Theses And Dissertations

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Browsing Theses And Dissertations by Author "Abrahams, Adrian Mark"

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    Potential role of photosynthetic and metabolic related proteins in the resistance mechanism of Tugela Dn against Russian Wheat Aphid-SA2
    (University of Fort Hare, 2016) Abrahams, Adrian Mark
    Continued increase in the world population increases the demand for food, which requires active intervention to ensure food security. Furthermore, greenhouse gases have resulted in global warming which has triggered climate change, with the consequence that food production systems have been negatively affected and this imparts further risk to food security. The agricultural sector, and thus global food security, therefore is directly affected by these changing climatic conditions since they result in an increase of insect outbreaks as well as related resistant breaking pathogens. The effect of carbon dioxide (CO2) on cereal’s most economically significant insect pests, the aphid, is not well researched. Due to the destructive nature of D. noxia’s feeding behaviour on crops, several methods have been devised to control the damages. In South Africa, the first wheat resistant cultivar, Tugela Dn which contained the Dn resistance gene was released in 1992. Initially it was reported that resistant cultivars reduce aphid population by inhibiting Russian Wheat Aphid (RWA) growth and reproduction. Recently, the presence of resistance breaking RWA biotypes in South Africa was confirmed and is thought to be more virulent on existing RWA-resistant wheat lines. This study focussed on gaining a greater understanding of the resistance mechanisms activated by the Russian Wheat Aphid resistance gene Dn (Dn1) and whether RWA-SA2 overcomes the resistance response of Tugela Dn with the aim of improving agronomical traits of wheat plants. Initial reports showed that RWA-SA2 not only bred faster but also caused more damage to wheat lines than did RWA-SA1. RWA-SA2 appeared unaffected by the Dn1 resistance gene and posed a serious threat to small grain production in South Africa. In the first part of this study, we evaluated whether RWA-SA2 damages/compromises the photosynthetic mechanisms of wheat by measuring chlorophyll fluorescence of infested RWA-SA2 susceptible and resistant wheat cultivars as well as whether there are any changes in stomatal conductance. Results obtained indicate that although both wheat isolines (Tugela and Tugela Dn) exhibited aphid injury, Tugela as expected appeared to be affected more and at an earlier stage. It would appear that the chlorophyll concentration in the uninfested Tugela leaves were significantly higher than RWA-SA2 infested Tugela leaves from 120 hours onwards. Tugela Dn infested leaves on the other hand appeared to have thrived quite well with negligible chlorophyll concentration loss. Stomatal conductance was enhanced considerably in Tugela by RWA-SA2 feeding suggesting increased stomatal apertures. However, stomatal conductance dropped after day 7 in Tugela which is a result from damage to the leaf tissues. The second part of this study focussed on identifying exclusive expressed proteins during RWA-SA2 infestation on RWA-SA1 susceptible and resistant wheat cultivars as well as to identify possible signalling pathways induced in cereals. The Tugela wheat cultivar provided more evidentiary support that during the initial hours of RWA-SA2 phloem feeding, several proteins were down-regulated that could possibly indicate an initial response to phloem feeding by “switching off” the plants metabolic mechanisms and preventing the flow of nutrients to the sieve elements for aphids and/or other phloem feeders to benefit. Exclusively identified proteins were largely involved in photosynthesis, metabolism and stress suggesting that the rate of incorporation and/or exportation of photosynthetic products decline, becoming restricted by feedback inhibition. It would appear that the pathways identified function in parallel to capitalize on more defense efforts rather than resistance against RWA-SA2 as these pathways seem to be interlinked. This study therefore provides evidence that Tugela Dn, seem to counteract deleterious effects of aphid (RWA-SA2) herbivory through up-regulation and faster regeneration of photosynthetic related molecule and does respond in a highly specific manner to infestation with RWA-SA2 by inducing unique pathways. It would also appear that RWA-SA2 partially overcomes the resistance response of Tugela Dn against RWA-SA1.