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

Abstract

The susceptibility of Medicago truncatula ssp. truncatula (barrel medic) cv. Jemalong and Lupinus angustifolius (lupin) cv. Illyarrie and Gungurru to cucumber mosaic virus (CMV) was reduced by prior inoculation of seedlings with commercial strains of root nodulating bacteria (Rhizobium or Bradyrhizobium, respectively). This effect was observed both with strains of CMV originating from legumes and with strains originating 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, and not the ability of CMV to replicate. In lupins, the reduction in susceptibility followed mechanical and graft inoculation of plants with virus. The effect was first noted at about the time that acetylene reduction activity (ARA) was first detected and just before nodules appeared. Virus-infected lupins showed a delay in the development of nodules and a reduced ARA, compared with uninfected plants, indicating that there was a two-way interaction between the virus and Bradyrhizobium. Different 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 nutrient solution partially reversed the effect of the nodulating bacteria, in that it increased the susceptibility of the plants. Medics not inoculated with Rhizobium and supplied with a high concentration of nitrate were the most susceptible to CMV. In a test to compare competent with incompetent bacteria, Rhizobium strain 1021 (which fixes N2) and its mutant, Rm 1491 (which does not fix N2), did not differ in their effect on susceptibility. Aggregates of virus particles and crystals of virus were found in nodule cells of medic plants, but virus particles and bacteroids were not found in the same cell. Individual and aggregated virus particles were commonly located along the cell wall of cells free of bacteroids. It is concluded that infection by root nodulating bacteria and external nitrogen supplementation have opposite effects on the susceptibility of barrel medic and lupin to CMV, and that CMV, in turn, reduces the effectiveness of the symbiosis between these bacteria and their host. Nodulation has little effect on virus replication, and both virus and bacteroids occur in the same nodule tissue.