INOCULATION WITH ROOT NODULATING BACTERIA REDUCES THE SUSCEPTIBILITY OF MEDICAGO-TRUNCATULA AND LUPINUS-ANGUSTIFOLIUS TO CUCUMBER MOSAIC-VIRUS (CMV) AND ADDITION OF NITRATE PARTIALLY REVERSES THE EFFECT

The susceptibility of Medicago truncatula ssp. truncatula (barrel medi c) cv. Jemalong and Lupinus angustifolius (lupin) cv. Illyarrie and Gu ngurru to cucumber mosaic virus (CMV) was reduced by prior inoculation of seedlings with commercial strains of root nodulating bacteria (Rhi zobium or Bradyrhizobium, respectively). This effect was observed both with strains of CMV originating from legumes and with strains origina ting from non-legumes. The virus content of the nodulated plants which were successfully inoculated was not markedly affected by nodulation, indicating that nodulation affected susceptibility to inoculation, an d not the ability of CMV to replicate. In lupins, the reduction in sus ceptibility followed mechanical and graft inoculation of plants with v irus. The effect was first noted at about the time that acetylene redu ction activity (ARA) was first detected and just before nodules appear ed. Virus-infected lupins showed a delay in the development of nodules and a reduced ARA, compared with uninfected plants, indicating that t here was a two-way interaction between the virus and Bradyrhizobium. D ifferent growth conditions did not affect the ARA values, the severity of symptoms or the susceptibility of lupin to CMV. A minimum level of nitrate was required for medic growth both with and without the root nodulating bacteria. The addition of higher levels of nitrate to the n utrient solution partially reversed the effect of the nodulating bacte ria, in that it increased the susceptibility of the plants. Medics not inoculated with Rhizobium and supplied with a high concentration of n itrate were the most susceptible to CMV. In a test to compare competen t with incompetent bacteria, Rhizobium strain 1021 (which fixes N2) an d its mutant, Rm 1491 (which does not fix N2), did not differ in their effect on susceptibility. Aggregates of virus particles and crystals of vinis were found in nodule cells of medic plants, but virus particl es and bacteroids were not found in the same cell. Individual and aggr egated virus particles were commonly located along the cell wall of ce lls free of bacteroids. It is concluded that infection by root nodulat ing bacteria and external nitrogen supplementation have opposite effec ts on the susceptibility of barrel medic and lupin to CMV, and that CM V, in turn, reduces the effectiveness of the symbiosis between these b acteria and their host. Nodulation has little effect on virus replicat ion, and both virus and bacteroids occur in the same nodule tissue.