The multiplicity of cellular infection in plant viruses: characterization and mechanisms of regulation – MOI

The interactions between mutant genomes within viral populations profoundly influences infection dynamics and viral evolution. Within eukaryotes, at the intra-host scale, the potential for these interaction events is defined in a large extent by one key parameter that is the number of viral genomes that co-infect each cell or, in other words, the multiplicity of infection of individual cells (here designated MOI).

High MOI values imply that recombination, mutual complementation, or any form of collective action within the cell can be possible and frequent. However, a high MOI could be a double-edged sword, favoring the intra-cellular competition for resources and potential invasion of the population by defective viral genomes. Beyond its influence on competition, collective actions, complementation and recombination, this parameter also impacts on the effective size of the viral population that infects the cell or, in other words, on the presence of bottlenecks during the successive cell infections, thereby impacting on the balance between two major evolution forces: genetic drift and selection. Consequently, it would not be surprising that different viruses have developed mechanisms to limit or facilitate the MOI, as obvious trade-offs are possible on this particular trait.

Despite the undisputable importance of the MOI in viral biology and evolution, theoretical studies rarely consider multiple infections at the scale of individual cells within a multi-cellular host, and this is particularly true for theoretical models addressing the evolution of virulence. Similarly, on the experimental side, tangible information is very scarce. There is only two formal analysis of this parameter in a eukaryotic host infected by a virus: a lepidoteran insect infected by a baculovirus, and a solanaceous leaf infected by a tobamovirus. Most importantly, both studies have estimated the cellular MOI at only one stage of the host invasion by the virus population, respectively the late and the initial stage, thus leaving the question of possible dynamic changes of this trait during host invasion totally unexplored.

Recently, our team has monitored for the first time the MOI in a plant-virus model, from the first signs of systemic infection until senescence of the host. We have shown that the cellular MOI of the Cauliflower mosaic virus (CaMV) is highly dynamic (changing with time and location within the host) and can reach maximum average values close to 13 genomes per cell in Brassica rapa plants. This high MOI value seems to differ from that observed during our preliminary exploration of the cellular MOI of the Turnip mosaic virus (TuMV) in the same host plant, that might constantly remain close to 1. Because these results (though preliminary for TuMV) indicate a very different situation depending on the viral species, one could imagine that the MOI is a life trait controlled by the virus and that very contrasted strategies, e.i. high or low MOI, might exist in nature.

We have carefully considered the conceptual originality of the question of the cellular MOI in virus models infecting eukaryotes, its fundamental interest, and the possible future practical applications, to construct an innovative and ambitious project with the following main objectives:

i) We will provide an extensive monitoring of the cellular MOI values during the invasion of various host plants by various viral species. This will establish a unique empirical set of data, uncovering the MOI scenarios in several natural virus-host associations.
ii) We will demonstrate that the cellular MOI is a life trait that is tightly controlled by the virus
iii) We will identify and characterize the plant/virus molecular and cellular interactions affecting and controlling the cellular MOI.
iv) We will evaluate a putative correlation between MOI and the effective size of a virus population during host plant invasion