Electrical penetration graphs indicate that tricin is a key secondary metabolite of rice, inhibiting phloem feeding of brown planthopper, Nilaparvata lugens

Tricin (5,7,4-trihydroxy-3,5-dimethoxyflavone) is a valuable secondary metabolite which is widely present in gramineous plants, including cultivated rice (Oryza sativa L.) (Poaceae). It can defend the rice plant against damage by the brown planthopper (BPH), Nilaparvata lugens (Stal) (Hemiptera: Delphacidae), one of the most important pests of rice. This study was conducted to elucidate the mechanisms of action of tricin on BPH feeding behavior. BPH feeding behavior in resistant (Rathu Heenati, RHT) and susceptible (Taichuang native 1, TN1) rice varieties and artificial diets was monitored using the electrical penetration graph (EPG) technique. Tricin concentrations in leaves of varieties RHT and TN1 were quantitatively analyzed by liquid chromatography, coupled to tandem mass spectrometric techniques. Six (NP and N1-5) and four (NP, N1, N2, and N4) types of waveforms occurred during feeding on rice plants and artificial diets, respectively. The tricin concentration of rice varieties was correlated with total and average durations of N4. Moreover, EPG data indicated that tricin significantly increased the duration of non-probing and pathway periods and strongly inhibited phloem ingestion (N4). The inhibition was strongly dose dependent, resulting in complete suppression of activity in the phloem region when the tricin concentration was increased to 1gl(-1). This study revealed that tricin disturbed the feeding behavior of BPH mainly by increasing the non-probe period and inhibiting phloem ingestion. We confirmed the hypothesis that tricin is a stylet probing stimulant' of rice planthoppers as proposed in previous studies. The information on the ecological effect of tricin from this study may be useful to clarify the resistance mechanism against BPH of RHT and other tricin-containing rice varieties.