Arbuscular mycorrhizal fungi protect soybean against growth penalties associated with Soybean mosaic virus infection

Plants modify their rhizosphere microbiome in response to pathogen infection and recruit microbes to their roots that can suppress disease. Such plant-beneficial microbes that are promoted and supported by the infected plant are potentially sustainable alternatives for agrochemicals. Here, we studied the fungal and bacterial rhizosphere microbiomes of soybean plants with disease symptoms on the leaves caused by natural infection by Soybean mosaic virus (SMV) in two commercial fields in the Netherlands. Although we set out to identify candidate beneficial microbes that were more abundant on roots of leaf-infected plants, we consistently found that arbuscular mycorrhizal fungi (AMF) were less abundant in the rhizosphere of virus-infected compared to healthy plants in the two fields. When SMV was inoculated prior to observable AMF colonization in subsequent greenhouse experiments, SMV did not affect the extent of AMF root colonization. Conversely, when SMV was inoculated after AMF had colonized soybean roots, AMF colonization did not affect disease incidence or severity, nor virus load in plant leaves. AMF root colonization did diminish the shoot growth reduction associated with SMV infection. These results suggest that AMF are robust plant-growth promoters of soybean, protecting plants against growth penalties associated with SMV infection.