Effects of cucumber mosaic virus infection on <i>Arabidopsis thaliana</i> in wild populations: from mutualism to antagonism

It is broadly accepted that viruses are often commensals or conditional mutualists of plants, a concept based on results of experiments under controlled conditions with scarce evidence from studies of plants infected in the field. Demonstrating that plant viruses are mutualists, commensals, or pathogens in nature, and identifying under which conditions is highly relevant to understand plant-virus interactions. We analysed the effect of cucumber mosaic virus (CMV) infection on the fitness of Arabidopsis thaliana plants in two wild populations. We monitored plant demography and CMV infection dynamics and, in each plant, quantified life-history traits related to survival and reproduction. We also analysed experimentally the effects of infection on plant tolerance to cold and drought. Data showed that the plant-virus interaction was neutral, mutualistic or antagonistic depending on the population, the plant life cycle (a winter or a spring annual) and the time of infection relative to plant development. Life-history trait modification, and increased survival to drought, upon infection were factors explaining infection-associated benefits. Results demonstrate that plant-virus interactions in nature move along the mutualistic-antagonistic continuum and allow identifying major factors conditioning the nature of the interaction, which is highly relevant to understand virus evolution, plant-virus co-evolution, and virus emergence.