A Saponin Correlated with Variable Resistance of Barbarea vulgaris to the Diamondback Moth Plutella xylostella

Two types of Barbarea vulgaris var. arcuata, the G-type and the P-type, differed in resistance to larvae of the diamondback moth (DBM) Plutella xylostella. Rosette plants of the G-type were fully resistant to the DBM when grown in a greenhouse or collected in the summer season, but leaves collected during the late fall were less resistant, as previously found for flea beetle resistance. The P-type was always susceptible. Extracts of resistant leaflets inhibited larval growth in a bioassay, and a growth-inhibiting fraction was isolated by activity-guided fractionation. A triterpenoid saponin (1) was isolated from this fraction and identified as 3-O-β-cellobiosyloleanolic acid from spectroscopic data and analysis of hydrolysis products. The decrease in resistance of the G-type in the fall was correlated with a decrease in the level of 1, from 0.6–0.9 to <0.2 μmol/g dry wt. Compound 1 was not detected in the susceptible P-type. We conclude that 1 is correlated with the variable resistance of B. vulgaris foliage to the DBM.