Studies of resistance to aphids and viruses in Arabidopsis thaliana and Capsicum annuum

Cucumber mosaic virus (CMV) causes significant losses in crop production. In this study, methods to enhance plant resistance to CMV infection and infestation by Myzus persicae, a vector that transmits CMV, were assessed. There were two parts to the project. In the first, the feasibility of using exogenous salicylic acid (SA) application to induce resistance to CMV infection in pepper (Capsicum annuum) plants were investigated. In the second, I investigated the basis of the heightened resistance to Myzus persicae infestation observed in Arabidopsis thaliana plants carrying mutant alleles for Argonaute (AGO) 1 to determine if it was due to accumulation of a cyanogenic compound, 4-hydroxyindole carbonyl nitrile (4-OH-ICN). Exogenous application of 1 mM SA to pepper plants prior to CMV inoculation induced resistance to CMV, which was exhibited as a delay in the onset of symptoms. The level of CMV disease incidence among the treated pepper plants was also lower than the control-treated CMV-inoculated plants by 22 days post inoculation. Thus, exogenous SA treatment may have potential to protect pepper plants. Mutations in the 4-OH-ICN biosynthetic genes cyp82C2 or fox1 did not diminish the resistance to aphids observed in Arabidopsis ago1.25 mutant plants. Aphid growth rates and fecundity on ago1/cyp82C2 or fox1/ago1 mutant plants did not significantly differ from aphid performance on wild-type plants. This indicates that deregulation of 4-OH-ICN biosynthesis does not contribute to aphid resistance in ago1 mutant Arabidopsis plants. Progeny of crosses involving ago1.25 mutant plants with mutant plants compromised in biosynthesis of SA (salicylic acid induction deficient 2, also called isochorismate synthase 1: ics1) were generated. Mutation in ICS1 affected the ago1.25 phenotype so that plants were less deformed and stunted and were less resistant to aphid infestation than ago1.25 plants. However, not all the ago1/ics1 progeny had milder phenotypes. The ago1.25/ics1 progeny that retained their ago1-like phenotypes also retained their heightened resistance to aphid infestation. However, the milder ago1-like phenotypes of the ago1.25/ics1 progeny cannot be explained by their ago1 and ics1 mutations. Results obtained with ago1.46/nahG cross indicated that SA accumulation is required for the ago1 developmental phenotype in Arabidopsis. Furthermore, a cross between ago1.25 mutant plant and a transgenic plant expressing the 2b RNA silencing suppressor protein of CMV were also generated. This cross resulted in plants which were severely deformed and with an even greater resistance to aphid infestation than their 2b transgenic or ago1.25 mutant parents. Many of the 2b/ago1 cross were sterile but a few of the cross formed siliques containing either few or no seeds at all.