Transgenic and natural resistance: Towards development of tomato (Lycopersicon esculentum Mill.) and pepper (Capsicum annuum L.) plants with broad resistance to virus infection

Viruses are a major limiting factor to the commercial production of tomato, Lycopersicon esculentum Mill., and pepper, Capsicum annumm L. The use of conventional breeding and cultural practices as control strategies has not been very effective in controlling the viral pathogens. The described concept of pathogen derived resistance (PDR) is a potential way of providing sustainable management of viral diseases. This research was aimed at utilizing PDR together with natural resistance to produce tomato and pepper plants that have broad resistance to virus infection. Transgenic resistance from tomato plants expressing the nucleocapsid protein (N) gene of tomato spotted wilt virus (TSWV) and natural resistance to TSWV from plants with the Sw-5 gene, were combined by sexual crossing as a way of obtaining broad resistance to infection by Tospoviruses, specifically TSWV and groundnut ringspot virus (GRSV). Our results show that there is potential to obtain broad resistance to tospovirus infection using this approach. The progeny of the crosses exhibited either a tolerant or resistant phenotype when inoculated with TSWV-BL, TSWV-H or GRSV-BR. The proportion of resistant to tolerant plants increased with each successive generation suggesting that increased natural and transgenic dosage contributed to increased resistance. Another approach of obtaining broad virus resistance to infection with Tospoviruses in tomato entailed the use of constructs carrying segments of the N gene from GRSV and TSWV to transform selected tomato cultivars. Preliminary results show that plants with resistance to either TSWV or GRSV can be produced using this approach. Attempts were also made to obtain virus resistance in peppers using PDR. Given the recalcitrance of pepper in tissue culture, an in vitro regeneration protocol needed to be developed. Using different pepper cultivars as sources of explants, a regeneration protocol with efficiencies of less than 10% was developed for cultivars Golden Tower and China 8212. However, no transgenic pepper plants could be produced using this established pepper regeneration protocol. It was therefore not possible to evaluate the full potential of PDR in controlling viruses that infect pepper.