Strigolactones are highly potent germination stimulants for seeds of the parasitic weeds Striga and Orobanche spp., which severely reduce the yield of economically important crops in tropical and semitropical areas. GR24 (1) is a widely used synthetic strigolactone able to stimulate parasitic seed germination in the absence of a host plant to reduce soil seed levels. However, S. gesnerioides seeds do not respond to GR24 and only germinate by exposure to cowpea (Vigna unguiclata) root exudates or their active ingredient, the structure of which has not been established. Our present study provides substantial insight into the plant metabolism and structural requirements of strigolactones for host recognition by parasitic weeds. First, we clarified the structural and stereochemical requirements of synthetic strigolactones for seed germination of S. gesnerioides through bioassays of 4-hydroxy-GR24 (HO-GR24, 2) and 4-acetoxy-GR24 (AcO-GR24, 3) and their stereoisomers. These results suggest that both an oxygenated substituent at C-4 and the configuration of the tricyclic lactone and the D-ring are essential structural requirements for induction of S. gesnerioides seed germination. Furthermore, GR24 exhibited inhibitory activity against seed germination of S. gesnerioides at concentrations that induce seed germination of S. hermonthica and 0. minor. Second, we isolated the germination stimulants from hydroponic culture filtrate of cowpea, and their structures were unambiguously determined as (-)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchol (11) and (+)-(3aR,4R,8bR,2'R)-ent-2'-epi-orobanchyl acetate (12), on the basis of mass, CD, and ^1H NMR spectra; optical rotator power; and chromatographic behavior on HPLC. These compounds are identical to products produced by red clover (Trifolium. pratense).
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