Changes in arbuscular mycorrhizal fungi between young and old Vitis roots

In perennial plants, root metabolic activity decreases as absorptive roots age and eventually senesce. How this change in activity influences fungi inhabiting those roots is not well known. Particularly for arbuscular mycorrhizal (AM) fungi that rely exclusively on plant-derived carbon, these changes may exert significant pressure on fungi, leading to functional or compositional changes. As part of a larger study examining the effects of vineyard groundcover vegetation on root-associated fungi in a model grapevine, we asked how AM fungi change both in terms of distribution of fungal structures and community structure as roots age and if this depends on the legacy effect of unique microbial communities associated with different groundcovers. AM fungal communities varied with groundcover vegetation but not between young and old roots. AM fungal structures, however, differed between young and old roots with more arbuscules in young roots and more vesicles and/or spores in older roots. This trend was consistent across the different groundcover-trained communities, although one groundcover led to more arbuscules overall. We discuss these changes in light of our current understanding of the temporal dynamics of the AM symbiosis in aging roots and pose several hypotheses and directions for future research.