Elucidation of possible virulence factors present in Russian wheat aphid (Diuraphis noxia) biotypes’ saliva

Abstract

The Russian wheat aphid (RWA) is a pest of cereals, such as wheat and barley. It feeds on these hosts by injecting saliva into the plants’ phloem tissue and consuming the mixture of saliva and photoassimilates. It has been proposed that the insect’s saliva contains elicitors or virulence factors, which cause the symptoms typically observed in susceptible wheat cultivars. These are leaf rolling, chlorotic streaking, a decrease in yield and death in cases of heavy infestation. In contrast, resistant plants display symptoms typical of defence responses, such as the formation of necrotic lesions and an increase in the expression of pathogenesis related proteins. But, most importantly, RWA feeding on these hosts does not result in their subsequent death. The objectives of the present study are thus to elucidate any putative virulence factors, present in insect saliva, that can result in the breakdown of resistance of cultivars and thus, lack of recognition and/or delayed onset of the plants’ defence responses. Thus, this thesis investigates the RWA on protein level to determine which components of these insects induce the different changes observed in the resistant and susceptible plants. Also, it examines whether or not the biotypes uniquely altered their elicitors in response to selective pressure. In Chapter 1 a brief introduction is presented on the Russian wheat aphid, its distribution and the effects of its feeding on resistant and susceptible wheat cultivars. In Chapter 2 a literature review provides insight on how the Russian wheat aphids feed and survive on wheat. It also outlines the control mechanisms which plants could employ to withstand attack from pests and pathogens. In Chapter 3 proteins were extracted from different parts of two Russian wheat aphid biotypes and separated on high pressure liquid chromatography (HPLC). Two biotypes were selected for the study to provide comparative information on the development of biotypes and/or their virulent elicitors. The presence of the potential elicitors was determined by examining the extent of leaf rolling chlorotic streaking/spots on injected plants’ leaves, determining the activity of defence related enzymes of the injected plants and visualizing the proteins extracted from these plants on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) gels. This was done in order to examine the plants on phenotypic, enzymatic and proteomic levels, which could confirm the results obtained on three different levels. It was found that resistant cultivars react similarly to the two biotypes, but that the RWA biotypes differ significantly on a protein level. Potential motivations for these variations are discussed. Results presented in this dissertation represent the outcomes of a study conducted from March 2005 to December 2006 in the Department of Genetics, Forestry and Agricultural Biotechnology Institute, University of Pretoria, under the supervision of Prof. A-M Botha-Oberholster. Chapter 3 is being prepared to be submitted for review in Insect Biochemistry and Molecular Biology.