## 2 Results
### Sequence Analysis of GmOCS  
As _GmOCS_ and _GmTPS21_ were the genes with the most substantially activated expressions in _S. litura_ and MeJA-treated soybean, their promoters were subjected to further analysis. Results showed that both _GmTPS_ genes might be responsive to several abiotic environmental cues such as light, drought, and phytohormones, including abscisic acid and gibberellin, etc. (Figure 3A). To better understand their roles in soybean stress response, _GmOCS_ was selected and cloned from soybean cultivar Wm82 by routine PCR as _GmTPS21_ was characterized to encode a sesquiterpene synthase, which was not able to produce (_E_)-\(\beta\)-ocimene. Online predictions indicated that _GmOCS_ encoded a protein with a molecular weight (MW) of 68.7 KDa and a theoretical PI at 6.25, and the protein had an instability index [(44)] of 43.83, which was predicted to be localized in chloroplast, implying that GmOCS might function as a monoterpene synthase and should be degraded soon as an instant and temporary response to external stimuli (Figure 3B). Detailed amino acid alignment revealed that GmOCS contained terpene-cyclization-related RR(X)\({}_{8}\)W motif, Mg\({}^{2+}\) or Mn  
Figure 2: **Volatile terpen and _GmTPS_ expressions response to MeJA, mechanical wounding, or _S. litura_ treatment. (A) Volatile terpen and expressions of _GmTPS_ genes detected in soybean leaves treated with MeJA. (B) Volatile compounds and expression of _GmTPS_ genes detected in soybean leaves under mechanical wounding condition. (C) Volatile compounds and expression of _GmTPS_ genes induced by _S. litura_. Healthy and intact leaves without damage were used as control. Each experiment was performed in three biological replicates, and only representative data were provided. The transcripts were normalized by _GmACTIN_ and _\(\beta\)-tubulin_ and compared with the expression level of a specific gene in control. All the data were calculated as \(\log_{2}(\text{N}+1)\). Red and blue boxes indicated high and low expression levels, respectively.**cofactor-binding-related DDXX (D/E) and (N,D) DXX (S,T,G) XXXE (NSE/DTE) motifs (Figure S3). Moreover, monoterpene synthases were generally predicted to play roles in plastids, whereas sesquiterpene synthases were usually localized in the cytoplasm. To further check the potential subcellular localization of GmOCS, the GmOCS-GFP fusion protein was expressed in _Arabidopsis_ (_Arabidopsis thaliana_) protoplasts. Consequently, it was primarily confined in chloroplasts, indicating its possible role in monoterpene biosynthesis (Figure 3C).  
Figure 3: **Sequence and subcellular analysis of GmOCS.****(A)** The _cis_-acting regulatory elements in promoter sequences of _GmOCS_ and _GmTPS21_. The 4254 bp and 3052 bp upstream sequences of the initiation codon ATG were selected as promoter sequences of _GmOCS_ and _GmTPS21_, respectively, and were analyzed by PlantCARE followed by being visualized with Simple BioSequence Viewer in TBtools [45]. (B) Basic sequence information of GmOCS. GmOCS sequence properties were analyzed with the ExPASy Protparam tool and Plant-mPLoc server with default parameters. (C) Subcellular localization of GmOCS in _Arabidopsis_ protoplasts. Green, GFP fluorescence; Red, chlorophyll fluorescence; Bright Light, brightfield image; Merged, merged green, red, and bright light images. Scale bar = 25 μm.