Transgenic Pest and Disease Resistant White Clover Plants

Abstract

Both insect pests and viral diseases have significant negative impacts on the yield and persistence of forage legumes, and often it is difficult to develop resistant germplasm using traditional plant breeding approaches. We illustrate some of the principles for the molecular breeding of forage legumes, by demonstrating how cloned resistance genes can be used to produce transgenic white clover plants, resistant to either an insect pest (Wiseana spp.) or white clover mosaic virus (WC1MV). Insect bioassays identified a Bacillus thuringiensis Cry1Ba toxin and the proteinase inhibitor, BPTI, as possible resistance factors for Wiseana larvae. Transgenic white clover plants expressing a modified crylBa gene, altered to give an increased proportion of G/C bases and mimic the codon use of plant genes, were toxic to feeding Wiseana larvae. While larvae feeding on white clover expressing the BPTI transgene survived, their growth rate was significantly reduced. Our strategy to develop resistance to WC1MV has been to express individual WC1MV genes in transgenic white clover plants. Expression of the coat protein gene of WC1MV in white clover gave low levels of resistance to the virus, but a substantial delay to systemic infection. Expression of a mutated form of the WC1MV 13K movement protein gene in white clover, resulted in plants with a high level of resistance and restricted systemic spread of the virus. Five of 22 transgenic white clover plants designed to express the WC1MV replicase gene showed immunity to WC1MV infection.