Cultivation of B. papyrifera samples for fungal research.

Based on the population structure, seeds of three B. papyrifera genotypes (8 replicates per genotype) were soaked in gibberellin and germinated on plates under sterile conditions. After 2 weeks, individual seedlings were transferred into separate pots containing sterilized field soil. In total, there were 24 pots (including 8 pots for each genotype) in our experiment. The field soil was collected from a farmland in Beijing (40°23′58″N, 116°45′39″E), which had not been treated with fertilizers. The sterilized field soil would provide a unified microbial environment for B. papyrifera individuals with different genotypes (61) and prevent interference in the influence of genotype from differences in the initial microbiome (62). After passing through a 2-mm sieve, the soil was dried at 200°C for 2 h. After being transferred into pots, the B. papyrifera plants were grown in a greenhouse without any nutritional supplements for 1 year (from November 2015 to November 2016). All pots were watered daily.