Doctoral Degrees (Plant Sciences)

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Browsing Doctoral Degrees (Plant Sciences) by Subject "3-glucanase"

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    Involvement of reactive nitrogen species in the Russian wheat: aphid resistance response of wheat
    (University of the Free State, 2010-05-31) Moloi, Makoena Joyce; Van der Westhuizen, Amie J.
    English: A recent accumulating body of evidence points to the importance of nitric oxide (NO) as a signal molecule involved in the regulation of many physiological and biochemical processes in plants. This study was aimed at investigating the involvement of reactive nitrogen species (RNS), in particular NO and peroxynitrite (ONOO-), in the Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), resistance response of wheat (Triticum aestivum L.). Resistant (cv. Tugela DN) and near isogenic susceptible (cv. Tugela) wheat plants were grown under the greenhouse conditions in trays at 24 oC (± 2 oC) and were infested at an early three leaf stage with approximately 20 RWAs (biotype RWASA1) per plant. RWA infestation induced an early accumulation (3-9 hours post infestation) of NO to a higher level in the resistant than susceptible plants. A corresponding increase in the activity of nitrate reductase (NR, EC 1.6.6.1) and inhibition studies using sodium tungstate (NR inhibitor) confirmed its involvement in NO production during the RWA resistance response. In addition, results indicated that the reduction of nitrate (NO3-) to nitrite (NO2-) involves NR and subsequently, nitrite reductase (NiR, EC 1.7.7.1) is responsible for the direct production of NO from NO2-. Inhibition studies using a nitric oxide synthase (NOS, EC 1.14.13.39) inhibitor, N-nitrilo-L-arginine-methyl ester (L-NAME), further proved that NOS has no significant effect on NO production during the RWA resistance response. In an experimental approach using sodium nitroprusside (SNP, NO donor) and / or sodium tungstate (NR inhibitor), it was discovered that NO acts as a signal for the induction of activities of secondary defense related enzymes such as the intercellular β-1,3-glucanase and peroxidase. Higher β-1,3-glucanase transcript accumulation after SNP treatment in resistant plants further supported the involvement of NO as signal molecule during the RWA defense response. In addition, results revealed that NO acts upstream of salicylic acid (SA) during the signaling events. High production of ONOO- [a product of an interaction between superoxide anion (O2-) and NO] in resistant plants after RWA infestation, suggests its involvement in the RWA resistance response. Inhibition of β-1,3-glucanase and peroxidase activities by urate (a specific inhibitor of ONOO- production), further suggests the involvement of ONOO- in the induction of the secondary RWA defense response. Furthermore, it was found that SNP root application of wheat plants resulted in transient resistance only. However, in contrast to root treatments, application of SNP as a seed dressing was more effective and has indeed a great potential of alleviating RWA damage in the susceptible plants by reducing aphid attraction as well as the intrinsic rate of aphid increase.