Expression and silencing of cowpea mosaic virus transgenes

Plant viruses are interesting pathogens because they can not exist without their hosts and exploit the plant machinery for their multiplication. Fundamental knowledge on viral processes is of great importance to understand, prevent and control virus infections which can cause drastic losses in crops. In this thesis, cowpea mosaic virus (CPMV) was studied. This virus consists of two, icosahedral particles that each carry a distinct single stranded RNA molecule of positive polarity. Several years of research have revealed much information on the genomic organisation, the strategy of gene expression and the multiplication processes of CPMV, which are described in Chapter 1, but also many aspects remain to be elucidated.To study individual viral processes, like replication, encapsidation or cell to cell movement, transgenic plants can be generated that express individual viral genes like the replicase, coat protein or movement protein gene. A prerequisite in this approach is the presence of an efficient and reliable plant regeneration and transformation system. (CPMV) 5 natural host is the tropical grain legume cowpea, Vigna unguiculata, a plant species that is recalcitrant at regeneration. Although in experiments described in Chapter 2 fertile plants could be regenerated from nodal thin cell layer segments, the explants were not competent for Agrobacterium-mediated transformation. Possibly in further studies, these nodal explants could prove suited for another transformation method.Therefore, tobacco, which is also a host for CPMV and highly competent for regeneration and transformation, was preferred as the species to generate transgenic plants carrying CPMV specific genes. Especially the CPMV movement proteins (MP) genes appealed to us for overexpression studies. CPMV cell to cell movement is enabled by the CPMV MPs that act to modify plasmodesmata. They are assumed to channel plasmodesmata with MP-containing tubular structures and through or with these tubules virus