## 3 Results
### VOCs-Induced NO Accumulation Promotes Fe Absorption and Availability  
NO functions as a reciprocal signaling molecule in plant-mic probe interactions, involving modulation of various physiological processes such as bacterial biofilm, plant growth, and development [55; 56; 57]. NO has been demonstrated to play an essential role in controlling Fe absorption in plants under Fe-limited conditions [58; 59; 60]. Here, DAF-FM DA was first used to analyze the accumulation of VOCs-induced NO in _Arabidopsis_ roots. As shown in Figure 7, only a slight NO-associated fluorescence was detected in the roots of control plants grown on an alkaline MS medium containing 50 \(\upmu\)M Fe(III)-EDTA, whereas the VOCs-exposed roots exhibited stronger fluorescence than the controls. Moreover, the VOCs-induced accumulation of root NO significantly decreased after treatment with the nitric oxide synthase (NOS) inhibitors, N\(\upomega\)-nitro-l-arginine methylene (L-NAME, 5 mM), or the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (c-PTIO,1 mM). NO has been demonstrated to be involved in the regulation of plant responses and tolerance to Fe-deficient conditions [59]. Moreover, NO exposure can promote the formation of nitrosyl-Fe complexes, thereby increasing the mobilization of Fe and delivery of NO to different tissues [61]. Reduced production of NO influences FIT-regulated Fe deficiency responses in plants. The _Arabidopsis_ FIT1 plays a vital role in mediating Fe deficiency responses, which is responsible for upregulating _IRT1_ and _FRO2_[35]. In this study, BF06 VOCs exposure significantly enhanced the expression of _FIT1_, _IRT1_ and _FRO2_ in plants under alkaline stress. As shown in Figure 8A-C, after 10 days of exposure to BF06 VOCs, the expression of _FIT1_ increased at least 6-fold in plants grown on an alkaline medium containing different Fe concentrations. The transcription of _IRT1_ and _FRO2_ was elevated more than 6-fold in the VOCs-exposed plants compared with the controls. Moreover, the activity of FCR in the VOCs-exposed plants was markedly higher than in the controls. However, the activity of FCR was significantly decreased in plants treated with L-NAME or c-PTIO. Similarly, the transcription levels of _FIT1_, _FRO2_, and _IRT1_ were also markedly lower in the non-treated plants than in the L-NAME- and c-PTIO-treated plants. Figure 6: Ultrastructular analyses of chloroplasts in the control and VOCs-exposed plants. Seven-day-old seedlings were cultured on an alkaline MS medium containing different concentrations of Fe(III)-EDTA (0, 5, 20, or 50 \(\upmu\)M) with or without exposure to BF06 VOCs. Then, transmission electron micrographs of chloroplasts were observed in both the control (**A–D**) and VOCs-exposed (**E–H**) plants when plants were treated with 0, 5, 20, or 50 \(\upmu\)M Fe(III)-EDTA, respectively. Scale bar = 1 \(\upmu\)m.  
We addressed the question of whether microbial VOCs-regulated plant Fe absorption was attributable to the action of NO. Seven-day-old _Arabidopsis_ seedlings were grown on an alkaline MS medium under different Fe conditions for 10 days with or without exposure to BF06 VOCs. Our results indicated that BF06 VOCs exposure failed to enhance the ability of plants to absorb Fe,  
Figure 8: Expression of Fe acquisition-related genes and FCR activity in _Arabidopsis_ plants after exposure to BF06 VOCs. _Arabidopsis_ plants were grown on an alkaline MS medium (pH = 7.6) for seven days, then transferred to a 1/2 MS medium containing 50 uM Fe(III)-EDTA with or without exposure to BF06 VOCs for 10 days. qRT-PCR analyses of the expression of _FIT1_ (**A**), _IRT1_ (**B**), and _FRO2_ (**C**) among non VOCs-exposed (control), VOCs-exposed, VOCs + L-NAME-treated, and VOCs + c-PTIO-treated plants, and their FCR activities were further examined (**D**). Different letters represent a significant difference between the control and VOCs-exposed plants at \(p<0.05\). Figure 7: Nitric oxide (NO) accumulation in _Arabidopsis_ roots after exposure to BF06 VOCs.