Seed microbiota are functionally diverse, including, but not limited to, pathogens, saprotrophs, growth-promoting microbes, and defensive mutualists. Despite the ecological importance of individual seed microbes, few studies have taken a holistic approach to characterizing taxonomic diversity and ecological function in the seed microbiome. We used DNA sequencing to identify culturable fungi and bacteria from a total of 900 surface sterilized seeds collected from ten different Populus trichocarpa trees growing in two contrasting environments (Westport, Oregon and Moscow, Idaho). In further greenhouse inoculation experiments we tested how a subset of the isolated seed microbes impacted P. trichocarpa seedling growth, survival, and disease severity caused by the tree’s major foliar rust pathogen, Melampsora.
Results/Conclusions
We found that P. trichocarpa seeds were characterized by low microbial diversity (62 isolates total, 11 fungal genera and 1 bacterial genus). The composition of the seed microbiome varied significantly among both tree genotypes and source environment (PERMANOVA). Inoculation with some of the fungal isolates (Diaporthe sp., Boeremia sp.) resulted in seedling mortality, indicating pathogenicity, while other isolates had no impact on seedling growth or survival. One seed endophyte (Cladosporium sp.) increased the severity of rust disease caused by Melampsora while another seed endophyte (Aureobasidium sp.) had no effect on disease severity. Our results thus demonstrate that while limited in taxonomic diversity, the seed microbiome contains functionally diverse microbes. Additional study is needed to further elucidate how seed microbiome structure and function varies among plants and environments.