## 3 Materials and Methods
### Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry (UHPLC-MS/MS) Analysis  
For the identification and quantification of SGs in _C. erythraea_ methanol extracts, Dionex Ultimate 3000 UHPLC system (Thermo Fisher Scientific, Bremen, Germany) equipped with a triple quadrupole mass spectrometer (TSQ Quantum access max, ThermoFisher Scientific, Basel, Switzerland) was employed. The samples were chromatographically separated on Hypersil gold C18 column (50 \(\times\) 2.1 mm) with 1.9 um particle size (Thermo Fisher Scientific, Waltham, MA, USA), thermostated at 30 degC. Mobile phase, consisting of water + 0.01% acetic acid (A) and acetonitrile (B), was eluted according to the gradient previously described in [45]. Acetonitrile was of LC-MS grade (Fisher Scientific, Leics, UK), and ultra-pure deionized water was generated while using Water Purification System (New Human Power I Integrate, Human Corporation, Seoul, Republic of Korea). The flow rate of themobile phase was set to 0.4 mL min\({}^{-1}\) and the injection volume to 10 \(\upmu\)L. All of the analyses were performed while using three biological replicates. A triple-quadrupole mass spectrometer with a heated electrospray ionization (HESI) source was set to the following parameters: vaporizer temperature 300 \({}^{\circ}\)C, spray voltage 4000 V, sheet gas (N\({}_{2}\)) pressure 28 AU, ion sweep gas (N\({}_{2}\)) pressure 1.0 AU and auxiliary gas (N\({}_{2}\)) pressure at 10 AU, capillary temperature 275 \({}^{\circ}\)C, and skimmer offset 0 V. Argon was used as the collision gas in the collision-induced fragmentation, and collision energy (cE) was set to 20 eV for all of the targeted compounds. Loganic acid (**1**), loganin (**2**), secologanin (**3**), sweroside (**4**), swertiamarin (**5**) and gentiopcrin (**6**) were quantified while using the selected reaction monitoring (SRM) mode of the instrument, and by tracking two diagnostic MS\({}^{2}\) fragments of each compound, which were previously defined in product ion scanning (PIS) experiment (Table 1). The identification of the targeted compounds in the samples was additionally confirmed while using DAD analysis (Figure 1), and the data were acquired at \(\uplambda\) = 240, 260, and 320 nm. The external standard method was used for the quantification of iridoids and secoiridoids. The preparation of stock-standard solutions was performed by dissolving 1 mg of compounds **1** (Extrasynthese, Genay, France), **2**, **3** (Sigma-Aldrich, Darmstadt, Germany), **4**, **5** (both 98% purity, [PERSON], Marktredwitz, Germany), or **6** (\(>\)90% purity, [PERSON], Karlsruhe, Germany) in 1 mL of 96% methanol. Stock solutions of six standards were mixed to obtain the working standard solution in concentration of 100 \(\upmu\)g mL\({}^{-1}\), which was then diluted with methanol to obtain further calibration levels, up to 5 ng mL\({}^{-1}\). For each of the calibration curves, the calculation of regression was performed. They all showed excellent linearity with correlation coefficients of r = 0.999, \(p<\) 0.001. Calculating peak areas was undertaken to obtain the total concentrations of the analyzed iridoids and secoiridoids and they are expressed as \(\upmu\)g per 100 mg of plant fresh weight (\(\upmu\)g 100 mg\({}^{-1}\) FW). Xcalibur software (version 2.2) was used for the instrument control, data acquisition, and analysis.