## 4 Materials and Methods
### Identification of VOCs by Means of SPME-GC/MS  
Each strain was cultured on tryptic soy agar (TSA) medium (BioLife, Sarasota, FL, USA) for 48 h at 30 \({}^{\circ}\)C [27]. Headspace vials were partially filled with the 2 g of solid medium containing colonies and sealed. Bacterial volatile organic compounds (VOCs) produced by _O. vulgare_ ssp. _vulgare_ endophytic strains were extracted from the vial headspace and concentrated by solid-phase microextraction (SPME) before desorption in the GC injection port. Headspace extraction was performed with a 2.5 mL Syringe-HS (0.64-57-R-H, PTFE, GERSTEL) conditioned and held at 40 \({}^{\circ}\)C from sample collection to injection. In the SPME, one Fibre Assembly was evaluated and used: 50/30 \(\upmu\)m divinylbenzene (DVB)/ carbowax (CAR)/ polydimethylsiloxane (PDMS) (Supelco, Bellefonte, PA, USA). Fiber was exposed to bacterial culture in 20 mL SPME vial (75.5 \(\times\) 22.5 mm) for 30 min at 40 \({}^{\circ}\)C, after 30 min of equilibration time. The desorption time was 5 min. Before use, fibre  
Figure 4: Schematic representation of the quantitative cross-streaking method. Target strain (blue) is streaked 10 times onto 3 half Petri dishes with septum. The cross-streaking plate (**a**) tests the ability of the tester strain (orange) to antagonize the growth of the target, while the growth control plate (**b**) permits one to verify its growth in the absence of the tester. In these last two cases, cells are recovered after incubation for 48 h at 30 \({}^{\circ}\)C (t\({}_{1}\)). The control plate (**c**) allows one to determine the number of cells streaked on the TSA medium; in this case, cells are immediately recovered for the viable count (t\({}_{0}\)). was conditioned and cleaned at 270 \({}^{\circ}\)C for 30 min, following instructions from Supelco(r). Splitless injection was used. Gas chromatographic analysis was performed using an Agilent 7000C GC (Agilent Technologies, Inc., Santa Clara, CA, USA) system equipped with a split/splitless injector, fitted with an Agilent HP5-MS UI capillary column (30 m \(\times\) 250 \(\upmu\)m; 0.25 \(\upmu\)m film thickness) coupled to an Agilent triple quadrupole Mass Selective Detector MSD 5973 (Agilent Technologies, Inc., Santa Clara, CA, USA), with ionization voltage, 70 eV; electron multiplier energy, 2000 V; transfer line temperature, 270 \({}^{\circ}\)C. Solvent Delay: 0 min. Helium was used as the carrier gas (1 mL min\({}^{-1}\)). The oven program was as follows: temperature was initially kept at 40 \({}^{\circ}\)C for 5 min and then gradually increased to 250 \({}^{\circ}\)C at a rate of 2 \({}^{\circ}\)C/min, which was held for 15 min and finally raised to 270 \({}^{\circ}\)C at 10 \({}^{\circ}\)C/min. Samples were injected at 250 \({}^{\circ}\)C automatically. Interval scan: 35-450 \(m/z\); Scan speed: 10,000 amu\(\cdot\)s\({}^{-1}\) (25 Hz). The GC-MS mass spectrum data were analysed using MassHunter Qualitative Analysis B.06.00, and the database of National Institute Standard and Technology (NIST) was used to interpret analysed data. Comparison of the mass spectrum of the unidentified components released by the bacterial isolates was carried out against the mass spectrum of already-known components available in the NIST 11 MS library. The molecular weight and peak area percentage of unknown compounds were evaluated by the software as observed from the chromatogram. The following are available online at [[https://www.mdpi.com/article/10.3390/microorganisms10071424/s1](https://www.mdpi.com/article/10.3390/microorganisms10071424/s1)]([https://www.mdpi.com/article/10.3390/microorganisms10071424/s1](https://www.mdpi.com/article/10.3390/microorganisms10071424/s1)). Table S1: Number of colony forming units of each target strain obtained at the beginning and at the end of the cross-streaking experiments in the absence or in the presence of the endophytic strains. Conceptualization, A.P.P., R.F. and A.M.P. ; methodology, G.S., G.P., A.P.P. and R.F. ; validation, S.D.D., A.V., S.C. and L.M.C.