# Rice Sesquierpene Plays Important Roles in Antixenosis against Brown Planthopper in Rice
## 2 Results and Discussion
### OsSTPS2 Is Responsible for the Production of E-\(\beta\)-Farnesene in RH and IL Infested by BPH  
Sesquiterpenes are the major plant volatile compounds released after damage by herbivore infestation [7; 8; 9]. The production of multiple volatile compound products in response to the expression of one sesquiterpene synthase gene is a natural phenomenon found in several plant species [10; 11; 12]. A total of 25 sesquiterpenes were identified by GC-MS in the mixture of volatile compounds emitted by KD, IL283, and RH rice plants infested by BPH (Figure 4, Tables S3-S5). The major sesquiterpene products of rice plants infested with BPH were \(\beta\)-ionone, \(\beta\)-ionone epoxide, E-\(\beta\)-farmesene, and linalool (Figure 5). E-\(\beta\)-farmesene (EBF) is the only major product that the BPH-resistant varieties RH and IL283 emitted in significantly higher amounts than the BPH-susceptible variety KD. Although RH emitted EBF at a lower level in the control condition, the emission was higher than KD in all the BPH treatment conditions. In addition, even though IL283 emitted EBF at a similar level to KD in the first six BPH feeding days, it started to emit at a higher level from day 7 to day 10 of BPH feeding. As RH and KD contain variations in _OsSTPS2_ at both the genomic and protein product levels, and as the KD allele seems to lose its function because of its disrupted protein chain, it may be assumed that the reduced EBF production in KD was the result of this abnormality. Our results suggest that, besides several sesquiterpenes, EBF is the major product of this gene, and the gene was named \"E-\(\beta\)-farmesene synthase (_OsEBFS_)\" due to its prominent property of producing EBF as the volatile compound induced by BPH infestation. Figure 2: BPH damage score reaction against brown planthopper (BPH) population from Ubon Rachathani (UBN-BPH) for six rice varieties/lines at seven days after BPH infestation (7 DAI). The error bars are the standard deviation (SD). Figure 3: Antixenosis of BPH feeding preference (AFP) test. (**A**–**D**) Comparisons between IL308 and other varieties; (**E**–**G**) comparisons between JHN and other varieties. A total of 10 second- and third-instar BPH nymphs were allowed to settle on the experimental plants, and five replicates were performed for each comparison. The y axis indicates the number of BPHs that settled on each plant as a percentage, and the \(x\) axis shows the progress between 1 and 96 h after BPH infestation. The error bars are the standard error (SE). HAI = hours after BPH infestation. The role of BPH-induced volatile compounds in the defense mechanisms of rice has long been studied [2; 13; 14]; however, only one sesquiterpene synthase gene (LOC_Os08g07100) has been reported to be induced by BPH [15]. Moreover, not only BPH feeding but also fall armyworm caterpillar infestation induces this gene; the major volatile products were found to be zingiberene and \(\beta\)-sesquiphellandrene, respectively [16]. In this present study, we demonstrated that the major volatile compound product of _OsSTPS2_, the novel BPH-feeding-induced sesquiterpene synthase, is EBF. We showed that the expression of the gene is controlled at the transcription level. Moreover, we also showed that two different processes can lead to the lesions in the protein product of the gene. EBF is a common constituent of the aphid alarm phenomone [17; 18; 19] and has been reported to be used by wild potato as a repellent against aphid attack [20]. Recently, the role of EBF in the indirect defense mechanism has been studied in transgenic Arabidopsis [21]. Unfortunately, no evidence of its role in the defense mechanism of transgenic plants against aphids has been found. This may be due to the gene having been studied in different host plants with different insect enemies. Our finding that rice emitted EBF in response to BPH attack may open a path to understanding how rice communicates with its attackers. Figure 4: Sesquiterpene products of rice plants after 10 days of BPH feeding treatment.