Host genotype and environment shape rhizosphere and root microbiome composition of pecan rootstocks

The rhizosphere and root-associated microbiomes play a crucial role in nutrient acquisition, stress tolerance, and overall plant performance. However, little is known about how microbial communities assemble and shift across environments in pecan (Carya illinoinensis). In this study, we compared the bacterial and fungal community compositions in the roots and rhizosphere of four pecan clonal rootstocks (NMU03, NMU04, NMU05, and NMU155) cultivated under greenhouse conditions, as well as their subsets that were subsequently transplanted to the field. Amplicon sequencing of 16S rRNA and ITS regions revealed significant differences in microbial diversity and taxonomic composition across environments and genotypes. Bacterial assemblages in greenhouse roots were typically dominated by a few families (e.g., Burkholderiaceae, Rhodanobacteraceae, and unclassified taxa). In contrast, field samples exhibited broader taxonomic distributions, with families such as Xanthobacteraceae, Haliangiaceae, and Geminicoccaceae emerging as dominant members. Fungal OTU abundance was consistently higher than bacterial abundance across all genotypes, likely reflecting mutualistic associations with mycorrhizal fungi, such as those in the Elaphomycetaceae family. Interestingly, Aspergillaceae dominated greenhouse and field fungal communities, suggesting ecological adaptability and potential contributions to plant stress tolerance. Comparisons with earlier greenhouse studies revealed that while some signature core microbiome members were retained following transplantation from the greenhouse to the field, the abundance of others decreased, highlighting successional shifts in community structure driven by environmental transitions. Together, these findings demonstrate the dynamic, genotype and environment-specific structuring of pecan microbiomes and highlight the importance of microbiome-informed breeding strategies to improve plant-microbe associations under variable growth conditions among pecan breeders.