TENTATIVE MOLECULAR MECHANISM FOR GERMINATION STIMULATION OF STRIGA AND OROBANCHE SEEDS BY STRIGOL AND ITS SYNTHETIC ANALOGS

Analysis of the bioactivity of strigol analogues revealed that the CD part of these molecules is primarily responsible for germination stimulation of seeds of parasitic weeds. Preparation and bioassays of a "reduced" analogue of GR24, in which the enol ether carbon double bond has been replaced by a carbon single bond, showed that this double bond is essential for stimulatory activity. A molecular mechanism for germination stimulation is proposed which can account for the bioactivity of all strigol analogues. This mechanism involves addition of a nucleophilic species in a Michael fashion, followed by elimination of ring D. Reactions of GR24 with nucleophiles support the suggested mechanism, but replacement of ring D by other leaving groups resulted in loss of bioactivity. A structural model stimulant containing the essential functional groups and steric requirements of substituents is proposed.