Plant oxylipins and their contribution to plant defence

Plants have evolved complex signalling pathways to cope with different biotic stresses. Complex interactions among these pathways permit a tight control between development and stress response. Oxylipin metabolism represents one of the main defence mechanisms employed by plants. It begins with the oxygenation of polyunsaturated fatty acids by lipoxygenases (LOX), to form fatty acid hydroperoxides. These fatty acids hydroperoxides are finally converted into an array of bioactive compounds, collectively known as oxylipins, by the action of several enzymes. Some of these, namely allene oxide synthase (AOS), hydroperoxide lyase (HPL) and divinyl ether synthase (DES) form a specialised cytochrome P450s subfamily, known as CYP74. CYP74 enzymes are specialised for the metabolism of hydroperoxides and, differently from other P450 enzymes, do not bind molecular oxygen but use already oxygenated fatty acids as oxygen donors and as source for reducing equivalents. AOS, together with allene oxide cyclase (AOC), catalyse the formation of 12-oxophytodienoic acid (OPDA) or dinor-OPDA, which are the first intermediates in jasmonic acid biosynthetic pathway. Alternatively, hydroperoxides can be cleaved into some other signalling molecules, such as volatile aldehydes and oxo-acids by HPL or converted into divinyl ethers by DES. At the end of these enzymatic reactions, an array of different oxylipins, which includes jasmonates, aldehydes, ketols, divinyl ethers, each showing specific biological functions, are produced. Levels of oxylipins are low in normal physiological conditions but increase rapidly in response to mechanical wounding, herbivore and pathogen attack and other environmental or developmental inputs. In the present review we will focus on recent advances related to oxylipins biosynthesis and their contribution to local and systemic defence mechanisms of plants.