Formation of (2E)-4-Hydroxy-2-nonenal and (2E)-4-Hydroxy-2-hexenal by Plant Enzymes: A Review Suggests a Role in the Physiology of Plants

It is demonstrated that (3Z)-nonenal (NON) and (3Z)-hexenal (HEX) are oxidized in a cascade by lipoxygenase (LOX) and hydroperoxide peroxygenase (HP peroxygenase) into (2E)-4-hydroxy-2- nonenal (HNE) and (2E)-4-hydroxy-2-hexenal (HHE), respectively. In turn, HNE inactivates LOX terminating the cascade. The hydroxy-alkenals produced serve to inhibit plant pathogens, which initiated the cascade. In addition to LOX, other unknown oxygenases may be involved in the cascade.

Oxylipin Pathway in Rice and Arabidopsis

Plants have evolved complex signaling pathways to coordinate responses to developmental and environmental Information. The oxylipin pathway Is one pivotal lipid-based signaling network, composed of several competing branch pathways, that determines the plant＇s ability to adapt to various stimuli. Activation of the oxyllpln pathway Induces the de novo synthesis of biologically active metabolltes called ＂oxyllplns＂. The relative levels of these metabolltes are a distinct indicator of each plant species and determine the ability of plants to adapt to different stimuli. The two major branches of the oxyllpln pathway, allene oxide synthase （AOS） and hydroperoxlde lyase （HPL） are responsible for production of the signaling compounds, jasmonates and aldehydes respectively. Here, we compare and contrast the regulation of AOS and HPL branch pathways In rice and Arabidopsis as model monocotyledonous and dicotyledonous systems. These analyses provide new Insights Into the evolution of JAs and aldehydes signaling pathways, and the complex network of processes responsible for stress adaptations In monocots and dicots.

An oriental melon 9-lipoxygenase gene CmLOX09 response to stresses, hormones, and signal substances

In plants, lipoxygenases （LOXs） play a crucial role in biotic and abiotic stresses. In our previous study, five 13-LOX genes of oriental melon were regulated by abiotic stress but it is unclear whether the 9-LOX is involved in biotic and abiotic stresses. The promoter analysis revealed that CmLOX09 （type of 9-LOX） has hormone elements, signal substances, and stress elements. We analyzed the expression of CmLOX09 and its downstream genes--CmHPL and CmAOS-in the leaves of four-leaf stage seedlings of the oriental melon cultivar ＂Yumeiren＂ under wound, hormone, and signal substances. CmLOX09, CmHPL, and CmAOS were all induced by wounding. CmLOX09 was induced by auxin （indole acetic acid, IAA） and gibberellins （GA3）; however, CrnHPL and CmAOS showed differential responses to IAA and GA3. CmLOX09, CmHPL, and CmAOS were all induced by hydrogen peroxide （H2O2） and methyl jasmonate （MeJA）, while being inhibited by abscisic acid （ABA） and salicylic acid （SA）. CmLOX09, CmHPL, and CmAOS were all induced by the powdery mildew pathogen Podosphaera xanthii. The content of 2-hexynol and 2-hexenal in leaves after MeJA treatment was significantly higher than that in the control. After infection with P. xanthii, the diseased leaves of the oriental melon were divided into four levels-levels 1,2, 3, and 4. The content of jasmonic acid （JA） in the leaves of levels 1 and 3 was significantly higher than that in the level 0 leaves. In summary, the results suggested that CmLOX09 might play a positive role in the response to MeJA through the hydroperoxide lyase （HPL） pathway to produce C6 alcohols and aldehydes, and in the response to P. xanthiithrough the allene oxide synthase （AOS） pathway to form JA.