvement has been suggested. To begin to address this knowledge lacuna, we developed a bioassay—using the strigolactone synthetic analogue GR24—as a high-throughput, inexpensive, and compact tool for monitoring the dissipation of strigolactones (GR24 in this case) in the soil by microorganisms and/or environmental conditions, including GR24 concentration, soil type, and temperature. As part of the bioassay, it was found that autoclaving the soil delayed the dissipation of GR24 versus sterilisation by gamma radiation and or no sterilisation. Analytical LC–MS/MS with a detection limit of 0.1 ppb confirmed this finding, with no GR24 being detected in non-sterilised soil after 24 h. Application of the bioassay to monitor GR24 dissipation in soil showed that the higher the GR24 concentration the slower the degradation, and the higher the temperature, the faster the degradation. It also showed that the organic matter content of the soil affected the GR24 dissipation rate. These findings were also confirmed by analytical LC–MS/MS, indicating the applicability of the methodology for studies of root exudes.