Biochemistry of Russian wheat aphid resistance in wheat: involvement of lipid-like products

[摘要] 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 LOXproteins were analysed by means of Western blots. Lipid peroxidation wasdetermined by measuring malondialdehyde levels spectrophotometrically. Toinvestigate the involvement of prostanoids in the RWA resistance response,prostanoid biosynthesis was inhibited with indomethacin where after the activities ofdefence related enzymes peroxidase (POD) and LOX were determinedspectrophotometrically. Lipid-like products were analysed by means of gas liquidchromatography (GC/MS) and mass spectrometry. Western blots were used toanalyse the cyclooxygenase (COX) proteins. LOX, which catalyzes the first step ofthe lipoxygenase pathway, was selectively induced in the infested resistant wheat.This increase in LOX activity coincided with lipid peroxidation. Several lipid-likecompounds were found to be newly induced after infestation and the synthesis ofothers were enhanced after infestation in the resistant wheat. Many of these lipidproducts were identified as hydroxyl and keto fatty acids. Some of these fatty acidscould be detected as early as 1 h.p.i. (hours post infestation). The levels of somefatty acids increased as infestation proceeded while the levels of others reached apeak 48 h.p.i. and declined towards 96 h.p.i. Hydroxyl and keto fatty acids areimportant signalling compounds during defence responses. A second pathway, COXpathway, which was believed to exist only in mammals, was also found during thewheat-RWA interaction. The COX protein was selectively induced in resistant wheatafter infestation . It has high homology to mammalian COX, which is responsible forprostanoid synthesis. The involvement of prostanoids in the RWA resistanceresponse was confirmed by inhibition studies of prostanoid biosynthesis.Downstream defence reactions e.g. LOX and peroxidase (POD) activities wereinhibited upon inhibition of the prostanoid biosynthetic pathway. These resultsemphasize the importance of two lipid biosynthesis pathways (LOX and COX) thatmay be essential for the establishment of a successful defence response in wheat tothe RWA.

[发布机构] University of the Free State