Investigation of the isomerization of trans- and cis-cinnamic acid in Arabidopsis using stable-isotope-labeled cinnamic acid isomers

Cinnamic acid (CA) is a widely distributed metabolite in plant species and is a precursor of many important plant molecules, including lignin, flavonoids, and coumarins. CA exists as both trans and cis isomers; the trans isomer is more stable and common in nature. Previous reports have revealed that the cis isomer of CA (cis-CA) has auxin-like activity when exogenously applied. However, it has also been reported that cis-CA does not function as an auxin but affects its transport. Although these reports suggest a crucial role for cis-CA as an endogenous signaling molecule, its exact function and mechanism of isomerization from trans-CA remain unclear. 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 LC-MS/MS. Moreover, we monitored the endogenous conversion from trans-to cis-CA using the labeled compounds, demonstrating the UV-dependent and UV-independent CA isomerization in Arabidopsis. Additionally, we identified cis-CA in diverse plant species, including liverwort.