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

Abstract

E-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 E-2-hexenal have been studied extensively, much less is known about the enzyme responsible for the conversion from Z-3- to E-2-hexenal. In this study we have identified two (3Z):(2E)-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 Z-3-hexenal to E-2-hexenal, but also (Z,Z)-3,6-nonadienal to (E,Z)-2,6-nonadienal, these last two representing major flavor volatiles of cucumber fruits. Transient expression of the cucumber HIs in Nicotiana benthamiana leaves drastically changed the GLV bouquet of damaged plants from a Z-3- to an E-2-enriched GLV profile. Furthermore, transcriptional analysis revealed that the two HIs showed distinct expression patterns. While HI-1 was specifically expressed in the flesh of cucumber fruits HI-2 was expressed in leaves as well. Interestingly, wounding of cucumber leaves caused only a slight increase in HI-2 transcript levels. These results demonstrate that cucumber HIs are responsible for the rearrangement of Z-3-aldehydes in both leaves and fruits. Future research will reveal the physiological importance of an increased conversion to E-2-aldehydes for plants and insects.

Silkworms suppress the release of green leaf volatiles by mulberry leaves with an enzyme from their spinnerets

“…1 ). On the other hand, ( Z )-3-hexenal stayed in the disrupted tissues is isomerized to form ( E )-2-hexenal in some plant species 37 , 38 . Thus, ( E )-2-hexenal and ( E )-2-hexen-1-ol were major volatiles in the disrupted mulberry leaves.…”

Arabidopsis thalianaSKP1 interacting protein 11 (At2g02870) negatively regulates the release of green leaf volatiles

“…14 Among the HPL downstream enzymes are included (3Z):(2E)-hexenal isomerase, responsible for the conversion of Z-3-hexenal to its E-2 isomer, and NAD-dependent alcohol dehydrogenase (ADH), that catalyzes the reduction of aldehydes to their respective alcohols in order to confer higher stability. [15][16][17] All these saturated and unsaturated aldehydes and their downstream products synthesized in the plant's vegetative tissue are volatile in nature and are collectively named as GLV. The role of GLV in the plant's response to various stresses, like microbial pathogens and herbivory attack, has been studied since the early 1990s.…”

Green leaf volatiles protect maize (Zea mays) seedlings against damage from cold stress

“…Z-3-HAL is reduced to form (Z)-3-hexen-1-ol (Z-3-HOL) by a NADPH dependent reductase (Matsui, Sugimoto, Mano, Ozawa, & Takabayashi, 2012), which can then be converted to (Z)-3-hexeny-1-yl acetate (Z-3-HAC) by acetyl-CoA:(Z)-3-hexen-1ol acetyltransferase (D'Auria, Pichersky, Schaub, Hansel, & Gershenzon, 2007). Z-3-HAL may also be converted to (E)-2-hexen-1-al spontaneously or enzymatically by (Z)-3:(E)-2-hexenal isomerase (Kunishima et al, 2016;Spyropoulou et al, 2017). GLVs are released almost immediately from freshly damaged leaves (Hatanaka, 1993;Matsui, 2006;Matsui et al, 2012), but can also be released systemically from undamaged leaves in response to feeding by insect herbivores (Turlings & Tumlinson, 1992).…”