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  • Author or Editor: A.J. Westhuizen x
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The Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is notorious for causing severe yield losses in many of the wheat producing areas of the world. The phenylpropanoid pathway is involved in many plant defence mechanisms. Phenylalanine ammonia-lyase (PAL) is a key enzyme leading to biosynthesis of phenolic acids, some of which are involved in plant defence mechanisms. The effect of RWA infestation on PAL activity and phenolic acid composition was studied in resistant (Tugela DN) and susceptible (Tugela) wheat cultivars. PAL activity was selectively induced in the infested resistant wheat. The increase in PAL activity was reflected in the selective increase in certain phenolic acid concentrations. The phenolic acids that were selectively induced in resistant wheat upon RWA infestation were identified using the authentic phenolic acids p-hydroxybenzoic acid, gallic acid, resorcylic acid, gentisic acid, caffeic acid, p-hydroxyphenylacetic acid, and ferulic acid. The induced levels of these phenolic acids corresponded to the peak PAL activities. It is evident that the phenylpropanoid pathway contributes to the resistance of wheat to the RWA.

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The involvement of nitric oxide (NO) in the Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov), resistance response of wheat (Triticum aestivum L.) was investigated. Resistant (cv. Tugela DN) and near isogenic susceptible (cv. Tugela) wheat plants grown under green-house conditions were used. Russian wheat aphid infestation induced an early accumulation of NO to a higher level in the resistant than susceptible plants. Nitric oxide accumulation corresponded with increased nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NiR, EC 1.7.7.1) activities, suggesting a possible association with NO production. Inhibition studies confirmed the involvement of NR in NO production during the RWA resistance response. The use of a NO donor or NR inhibitor revealed that NO could act as another early signal for induction of the secondary defence enzymes, intercellular β-1,3-glucanase and peroxidase.

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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.

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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.

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