Transgenic plants expressing the coat protein gene of cowpea aphid-borne mosaic potyvirus predominantly convey the delayed symptom development phenotype

Cowpea aphid-borne mosaic virus (CABMV) is a potyvirus that infects cowpea causing significant yield reduction. However, there is no durable natural resistance to the virus within the crop and genetic engineering for virus resistance was not possible because of a lack of an efficient, reliable and reproducible cowpea transformation and regeneration protocol. Coat protein-mediated resistance to CABMV was evaluated in Nicotiana benthamiana, a model host for the virus. The CABMV coat protein gene from a Zimbabwean isolate of the virus was optimised for expression in plants under a CaMV 35S promoter and cloned into the Hind III site of the binary vector plasmid pBI121 to result in the plasmid pBI121-CPk. The plasmid pBI121-CPk was used in Agrobacterium-mediated transformation of N. benthamiana leaf sections following the co-cultivation method. Regenerated plants were analysed by PCR and Southern blot hybridisation. R1 seedlings were assayed for kanamycin resistance and for presence of the coat protein and challenged with CAMBV-infected sap. Lines showing delayed symptom development were identified but no line showing immunity was identified. Delayed symptom development is significant resistance since it affords protection to the plants during the crucial early stages of development and exerts little evolutionary pressure on the virus to evolve new strategies.