Move over, bacteria! Viruses make their mark as mutualistic microbial symbionts

Abstract

Viruses are being redefined as more than just pathogens. They are also critical symbiotic partners in the health of their hosts. In some cases, viruses have fused with their hosts in symbiogenetic relationships. Mutualistic interactions are found in plant, insect, and mammalian viruses, as well as with eukaryotic and prokaryotic microbes, and some interactions involve multiple players of the holobiont. With increased virus discovery, more mutualistic interactions are being described and more will undoubtedly be discovered.

FIG 1

A simple model of biotic stress impacts on quality-selected mutualistic symbioses. The host isocline is a curve that intersects the x axis at the environmental carrying capacity (value K). The viral symbiont isocline is a line with a slope of the maximum titer, when the growth rate of the virus is much lower than the decay rate. The host and virus isoclines intersect at an equilibrium point that a pair of symbiotic partners can approach. (a) When an acute plant-pathogenic virus infects a plant (plant virus-plant host), the plant becomes tolerant to drought stress; without stress, the uninfected plant does better, but under stress, a decrease in K encourages a pathogenic virus to become mutualistic. (b) In a three-way mutualistic symbiosis (fungal virus-fungal endophyte-plant), the virus has no negative impact on the plant under normal conditions and is required for survival at high soil temperatures. Extremely low K and low viral virulence result in a stronger mutualistic symbiosis effect. Reproduced with permission from reference .