**Abbreviations**

Aromatic amino acid aminotransferase (ArAT), atmospheric pressure chemical ionization (APCI), coenzyme A (CoA), correlation spectroscopy (COSY), Dalton (Da), desorption ESI (DESI), dimethylaminopurine (cytokinin) (DMAA), direct analysis in real-time (DART), distortionless enhancement via polarization transfer (DEPT), electron ionization (EI), electrospray ionization (ESI), planar electrochromatography or thin-layer electrochromatography (ETLC), gas chromatography-mass spectrometry (GC-MS), heteronuclear multiple bond correlation (HMBC), heteronuclear single quantum coherence (HSQC), high-performance liquid chromatography (HPLC), high-performance TLC (HPTLC), hyoscyamine 6β-hydroxylase (H6H), indole-3-acetic acid (auxin) (IAA), infrared (IR), ionmobility spectrometry (IMS), laser ablation dart imaging (LADI), liquid chromatography-mass spectrometry (LC-MS), mass spectrometry (MS), methylputrescine oxidase (MPO), multiple reaction monitoring (MRM), tandem mass spectrometry (MS/MS), nuclear magnetic resonance (NMR), nuclear Overhauser effect spectroscopy (NOESY), putrescine *N*-methyltransferase (PMT), quadrupole timeof-flight mass spectrometry (QToF), rotating frame Overhauser enhancement spectroscopy (ROESY), selective reaction monitoring (SRM), time-of-flight mass spectrometry (TOF), thin-layer chromatography (TLC), total correlation spectroscopy (TOCSY), tropinone reductase (TR), ultrahigh-performance liquid chromatography (UHPLC), ultraviolet (UV), vibrational circular dichroism (VCD).
