Rhizosphere microbiome and boxwood cultivar disease trait relation
Background The rhizosphere microbiome is known to play an important role in supporting plant growth and health. However, how it may be associated with plant foliar health remained unclear, particularly under field conditions. Here we used boxwood as a model crop and collected the rhizosphere soils from four cultivars with tolerant, intermediate, and susceptible boxwood blight resistance levels at Oregon and Virginia nurseries in May, August, and November. Amplicon sequencing was then applied to characterize the bacterial and fungal communities.  Results Actinobacteriota and Ascomycota were the most abundant bacterial and fungal phyla, and Bacillus and Clonostachys were the most abundant bacterial and fungal genera across all cultivars, locations, and sampling times. Presumptive mycorrhizal fungi were also identified, with Ceratobasidium, Hyaloscypha, and Sistrema being dominant across the two locations. Additionally, 31 and 32 core bacterial genera were identified among all cultivars in Oregon and Virginia, respectively, with 27 being common between the two locations. Similarly, eight core fungal genera were identified at each location and six were shared. Further, boxwood blight resistance traits influenced the structure of the fungal community in May and August, and that of the bacterial community in August at both locations. Importantly, the cross-kingdom network of the tolerant cultivar between mycorrhizal fungi and bacteria had more positive interconnections and higher modularity than those of the susceptible and intermediate cultivars. In particular, seven mycorrhizal fungi were closely connected to form a prominent cluster in the tolerant cultivar network, with Amanita identified as a hub taxon.  Conclusions Our study revealed that the boxwood rhizosphere harbored a wealth of microbes that may play a role in promoting plant growth and health, providing microbiome evidence supporting boxwood as a low maintenance plant. The structure of rhizosphere fungal community was greatly influenced by boxwood blight resistance traits, underpinning fungi are specialized in response to plant phenotypic and genotypic cues to affect plant health. Importantly, the interactions within the putative mycorrhizal fungal community may be associated with boxwood blight tolerance. Collectively, these findings provide important information that will be useful in boxwood breeding programs to enhance plant resilience to disease and environmental stress.