Characterization of a new (Z)-3:(E)-2-hexenal isomerase from tea (Camellia sinensis) involved in the conversion of (Z)-3-hexenal to (E)-2-hexenal

Two major green leaf volatiles (GLVs) in tea that contribute greatly to tea aroma, particularly the green odor, are (E)-2-hexenal and (Z)-3-hexenal. Until now, their formation and related mechanisms during tea manufacture have remained unclear. Our data showed that the contents of (E)-2-hexenal and (Z)-3-hexenal increased more than 1000-fold after live tea leaves were torn. Subsequently, a new (Z)-3:(E)-2-hexenal isomerase (CsHI) was identified in Camellia sinensis. CsHI irreversibly catalyzed the conversion of (Z)-3-hexenal to (E)-2-hexenal. Abiotic stresses including low temperature, dehydration, and mechanical wounding, did not influence the (E)-2hexenal content in intact tea leaves during withering, but regulated the proportions of (Z)-3-hexenal and (E)-2hexenal in torn leaves by modulating CsHI at the transcript level. For the first time, this work reveals the formation of (E)-2-hexenal during tea processing and suggests that CsHI may play a pivotal role in tea flavor development as well as in plant defense against abiotic stresses.

Automated summary

This study reveals the formation and mechanisms of (E)-2-hexenal during tea processing, and identifies the role of CsHI in this process. Additionally, it shows that abiotic stresses can regulate the proportions of (Z)-3-hexenal and (E)-2-hexenal in torn tea leaves by modulating CsHI expression levels. The findings of this study are important for the development of tea flavor and plant defense against abiotic stresses.