SUMMARYA pre-infestation of the white-backed planthopper (WBPH), Sogatella furcifera Horvá th, conferred resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) in rice (Oryza sativa L.) under both laboratory and field conditions. The infestation of another planthopper species, the brown planthopper (BPH) Nilaparvata lugens Stå l, did not significantly reduce the incidence of bacterial blight symptoms. A large-scale screening using a rice DNA microarray and quantitative RT-PCR revealed that WBPH infestation caused the upregulation of more defence-related genes than did BPH infestation. Hydroperoxide lyase 2 (OsHPL2), an enzyme for producing C 6 volatiles, was upregulated by WBPH infestation, but not by BPH infestation. One C 6 volatile, (E)-2-hexenal, accumulated in rice after WBPH infestation, but not after BPH infestation. A direct application of (E)-2-hexenal to a liquid culture of Xoo inhibited the growth of the bacterium. Furthermore, a vapour treatment of rice plants with (E)-2-hexenal induced resistance to bacterial blight. OsHPL2-overexpressing transgenic rice plants exhibited increased resistance to bacterial blight. Based on these data, we conclude that OsHPL2 and its derived (E)-2-hexenal play some role in WBPH-induced resistance in rice.

In order to clarify the mechanism of induced resistance to blast disease in rice, Oryza sativa, that had been previously infested by the white-backed planthopper, Sogatella furcifera Horváth, we first investigated the accumulation of salicylic acid (SA) and jasmonic acid (JA) in rice plants infested by the planthopper. The results confirmed that infestation of S. furcifera strongly stimulates the production of SA and JA in rice. These results indicate that both salicylate-and jasmonate-mediated pathways (SA and JA pathways), which are involved in the general defense system in plants, were activated in rice infested by S. furcifera. Further results confirmed that S. furcifera infestation induces accumulation of a major rice diterpenoid phytoalexin, momilactone A, and a flavonoid phytoalexin, sakuranetin, which are well known as antimicrobial chemicals, particularly in blast disease caused by the blast fungus, Magnaporthe oryzae B. Couch. All these results strongly suggest the following hypothetical mechanism of induced-resistance to M. oryzae in rice infested by S. furcifera. First, S. furcifera releases some elicitor-active compounds, which might be produced in the salivary glands, into the rice plant during feeding. Next, the defense signal systems, SA-and JA-mediated pathways, are activated by the elicitor. Finally, phytoalexins are induced in rice as antimicrobial compounds mainly through activation of the JA-mediated pathway.

The effect of previous rice plant infestation of the white-backed planthopper, Sogatella furcifera, on the incidence of rice blast disease caused by the fungal pathogen, Magnaporthe grisea, was investigated. The fungal pathogen M. grisea was less likely to cause symptoms of leaf blast on rice plants that had been previously infested with S. furcifera than on uninfested plants. This indicates that resistance to M. grisea was induced in rice plants by infestation of S. furcifera. The number of blast lesions per plant decreased with increasing numbers of infesting S. furcifera from 0 to 10 pairs. The active period of induced resistance was maintained for a relatively long time being greater than 15 d. The difference in the incidence of leaf blast between S. furcifera infested and uninfested plants in the booting stage became smaller than that in the tillering stage, although there was a still significant difference in blast incidence between both infested and uninfested plants in booting stage. There was no significant difference in the incidence of leaf blast between damaged plants treated with needling and untreated control plants. This suggests that the simple mechanical effect to the plants caused by stylet insertion (penetration) and subsequent probing with the feeding behavior of S. furcifera was not related to the observed phenomenon. In S. furcifera infested plants, the expression of two genes regarding b-1,3-glucanase, Gns4 and Gns5, was confirmed by RT-PCR analysis. These results show that infestation of S. furcifera apparently induced physiological changes including gene expression that were related to pathogen resistance to M. grisea in rice plants.