Biochemistry of Russian wheat aphid resistance in wheat: involvement of lipid-like products
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Physical damage and disease are known to cause changes in the lipid composition of plants. The biochemical pathways that lead to changes in the lipid composition were investigated in a comparative study using Russian wheat aphid (RWA) (Diuraphis noxia) infested and uninfested resistant (‘Tugela DN’, ‘Gariep’ and ‘Betta’ DN) and near isogenic susceptible (‘Tugela’, ‘Molopo’ and ‘Betta ’) wheat cultivars. Lipoxygenase (LOX) activity was determined spectrophotometrically while LOX proteins were analysed by means of Western blots. Lipid peroxidation was determined by measuring malondialdehyde levels spectrophotometrically. To investigate the involvement of prostanoids in the RWA resistance response, prostanoid biosynthesis was inhibited with indomethacin where after the activities of defence related enzymes peroxidase (POD) and LOX were determined spectrophotometrically. Lipid-like products were analysed by means of gas liquid chromatography (GC/MS) and mass spectrometry. Western blots were used to analyse the cyclooxygenase (COX) proteins. LOX, which catalyzes the first step of the lipoxygenase pathway, was selectively induced in the infested resistant wheat. This increase in LOX activity coincided with lipid peroxidation. Several lipid-like compounds were found to be newly induced after infestation and the synthesis of others were enhanced after infestation in the resistant wheat. Many of these lipid products were identified as hydroxyl and keto fatty acids. Some of these fatty acids could be detected as early as 1 h.p.i. (hours post infestation). The levels of some fatty acids increased as infestation proceeded while the levels of others reached a peak 48 h.p.i. and declined towards 96 h.p.i. Hydroxyl and keto fatty acids are important signalling compounds during defence responses. A second pathway, COX pathway, which was believed to exist only in mammals, was also found during the wheat-RWA interaction. The COX protein was selectively induced in resistant wheat after infestation . It has high homology to mammalian COX, which is responsible for prostanoid synthesis. The involvement of prostanoids in the RWA resistance response was confirmed by inhibition studies of prostanoid biosynthesis. Downstream defence reactions e.g. LOX and peroxidase (POD) activities were inhibited upon inhibition of the prostanoid biosynthetic pathway. These results emphasize the importance of two lipid biosynthesis pathways (LOX and COX) that may be essential for the establishment of a successful defence response in wheat to the RWA.
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