Comparative analyses of transcriptionally regulated downstream genes between arbuscular mycorrhizal and ectomycorrhizal symbioses in the single host plant Eucalyptus grandis.

In nature, some tree species interact with both arbuscular mycorrhizal (AM) Glomeromycotina fungi and ectomycorrhizal (ECM) Basidiomycota/Ascomycota fungi, and are termed dual mycorrhizal plants. Although the AM-upregulated genes and their functions have been well studied, those of ECM symbiosis remain unclear, despite their essential roles in forest ecosystems. Therefore, this study aimed to compare symbiosis-regulated downstream genes in the dual mycorrhizal model tree, Eucalyptus grandis, during fully developed AM and ECM symbioses. First, we conducted a comparative transcriptomic analysis and found a distinct transcriptional profile between E. grandis AM and ECM roots. Notably, none of the examined AM-related downstream genes were upregulated in the ECM roots. To identify ECM-specific genes and their expression patterns, comparative genomic analysis was performed. This study identified several gene families, including NAC transcription factors, that significantly expanded across the examined ECM lineages. Interestingly, we identified some ECM-promoted NAC transcription factors in the ECM roots of E. grandis, Populus trichocarpa, and Castanea mollissima. Moreover, none of the Eucalyptus NAC genes were transcriptionally promoted during AM symbiosis. Taken together, our results indicate that the downstream pathways necessary for the establishment of AM and ECM symbioses would be distinct.