The cucumber mosaic virus (CMV) 2b multifunctional protein is a suppressor of plant defences and an important determinant of viral pathogenesis. The effect of the 2b protein on symptom induction is due in large part to its physical interaction with the host RNA silencing factor Argonaute 1 (AGO1). The consequent inhibition of AGO1 activity induces developmental symptoms such as stunting and organ deformation in CMV-infected plants. In Arabidopsis thaliana, if 2b-induced inhibition of AGO1 is too strong this de-represses resistance against aphids, the insect vectors of CMV, which will be deleterious to virus transmission. In my initial work, I helped to establish that another CMV protein, the 1a replication protein, also binds to the 2b protein and modulates the 2b-mediated inhibition of AGO1 activity. This prevents induction of aphid resistance in CMV-infected A. thaliana plants and represents a novel regulatory system in which viral counter-defence protein functions are selectively modulated by another viral protein. I investigated why 2b proteins of Subgroup IA and IB CMV strains seemed able to interact with AGO1 and induce strong developmental symptoms and aphid resistance whereas 2b proteins of CMV strains of Subgroup II could not. Challenging established ideas, I demonstrated that a Subgroup II strain (LS-CMV) 2b protein can physically interact with and inhibit AGO1. I also found that the 2b protein of an atypical CMV Subgroup IA strain (Ho-CMV) that does not induce symptoms also binds to and inhibits AGO1. I showed that the property that distinguishes in vivo complex formation between AGO1 and the Ho-CMV or LS-CMV 2b protein is that it occurs predominantly in host cell nuclei. However, most Subgroup IA or IB CMV 2b proteins complex with AGO1 to a greater extent in the cytoplasm. I identified specific 2b residues that appear to facilitate 2b-1a protein-protein interactions, and in the process of investigating potential regions of the 2b protein needed for 2b-AGO1 complex formation I found evidence that the 2b protein may contain intrinsically disordered regions. Such disorder may explain the ability of the 2b protein to interact with a wide range of factors.