Authors:M.J. Moloi, A.J. van der Westhuizen, and A. Jankielsohn
The Russian wheat aphid (RWA) is serious pest of wheat in South Africa since its discovery in 1978. Nitric oxide (NO) plays an essential role in the RWA resistance response of wheat. This study was conducted to establish whether NO acts upstream or downstream of salicylic acid (SA) during the RWA defence response and also to investigate the effect of NO application on RWA control. In addition, the involvement of peroxynitrite in the RWA resistance response of wheat was studied. Resistant and/or susceptible plants grown under controlled conditions (25 ± 2°C) were used. Using a NO donor, sodium nitroprusside (SNP), and a NO production inhibitor, sodium tungstate (NaWO4), it was proved that NO acts upstream of SA during the RWA resistance response of wheat. Furthermore, a significant decrease in RWA intrinsic increase rate (rm) and disease symptom development after SNP application emphasized the role of NO in the RWA resistance responses. High levels of peroxynitrite (by-product of NO) content in the RWA infested resistant plants and inhibition of secondary defence enzymes (β-1,3-glucanase and peroxidase) after urate (inhibitor of peroxynitrite production) application proposed the involvement of this molecule in the signalling events of the RWA resistance.
The oxidation of fatty acids, which is responsible for changes in lipid composition, were investigated in a comparative study using the Russian wheat aphid (RWA) (Diuraphis noxia), infested and un-infested resistant (Tugela DN) and close isogenic-susceptible (Tugela) wheat cultivars. LOX, which catalyzes the first step of the lipoxygenase pathway, was selectively induced in the infested resistant wheat. A pathogen-induced oxygenase protein was also found to be induced during the wheat-RWA interaction. The involvement of oxylipins in the RWA resistance response was confirmed by inhibition studies using indomethacin, which is known to inhibit prostanoid biosynthesis in mammalian tissue. Downstream defense reactions, e.g. LOX and POD activities, were inhibited upon indomethacin treatment. These results emphasize the importance of fatty acid oxidation as an essential process for the establishment of a successful defense response in wheat to the RWA.