Relationships between Sphaerulina musiva infection and the Populus microbiome and metabolome

Pathogenic fungal infection in plants may, in some cases, lead to downstream systematic impacts on the plant metabolome and microbiome that may inhibit or exacerbate the effects of fungal infection. While Sphaerulina musiva is a well-characterized fungal pathogen, which infects Populus species, important wood fiber and biofuel feedstock crops, little is known about its systematic effects on the metabolome and microbiome of Populus. Here, we investigate the microbiome and metabolome of P. trichocarpa and P. deltoides leaves and roots and the microbiome of their associated surfaces (i.e., phylloplane and rhizosphere, respectively) to understand the systematic impact of S. musiva abundance and infection on Populus species in a common garden field setting. We find that S. musiva is indeed present in both P. deltoides and P. trichocarpa, but S. musiva abundance is unrelated to stem canker onset and development. We also find that the leaf and root metabolomes significantly differ between the two Populus species and that certain leaf metabolites, particularly the phenolic glycosides salirepin and salireposide, are diminished in canker-infected P. trichocarpa trees compared to their uninfected counterparts. Furthermore, we found significant associations between the metabolome, S. musiva abundance and the microbiome, particularly in P. trichocarpa leaves. This suggests that the effects of S. musiva on P. trichocarpa trees are widespread and not confined to the site of canker infection.