## 2 Results and Discussion
### Discovery of Potential Markers of HT and MT Strains  
The potential markers discovery is a critical step for metabolomics studies [28]. The process of selecting informative metabolites was important for finding the differences between HT and MT strains and it could provide clues of their different metabolism pathways. Potential markers were then selected using VIP values based on the untargeted profiling method. The plot of VIP value (first 100 variables) with standard error is shown in Figure 5a. The potential markers were selected based on VIP value higher than 1.5 [21; 22] and \(p<0.05\) according to the \(t\)-test. Besides, metabolites with error bars extending beyond zero, which showed no statistic meaning, were also excluded. Finally, 39 extracted ion variables were obtained and these variables were identified using ion information and retention times. In total, 12 compounds were identified and the relative content (normalized by the internal standard ion fragment) is shown in Table 2.  
Figure 4: OPLS-DA score plots of two models, (**a**) untargeted profiling; (**b**) quantitative analysis. Figure 3: PCA score plot for the metabolic profile of four strains. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**NO.**} & \multirow{2}{*}{**Potential Markers**} & \multirow{2}{*}{
\begin{tabular}{c} **Retention** \\ **Time/Min** \\ \end{tabular} } & \multirow{2}{*}{**Ion Information**} & \multicolumn{2}{c}{**Relative Content 1** \\ \hline
1 & Styrene & 8.001–8.004 & 103, 78, 77, 104, 51, 105 & **0.13-29.77 *** & 0.08–13.21 \\
2 & 1-Octen-3-one & 10.627–10.672 & 97, 70, 111, 98, 83, 55 & **1.46–114.32 *** & 4.00–86.63 \\
3 & Octanal & 11.378 & 55 & **0.03-2.95 *** & 0.04–1.43 \\
4 & Limonene & 12.232 & 91 & **0.17-9.21 *** & 0.04–0.63 \\
5 & 2-Octen-1-ol & 13.408–13.466 & 68, 95, 58, 81, 54, 110, 82, & **0.51–75.13 *** & 2.00–57.36 \\
6 & Methyl octanoate & 15.091 & 74 & 0.03–0.14 & **0.03-0.56 * \\
7 & Unknown & 15.438–15.446 & 69, 84, 55 & 0.02–0.44 & **0.03–1.08 * \\
8 & Unknown & 20.402 & 91 & **0–0.82 *** & 0–0.25 \\
9 & Unknown & 21.057 & 91 & **0–0.27 *** & 0–0.07 \\
10 & Thujopsene & 23.718–23.756 & 204, 121, 105 & 0–0.67 & **0–4.1 *** \\
11 & Unknown & 24.599 & 165 & **0.05–0.47 *** & 0.02–0.23 \\
12 & Cuparene & 25.542 & 132 & 0–0.01 & **0–1.3 * \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Potential markers selected by VIP values and \(t\)-test. Figure 5: Variable importance on projection (VIP) plot scores for two models, (**a**) untargeted profiling; (**b**) quantitative analysis. These volatile compounds included, 1 alcohol, 1 aldehyde, 1 ketone, 1 ester, 3 hydrocarbons, 2 sesquiterpenenes and 4 unidentified compounds. Of these, (_E_)-2-octen-1-ol, octanal, 1-octen-3-one, styrene, limonene and 3 unidentified compounds (\(m/z\) was 91, 91 and 165) were selected as the important metabolites for AC44 and AC46 strains. The abundance of these compounds was significantly higher than those generated by high toxigenic strains. The result was similar to previous studies, that the non-toxic strains synthesized more volatile compounds than the toxigenic strains [5]. The reason for abundant C8-compounds, (_E_)-2-octen-1-ol, octanal and 1-octen-3-one, in MT strains may be explained by the metabolic pathway leading to the formation of MVOCs and OTA, which provides important clues to the relationship between mycotoxin formation and various groups of volatiles (Figure 6) [41]. The polyketide skeleton formation (marked in red) is a critical step of OTA biosynthesis, which requires acetate and malonate with the activity of polyketide synthases [48]. Meanwhile, the fatty acid formation pathway (marked in blue) is also derived from acetate and malonate via the acetate-malonate pathway, which forms a competitive relationship with polyketide skeleton formation [41]. According to that, we speculate that less OTA biosynthesis may lead to more fatty acid formation. As a result, more eight carbon compounds, octanal, (_E_)-2-octen-1-ol and 1-octen-3-one, are synthesized from fatty acid [38].