Medicago truncatula is used as a pasture legume and a source of nitrogen for grain crops in southern Australia. Alfalfa mosaic virus (AMV) infection reduces herbage production and nodulation. The coat protein gene of a South Australian strain of AMV (AMV N20) has been transferred to Medicago truncatula cv. Jemalong 2HA using Agrobacterium-mediated transformation. The most detailed investigations were carried out with the coat protein gene in the sense orientation (CP+). Progeny (T₁, T₂, T₃) raised from self-pollinated primary transformants (T₀) containing the coat protein CP+ gene were resistant to AMV. Based on Southern analysis and segregation, the transformants contained a single gene copy. In the T₃ generation, one line was immune and one line showed resistance to AMV N20. The immune line contained no detectable virus when plant sap from either inoculated or systemic leaves was bioassayed on Phaseolus vulgaris. This line was also immune to the heterologous AMV S40 isolate. A line with the coat protein gene in antisense orientation (CP⁻) showed delayed systemic infection but was not immune. We conclude that coat protein mediated protection (CPMP) is an effective strategy for controlling AMV infection and should be further evaluated in the field.
Australian Journal of Agricultural Research Vol. 52, p. 67-72