## 3 Experimental Section
### Headspace Collection and HS-SPME-GC-MS Analysis  
We used two methods and assayed to collect the HS of _Arabidopsis_ seedlings after a 7 day-incubation. To quench enzyme activity in _Arabidopsis_, we treated the seedlings in the following manner: (i) a set of HS vials wrapped in aluminum foil were incubated at 80 \({}^{\circ}\)C for 30 min in a windy oven (WFO-600 ND; Eyela, Tokyo, Japan) and (ii) 0.25-M EDTA-NaOH water solution (pH 7.5) was added into another set of vials to attain a final EDTA concentration of 50 mM. Solid CaCl\({}_{2}\) was then immediately added to a final concentration of 5 M [39]. Next, 10-\(\upmu\)L of \(n\)-alkane standard solutions C8-20 (0.8 mg/L) was added to each vial as an internal standard. The vials were closed with magnetic screw-caps and then sonicated (US-108; NSD, Suwa, Japan) at a frequency of 38-Hz for 5 min. Vials without any treatment were prepared and used as controls.  
Nine types of SPME fibers were purchased from Supelco (Supelco, PA, USA). A SPME fiber coated with a 100-\(\upmu\)m-thick layer of polydimethylsiloxane (PDMS) metal alloy (100-\(\upmu\)m PDMS) was finally chosen for VOC profiling (see Results and Discussion section). SPME fibers were coated with (i) 30-\(\upmu\)m- or (ii) 7-\(\upmu\)m-thick layers of PDMS-fused silica (FS) fiber/stainless steel (SS), (iii) 75-\(\upmu\)m-thick layer of carboxen/polydimethylsiloxane (CAR/PDMS) FS/SS, (iv) 85-\(\upmu\)m-thick layer of CAR/PDMS StableFlex (SF) fiber/SS, (v) 65-\(\upmu\)m-thick layer of PDMS/divinylbenzene (DVB) SF/SS, (vi) 50/30-\(\upmu\)m-thick layer of DVB/CAR on PDMS (DVB/CAR/PDMS) SF/SS, (vii) 85-\(\upmu\)m-thick layer of polyacrylate (PA 85) or (viii) carboxax-polyethylene glycol (C-PEG) for HS collection. Before the analysis, all fibers were conditioned at the appropriate conditioning temperature and time as shown in Table 3. Each fiber was exposed to the vial headspace for 20 min at 60 \({}^{\circ}\)C with continuous agitation.  
After HS collection, the volatiles were thermally desorbed in splitless mode on a CTC CombiPAL autosampler (CTC analytics, Zwingen, Switzerland) connected to an Agilent 6890N gas chromatograph (Agilent Technologies, Wilmington, USA) for 0.1 min at the appropriate inlettemperature, as shown below. Each fiber was baked for 5 min by applying the appropriate conditioning temperature (see below).  
The capillary column used for the analysis was a 30-m\(\times\)0.25-mm inner diameter fused-silica capillary column with a chemically bound 0.25-\(\upmu\)L film Rxi-5 Sil MS stationary phase (RESTEK, Bellefonte, USA) with a tandem connection to a fused silica tube (1 m, 0.15 mm). A mass spectrometer column change interface (ms NoVent-J; SGE, Yokohama, Japan) was used to prevent air and water from entering the MS during column change over. Helium was used as the carrier gas at a constant flow rate of 1.0 ml/min. The temperature program started with a 2 min isothermal step at 50 \({}^{\circ}\)C, followed by temperature ramping at 15 \({}^{\circ}\)C to a final temperature of 260 \({}^{\circ}\)C, which was then maintained for 2 min. The transfer line to the mass spectrometer was set to 250 \({}^{\circ}\)C. The TOF mass spectrometer was a Pegasus 4D MS system (Leco, MI, USA) with an EI source set to 200 \({}^{\circ}\)C. The acceleration voltage was turned on after a solvent delay of 200 s. Mass spectra were monitored with an acquisition rate of 30 spectra/s and over a mass-to-charge ratio range of \(m/z\) = 30-550.