## 3 Discussion  
Many metabolites produced during the growth of microorganisms and plants display nematicidal activity, so these metabolites have high potential as biological nematicides [23; 24]. Among metabolites, volatile organic compounds (VOCs) have received wide-spread attention because of their potential to control RKNs and other plant pathogens (such as insects, fungi, and bacteria) [15]. Compared with non-volatile nematicidal metabolites, the advantage of volatile nematicidal metabolites is that they have good diffusion and permeability. Previous studies showed that various VOCs produced by _Bacillus_ spp. (_Bacillus cereus_, _Bacillus atrophaeus_, _Bacillus atlutdinis_) in soil showed significant nematicidal activity against root-knot nematodes [19; 20]. Therefore, the VOCs produced by _Bacillus_ spp. have great application prospects in controlling root-knot nematodes. The _B. velezensis_ GJ-7 was isolated from the rhizosphere soil of _P. notoginseng_, which acts against _M. hapla_[22]. In our previous study, _B. velezensis_ GJ-7 fermentation broth showed strong nematicidal activity against _M. hapla_ J2s and suppressed the hatching of eggs in vitro, which means it produced metabolites against root-knot nematodes. However, little was known about whether the volatile organic compounds (VOCs) produced by _B. velezensis_ GJ-7 were effective in biocontrol against _M. hapla_. Further study of the main nematicidal metabolites of _B. velezensis_ GJ-7 is of great significance for the development of biological pesticides. In this study, we tested the nematicidal activity of the GJ-7 strain VOCs against _M. hapla_ through a three-compartment Petri plate experiment. The results suggested that the GJ-7 strain may produce VOCs that act against _M. hapla_ J2s. Volatile organic compounds (VOCs) usually exist as complex mixtures of low molecular weight lipophilic compounds [25]. These can be alcohols, ketones, hydrocarbons, terpen, fatty acids, or heteroatom-containing compounds [26]. So far, more than 1000 VOCs produced by more than 400 species of bacteria and fungi have been described [27; 28]. The solid-phase micro-extraction gas chromatography-mass spectrometry (SPME-GC/MS) is widely used for the collection and identification of VOCs. In this method, VOCs are adsorbed by the SPME fiber from the headspace of a culture medium. The adsorbed com \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multicolumn{4}{c}{**Chemotaxis Index (C.I)**} \\ \cline{2-5}  & **0.1**\(\mu\)**L/mL** & **1**\(\mu\)**L/mL** & **10**\(\mu\)**L/mL** & **100**\(\mu\)**L/mL** \\ \hline
3-methyl-1-butanol & \(-\)0.044 \(\pm\) 0.0006 b & \(-\)0.274 \(\pm\) 0.025 a & \(-\)0.284 \(\pm\) 0.042 a & \(-\)0.299 \(\pm\) 0.037 a \\
3-methyl-2-pentanone & \(-\)0.019 \(\pm\) 0.006 b & \(-\)0.033 \(\pm\) 0.015 b & \(-\)0.103 \(\pm\) 0.007 a & \(-\)0.112 \(\pm\) 0.018 a \\
5-methyl-2-hexanone & \(-\)0.035 \(\pm\) 0.007 b & \(-\)0.057 \(\pm\) 0.028 b & \(-\)0.095 \(\pm\) 0.012 ab & \(-\)0.148 \(\pm\) 0.037 a \\
2-heptanone & \(-\)0.111 \(\pm\) 0.017 b & \(-\)0.216 \(\pm\) 0.022 a & \(-\)0.270 \(\pm\) 0.034 a & \(-\)0.258 \(\pm\) 0.003 a \\
2,5-dimethylpyrazine & \(-\)0.064 \(\pm\) 0.017 a & \(-\)0.048 \(\pm\) 0.003 a & \(-\)0.052 \(\pm\) 0.013 a & \(-\)0.079 \(\pm\) 0.003 a \\
6-methyl-2-heptanone & \(-\)0.022 \(\pm\) 0.015 b & \(-\)0.023 \(\pm\) 0.018 b & \(-\)0.039 \(\pm\) 0.023 b & \(-\)0.120 \(\pm\) 0.036 a \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemotaxis index values of commercial VOCs toward _M. hapla_. Different lowercase letters indicate significant differences among different concentrations of the same VOC. (\(n=3\)). pounds are then separated with gas chromatography and further identified with mass spectrometry. In this study, SPME-GC/MS technology was also used to analyze and identify the VOCs produced by _B. velezensis_ GJ-7.