Investigation of <i>trans-</i>to-<i>cis</i> isomerization of cinnamic acid in Arabidopsis using stable-isotope-labeled cinnamic acid

Cinnamic acid (CA) is a widely distributed metabolite in plant species and is a precursor of many important plant molecules such as lignin and flavonoids. CA exists as both trans and cis isomers; the trans isomer is more common in nature. Previous reports have revealed that the cis isomer of CA (cis-CA) has auxin-like activity when exogenously applied. Moreover, cis-CA was found as an endogenous compound in planta. Here, we report the chemical synthesis of stable-isotope-labeled trans- and cis-CA. Using these labeled compounds as internal standards, we developed a quantification method of CA using liquid chromatography-quadrupole/time-of-flight tandem mass spectrometry (LC-MS/MS). We identified cis-CA in diverse plant species, including liverwort, moss, and lycophyte, implying an important role of cis-CA in the terrestrial plant kingdom.