Browsing by Author "Swiegers, Hendrik Willem"

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    Comparing different siRNA delivery systems to target Diuraphis noxia
    (Stellenbosch : Stellenbosch University, 2018-12) Swiegers, Hendrik Willem; Botha-Oberholster, Anna-Maria; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Diuraphis noxia, also known as the Russian wheat aphid, is a major pest of wheat. Breeding for resistance against D. noxia has been relatively successful in wheat as there has been many resistance genes incorporated into wheat in the past. However, this resistance has more often than not been counteracted by D. noxia through the development of a new biotype. The mechanism with which D. noxia is able to do this is not well understood. Previously, a highly virulent, laboratory generated biotype, known as SAM (South African Mutant), was compared to its avirulent progenitor, SA1, through proteome analysis of the salivary glands and complete genome sequence analysis. It was found that, among other differences, the cuticle protein, Dncprr1-8, containing a Rebers and Riddiford consensus was present in the salivary gland of SAM but not SA1. The gene also contained single nucleotide polymorphisms (SNPs) between the biotypes. In this study the function of Dncprr1-8 was investigated through RNA interference (RNAi). As RNAi has never been performed in D. noxia, several methods of siRNA delivery to this organism were compared. Injection of siRNA into the aphid haemolymph and ingestion of siRNA through artificial feeding medium was not successful. Allowing D. noxia to feed on wheat inoculated with a virus-induced gene silencing (VIGS) vector modified to contain D. noxia transcript sequence was partly effective, but overall had variable results. Finally, siRNA delivery through injection into wheat and allowing D. noxia to feed around the injection site, proved to be the most effective. Delivery of Dncprr1-8-siRNA using this method resulted in reduced survival and fecundity of biotype SAM while feeding on resistant wheat. The phenotypic responses were then compared to that of another aphid species, Myzus persicae, feeding on Arabidopsis thaliana injected siRNA targeting the same gene. M. persicae did not display reduced survival, but did produce fewer nymphs. Collectively, the results were then used to draw conclusions on the putative function of Dncprr1-8 in the plant-aphid interaction.
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    Elucidating mechanisms that underpin aphid-plant interactions
    (Stellenbosch : Stellenbosch University, 2024-12) Swiegers, Hendrik Willem; Botha-Oberholster, Anna-Maria; Foyer, Christine Hellen; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics & Institute of Plant Biotechnology.
    Aphids are global pests of both crop and ornamental plants. Aphid infestations of crop plants result in yield losses through nutrient depletion and the spread of viruses. Although often effectively controlled by traditional chemical insecticides, these have deleterious effects on both the environment and human health. As such the use of insecticides have been restricted in some parts of the world. Secondly, many aphid populations have developed resistance to various insecticide classes. Host plant resistance offers an alternative to control aphid pests, however as with other control methods, aphids often overcome host resistance through the development of virulent biotypes. The aim of this project was to investigate the aphid-plant interaction by firstly examining differential gene regulation of aphid and host using virulent and avirulent Diuraphis noxia (Russian wheat aphid) biotypes feeding on a resistant Triticum aestivum (bread wheat) cultivar containing the D. noxia resistance gene, Dn7. The virulent D. noxia biotype SAMv2 exhibited distinct gene expression compared to the genealogically linked avirulent SAl, suggesting adaptations to overcome host resistance. SAMv2 upregulated transcripts implicated in detoxification (e.g. L-xylulose reductase) as well as other transcripts of unknown function. Transposable elements were highly active in all biotypes and to a lesser extent, epigenome modifying enzymes, potentially contributing to rapid adaptation to its host. T. aestivum responded to aphid feeding with jasmonic acid signalling and terpene biosynthesis. However, virulent SAMv2 appeared to modulate these defence mechanisms as monoterpenoid and diterpenoid biosynthesis were only upregulated following feeding by avirulent biotypes SAS and SAl, respectively. It was also found that the strigolactone synthesis gene, CYP711A1 was upregulated following feeding of biotype SAS. Secondly, the effect of elevated CO2 and strigolactones on Pisum sativum (pea) susceptibility to Acyrthosiphon pisum (pea aphid) were investigated. A. pisum performed better on strigolactone synthesis and signalling P. sativum mutants while [CO2] had no effect on aphid fecundity. The lower gibberellic acid levels observed in the strigolactone mutants were significantly correlated with increased A. pisum fecundity. Lastly, the function of cuticle protein, CpRRl-8, in the formation of virulence in D. noxia was also investigated. This work provides a foundation for developing more effective and sustainable aphid control strategies. Future research building on these findings will continue to uncover the dynamics of aphid-plant interactions and contribute to the development of resilient agricultural systems.