Synthesis and Evaluation of New Halogenated GR24 Analogs as Germination Promotors for Orobanche cumana

Orobanche and Striga are parasitic weeds extremely well adapted to the life cycle of their host plants. They cannot be eliminated by conventional weed control methods. Suicidal germination induced by strigolactones (SLs) analogs is an option to control these weeds. Here, we reported two new halogenated (+)-GR24 analogs, named 7-bromo-GR24 (7BrGR24) and 7-fluoro-GR24 (7FGR24), which were synthesized using commercially available materials following simple steps. Both compounds strongly promoted seed germination of Orobanche cumana. Their EC(50) values of 2.3±0.28×10(−8)M (7BrGR24) and 0.97±0.29×10(−8)M (7FGR24) were 3- and 5-fold lower, respectively, than those of (+)-GR24 and rac-GR24 (EC(50)=5.1±1.32–5.3±1.44×10(−8); p<0.05). The 7FGR24 was the strongest seed germination promoter tested, with a stimulation percentage of 62.0±9.1% at 1.0×10(−8)M and 90.9±3.8% at 1.0×10(−6)M. It showed higher binding affinity (IC(50)=0.189±0.012μM) for the SL receptor ShHTL7 than (+)-GR24 (IC(50)=0.248±0.032μM), rac-GR24 (IC(50)=0.319±0.032μM), and 7BrGR24 (IC(50)=0.521±0.087μM). Molecular docking experiments indicated that the binding affinity of both halogenated analogs to the strigolactone receptor OsD14 was similar to that of (+)-GR24. Our results indicate that 7FGR24 is a promising agent for the control of parasitic weeds.