## 3 Results
### BF06 VOCs Enhance the Tolerance of Arabidopsis to Fe Deficiency  
The solubility of soil Fe in the plant rhizosphere is much lower under high pH conditions, which contributes to Fe deficiency-induced chlorosis in plants [30]. Fe deficiency often caused a reduction  
Figure 3: List of the top 10 significant GO terms for upregulated differentially expressed genes (DEGs) of VOCs-exposed _Arabidopsis_ plants based on GO classifications. GO terms were categorized into three functional groups including biological process (BP), cellular component (CC), and molecular function (MF). The \(p\) value indicates the significance of the comparison between the control and VOCs-exposed plants. of chlorophyll content, and severely interfered with the structure and functions of chloroplasts [14]. In this study, we explored the effects of BF06 VOCs on the responses of plants to alkaline stress. Seven-day-old _Arabidopsis_ seedlings were grown on an alkaline MS medium (pH = 7.6) under different Fe conditions, for 10 days with or without exposure to BF06 VOCs. We found that VOCs exposure evidently alleviated the leaf chlorosis caused by Fe deficiency (Figure 4A), indicating increased Fe levels in the VOCs-exposed plants. Total chlorophyll content was much higher in the VOCs-exposed plants than in the controls under alkaline stress (Figure 4B). Accordantly, BF06 VOCs exposure obviously improved photosynthetic capacity in plants. Several photosynthetic parameters including \(Pn\), \(Fo/Fm\), and \(\Phi\)PSII were significantly elevated in the VOCs-exposed plants compared with the controls (Figure 4C-E). Considering that Fe is an essential element in photosynthesis, the levels of Fe might be correlated with BF06 VOCs exposure. Thus, we measured the content of Fe in both the control and VOCs-exposed plants under alkaline stress. Our data revealed that BF06 VOCs exposure induced a great elevation of Fe content in alkaline-treated plants (Figure 5A). Especially at 5 \(\upmu\)M Fe, total Fe content of VOCs-exposed plants was about double that of the controls. When plants were grown on the medium without the Fe source, BF06 VOCs exposure obviously ameliorated plant Fe deficiency. However, no significant difference in total Fe content was observed  
Figure 4: BF06 VOCs promoted the growth of _Arabidopsis_ plants under Fe-limited conditions. Seven-day-old seedlings were cultured on an alkaline MS medium containing different Fe levels (0, 5, 20, or 50 \(\upmu\)M Fe(III)-EDTA) for 10 days with or without exposure to BF06 VOCs. The control and VOCs-exposed plants were used to analyze plant growth phenotype (**A**); chlorophyll content (**B**); photosynthetic rate (**C**); \(Fo/Fm\) (**D**); and \(\Phi\)PSII (**E**). Different letters represent the significant difference between the control and VOCs-exposed plants at \(p<0.05\). between the control and VOCs-exposed plants (Figure 5A). To account for this, the soluble and cell wall Fe was analyzed in the control and VOCs-exposed plants. The soluble Fe content of shoots or roots in the controls was markedly lower than in the VOCs-exposed plants (Figure 5B,C). It has previously been indicated that the cell wall plays an important role in Fe stores in plants under Fe-deficient conditions [45]. Thus, we assessed whether BF06 VOCs exposure promoted the release of root cell wall Fe under Fe-limited conditions. In this study, the cell wall of control roots had less bound Fe than the VOCs-exposed roots (Figure 5D). Similar phenomena were observed where VOCs exposure remarkably increased the soluble Fe in plants under different Fe conditions (Figure 5B,C). Our results implied that the VOCs-induced increase of soluble Fe contributed to enhancing the availability of cell wall Fe. Furthermore, ultrastructural observation of mesophyll cells revealed that few grana lamellae rudimentary grana occurred in the chloroplasts of control plants, displaying the classic traits of thylakoid disorganization caused by Fe deficiency (Figure 6A-D).