Isomerases determine

green odor of plants

Published September 01 2016

Researchers have identified hexenal isomerases that give plants their characteristic smell.

When a plant’s tissue gets damaged, it gives off a green, leafy odor. In an effort to make crops like tomatoes more appealing to consumers, breeders of fruits and vegetables are interested in masking or sweetening this odor, which can affect a food’s flavor. Conventional breeding methods have been tried, but with limited results. Some researchers think genetic identification methods may hold the key to odor and flavor enhancement.

The leafy odor of plants is characterized by green leaf volatiles. GLVs are composed of six-carbon compounds that provide not only the characteristic green leaf odor but also characteristic aromas and flavors to fruits and vegetables.

One compound, (E)-2-hexenal, was identified early on in GLV research and named the leaf aldehyde. But the enzyme or enzymes involved in (E)-2-hexenal production had yet to be identified.

In a recent article in the Journal of Biological Chemistry, Yasuo Yamauchi and his team at the Graduate School of Agricultural Science at Kobe University in Japan discuss how they identified (Z)-3:(E)-2-hexenal isomerases, or HIs, as essential to the production of the (E)-2-hexenal in plants. The team found that various plant species have homologous HIs and that red paprika, a bell pepper variant, has especially robust HI activity. They extracted and purified HI from red paprika to determine its enzymatic role in the production of the (E)-2-hexenal and to evaluate its ability to enhance the odor of tomato plants.

The researchers performed alignment and phylogenetic tree analysis and found that HIs belong to a family of a functionally diverse proteins containing a conserved barrel domain. They discovered that the three catalytic amino acids histidine, lysine and tyrosine are conserved in various plant species and form a catalytic site known as catalytic HKY in HIs. HIs enzymatically isomerize a six-carbon compound called (Z)-3-hexenal to (E)-2-hexenal; 3-hexyn-1-al, another six-carbon compound, acts as a suicide substrate for HIs by binding irreversibly to histidine. This suggests that the catalytic mode of HI is a keto-enol tautomerism reaction, which is a chemical equilibrium between a ketone or aldehyde and an alcohol mediated by a catalytic histidine residue.

The researchers compared transgenic tomato plants overexpressing the red paprika HI with wild-type tomato plants. (Z)-3-hexenal is the main volatile that determines wild-type tomato flavor and is responsible for the green, leafy odor of tomatoes. They showed that transgenic tomato plants accumulate the (E)-2-hexenal, in contrast to wild-type tomato plants that mainly accumulate (Z)-3-hexenal. This study demonstrates that the conversion of (E)-2-hexenal to (Z)-3-hexenal by HI contributes to a sweeter green odor of tomatoes. This genetic identification of volatile emissions of tomato fruits made by Yamauchi’s team may provide tomato breeders with an opportunity to abandon conventional breeding methods and turn to genetic manipulation for improvement of tomato flavor.

Lee D. Gibbs Lee D. Gibbs is a doctoral candidate at the University of North Texas Health Science Center.