Germination Stimulant Activity of Isothiocyanates on Phelipanche spp.

The root parasitic weed broomrapes, Phelipanche spp., cause severe damage to agriculture all over the world. They have a special host-dependent lifecycle and their seeds can germinate only when they receive chemical signals released from host roots. Our previous study demonstrated that 2-phenylethyl isothiocyanate is an active germination stimulant for P. ramosa in root exudates of oilseed rape. In the present study, 21 commercially available ITCs were examined for P. ramosa seed germination stimulation, and some important structural features of ITCs for exhibiting P. ramosa seed germination stimulation have been uncovered. Structural optimization of ITC for germination stimulation resulted in ITCs that are highly active to P. ramosa. Interestingly, these ITCs induced germination of P. aegyptiaca but not Orobanche minor or Striga hermonthica. P. aegyptiaca seeds collected from mature plants parasitizing different hosts responded to these ITCs with different levels of sensitivity. ITCs have the potential to be used as inducers of suicidal germination of Phelipanche seeds.

Structure-activity relationship of the aromatic moiety of 6-substituted 5,6-dihydro-2H-pyran-2-one to find the novel compound showing higher plant growth inhibitory activity.

In the course of our research on the structure-activity relationship of 5,6-dihydro-2H-pyran-2-one, (S)-6-[(R)-2-hydroxy-6-(4-fluorophenyl)hexyl]-5,6-dihydro-2H-pyran-2-one was found to show 2-3-fold more potent plant growth inhibitory activity against Italian ryegrass shoots (IC50 = 95 µm) and roots (IC50 = 17 µm) than compound bearing unsubstituted phenyl group. The small and electron withdrawing atom at 4-position of the benzene ring caused the higher activity.

Syntheses and Phytotoxicity of All Stereoisomers of 6-(2-Hydroxy-6-phenylhex-1-yl)-5,6-dihydro-2H-pyran-2-one and Determination of the Effect of the α,ß-Unsaturated Carbonyl Structure and Hydroxy Group Bonding to Chiral Carbon.

All four stereoisomers of naturally occurring 6-(2-hydroxy-6-phenylhex-1-yl)-5,6-dihydro-2H-pyran-2-one (1) were synthesized by employing yeast-reduction products with high optical purity [from 95% enantiomeric excess (ee) to more than 99% ee], and then their phytotoxicities against lettuce and Italian ryegrass were evaluated. In the Italian ryegrass seedlings test, (6S,2'R)-1 showed the most potent and stereospecific activity against the shoots (IC50 = 260 µM) and roots (IC50 = 43.2 µM), with a significant difference from other stereoisomers. The highest seed germination inhibitory activity against Italian ryegrass seed was also observed in (6S,2'R)-1, showing a 53% germination ratio from the control at 1000 µM. This advantageous (6S,2'R) stereochemistry was employed in the syntheses of α,ß-dihydro, 2'-dehydroxy, and 2'-methoxy derivatives 13-15. By the test using these derivatives, the importance of the α,ß-unsaturated double bond and hydroxy group bonding to a chiral center on the 6-alkyl chain of 5,6-dihydro-α-pyrone for phytotoxicity was determined. In the test against lettuce, the 6S configuration and (6S,2'S) configuration were necessary for growth inhibition (IC50 = ca. 60 µM) and germination inhibition (63% germination ratio at 1000 µM), respectively.