Evaluating the stability of nursery-established arbuscular mycorrhizal fungal associations in apple rootstocks

Arbuscular mycorrhizal fungi (AMF) are promoted as commercial bioinoculants for sustainable agriculture. Little is known, however, about the survival of AMF inoculants in soil and their impacts on native or pre-established AMF communities in root tissue. The current study was designed to assess the stability of pre-existing/nursery-derived AMF in apple rootstocks after being planted into soil containing a known community of AMF with a limited number of species. Root-associated endophytic communities (bacteria and fungi) are known to differ depending on apple rootstock genotype. Thus, an additional aim of this study was to explore the effect of apple rootstock genotype on AMF community structure. A greenhouse experiment was conducted in which a variety of apple rootstock genotypes (G.890, G.935, M.26, and M.7) were inoculated with a commercially available, multi-species AMF consortium. Nursery-derived AMF communities were sequenced, and changes to AMF community structure following cultivation in pasteurized soil (inoculated and non-inoculated) were assessed using a Glomeromycota-specific phylogenetic tree, which included 91 different AMF species from 24 genera. Results show that inoculant colonization potential was limited and that apple rootstocks serve as a significant source of inoculum from the nursery where they are produced. Rootstocks established relationships with introduced AMF in a genotype-specific manner. Regardless of colonization success, however, the inoculant caused alterations to the resident AMF communities of both Geneva and Malling rootstocks, particularly low abundance taxa. In addition, phylogeny-based analysis revealed a unique, well-supported clade of unknown taxonomy, highlighting the importance of using phylogenetic-based classification for accurate characterization of AMF communities.IMPORTANCEUnderstanding the impacts of introduced AMF on residential AMF communities is essential to improving plant productivity in nursery and orchard systems. In general, there is a dearth of data on the interactions of commercial AMF inoculants with pre-established AMF communities living in symbiosis with the host plant. The interplay between apple rootstock genotype and the endophytic root microbiome is also an area where more research is needed. This study demonstrates the potential for nursery-established AMF associations to be maintained when transplanted into the field. In addition to providing insight into rootstock/AMF associations, our study calls attention to the current issues attendant with relying on web-based databases for determining AMF identity. The use of phylogenetic tools represents one possible solution and may be of value to industry practitioners in terms of improving product composition and consistency.