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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.

Dot blot and tissue print hybridisation assays were developed for the rapid and sensitive detection of pea seed borne mosaic virus (PSbMV). Radioactive (32P-labelled) and non-radioactive (DIG-labelled) random primed cDNA probes representing the entire genome of PSbMV were used in hybridisation assays. A comparison of detection sensitivity was made between these probes and they were found equally sensitive. PSbMV was readily detected in dot blots by their homologous cDNA probes to 50 fg in purified preparations and to a 1 : 3125 dilution in purfied total nucleic acid or crude sap from infected plants. The sensitivity of heterologous probes was significantly less than that of homologous probes in purified preparation but nearly the same in purified total nucleic acid or crude sap from infected plants. The virus was also specifically detected in tissue squashes by tissue print hybridisation using cDNA probes. Non-radioactive probes were found suitable for PSbMV diagnosis without significant loss of detection sensitivity. There was no non-specific hybridisation against the healthy control in hybridisation assays using either radioactive or non-radioactive probes.

The coat protein gene of a South Australian strain of alfalfa mosaic virus (AMV-N20 [NcS]) has been cloned, sequenced, and transferred into Nicotiana tabacum L. cv. Xanthi via Agrobacterium tumefaciens under the control of the CaMV 35S promoter. A number of lines (T0 generation) were selected with the coat protein gene either in sense orientation (CP+) or in antisense orientation (CP–). The T0 plants were tested for their gene expression and susceptibility to the homologous AMV strain. A significant delay in the onset of symptoms and a reduction in virus accumulation was observed in CP+ plants mechanically inoculated with AMV. CP– plants were also significantly protected but less so than the CP+ plants. Plants transformed with the expression vector only (CP0) showed a minor resistance to local infection on inoculated leaves compared with untransformed plants. The strategy of coat protein mediated protection (CPMP) using the CP gene in either messenger sense or antisense would therefore be appropriate for testing on economically important pasture legumes.

Alfalfa mosaic virus (AMV) infection of the annual barrel medic, Medicago truncatula, has been shown to cause significant reductions in growth and productivity in field and pot trials. The degree of reduction was dependent on the medic cultivar and the virus isolate. In some experiments herbage production was reduced by more than 50%. Although infection did not appear to be associated with significant changes in root growth, root nodulation of infected plants was about one-third less than that of healthy plants. Two AMV isolates showed different effects on seed production. Infection with one virus isolate was associated with a 15-30% decrease in seed production, and the virus was detected in more than 2% of seed from the infected plants, whereas the other isolate failed to reduce seed yield and did not appear to infect any seeds.