## 3 Discussion  
Safeguarding plants against pests, microorganisms, and weeds is always the focus of the plant protection field. Plants produce a large variety of specialized metabolites that can function as active anti-stress molecules, and how to explore and employ these natural molecules has attracted much attention. It is widely documented that terpen are among the most reported bioactive compounds with anti-insect, anti-microbial (phytoalexin), and allelopathic activities [17; 46; 47]. Especially, plant volatile terpen, mainly composed of monoterenes and sesquiterpenes, play pivotal roles in plant protection, perhaps primarily  
Figure 6: **The overexpression of _GmOCS_ in _N. tabacum_ led to enhanced resistance against S. litura._ (A) Expression analysis of the _GmOCS_ gene by RT-PCR in the wild type and transgenic tobaccos. (B) The terpene release pattern detected from the control and transgenic tobacco leaves overexpressing _GmOCS_. (C) Relative contents of volatile terpen detected in the control and transgenic soybean leaves overexpressing _GmOCS_. (D) _S. litura_ attacked wild leaves and transgenic leaves, overexpressing _GmOCS_ in a dual-choice feeding-preference experiment. (E) Leaves of wild and _GmOCS_ overexpressed lines eaten by _S. litura_ in a dual-choice feeding-preference experiment. Data represented the mean \(\pm\) SD of three biological replicates, and the student’s \(t\)-test was used to analyze the significance of differences (**_p_ \(<\) 0.01) in (B,E). **by repelling insect herbivores, attracting the natural enemies of the herbivores, and increasing the resistance of plants by signaling within or between plants [25; 48; 49; 50; 51]. Intriguingly, the selective modulation nature that terpenes seem to be more toxic to the enemy insects but with only little effects on beneficial insects, making terpenes an amazing candidate for pesticide development [52]. However, research on herbivore-induced plant volatiles has mainly focused on model plants instead of agricultural crops. Among these, the monoterpene (\(E\))-\(\beta\)-ocimene is perhaps one of the most common volatiles whose release is induced by herbivory and is thought to protect plants by attracting natural enemies or repelling the insects [23; 53; 54; 55]. Since the first (\(E\))-\(\beta\)-ocimene synthase gene (\(AtTPS03\)) was cloned from _Arabidopsis_, more and more \(TPS\) genes responsible for (\(E\))-\(\beta\)-ocimene synthesis have been characterized, and their anti-insect functions in some plants, such as tomato, Chinese cabbage, and lima bean were also deciphered [56; 57; 58]. In the present study, soybean \(GmOCS\) was proved to encode (\(E\))-\(\beta\)-ocimene synthase by both in vitro enzymatic assay and transient \(N\). _penthamiana_ overexpression assay, in which \(\beta\)-ocimene, especially (\(E\))-\(\beta\)-ocimene, was the most obvious product (Figure 4). Elicitor-induced assay and dual-choice feeding-preference assay indicated that \(GmOCS\) was substantially activated by a \(S\). _litura_ attack and, in turn, repelled the insect with its product (\(E\))-\(\beta\)-ocimene (Figures 2 and 5). Generally, insects may damage plant leaves during an attack. Simultaneously, JA increases significantly and is proposed to induce an increase in \(\beta\)-ocimene [50; 59; 60]. Hereto, MeJA and mechanical wounding are generally employed to simulate the herbivore attack in these assays. Notwithstanding, they cannot be replaced with each other as different volatiles and gene responses were observed in our results (Figure 2). This could be understandable as even the bouquets of volatiles were different when referring to different herbivore attacks. For example, _Helicoverpa zeadamaged \(N\). _tubacum_ released more (\(E\),\(E\))-TMTT (4,8,12-trimethyl-1,3,7,11-tridecattarene) and (\(E\),\(E\))-\(\alpha\)-farnesene than \(H\). _virescens_-attacked tobacco, which released more \(\alpha\)-pinene instead [61].