AUTHOR=Spyropoulou Eleni A. , Dekker Henk L. , Steemers Luuk , van Maarseveen Jan H. , de Koster Chris G. , Haring Michel A. , Schuurink Robert C. , Allmann Silke 

TITLE=Identification and Characterization of (3Z):(2E)-Hexenal Isomerases from Cucumber

JOURNAL=Frontiers in Plant Science

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2017.01342

DOI=10.3389/fpls.2017.01342

ISSN=1664-462X

ABSTRACT=<p><italic>E</italic>-2-hexenal is a volatile compound that is commonly emitted by wounded or stressed plants. It belongs to the group of so-called green leaf volatiles (GLVs), which play an important role in transferring information to plants and insects. While most biosynthetic enzymes upstream of <italic>E</italic>-2-hexenal have been studied extensively, much less is known about the enzyme responsible for the conversion from <italic>Z</italic>-3- to <italic>E</italic>-2-hexenal. In this study we have identified two (3<italic>Z</italic>):(2<italic>E</italic>)-hexenal isomerases (HIs) from cucumber fruits by classical biochemical fractionation techniques and we were able to confirm their activity by heterologous expression. Recombinant protein of the HIs did not only convert the leaf aldehyde <italic>Z</italic>-3-hexenal to <italic>E</italic>-2-hexenal, but also (<italic>Z,Z</italic>)-3,6-nonadienal to (<italic>E,Z</italic>)-2,6-nonadienal, these last two representing major flavor volatiles of cucumber fruits. Transient expression of the cucumber HIs in <italic>Nicotiana benthamiana</italic> leaves drastically changed the GLV bouquet of damaged plants from a <italic>Z</italic>-3- to an <italic>E</italic>-2-enriched GLV profile. Furthermore, transcriptional analysis revealed that the two HIs showed distinct expression patterns. While <italic>HI-1</italic> was specifically expressed in the flesh of cucumber fruits <italic>HI-2</italic> was expressed in leaves as well. Interestingly, wounding of cucumber leaves caused only a slight increase in <italic>HI-2</italic> transcript levels. These results demonstrate that cucumber HIs are responsible for the rearrangement of <italic>Z</italic>-3-aldehydes in both leaves and fruits. Future research will reveal the physiological importance of an increased conversion to <italic>E</italic>-2-aldehydes for plants and insects.</p>