Formations of mycorrhizal symbiosis alter the phenolic heteropolymers in roots and leaves of four temperate woody species

The decomposition rates of senesced tissues from plants associated with ectomycorrhizal (EcM) fungi tend to differ from that associated with arbuscular mycorrhizal (AM) fungi. However, the chemical underpinnings that could drive the observed differences in decomposition are less explored.

Here, we characterized the content, composition, and spatial organization of phenolic heteropolymers in roots and leaves of four temperate tree species across eight plant-fungus combinations, forming either AM or EcM associations.

Colonization by either AM or EcM fungi tended to decrease the abundance of lignin, condensed tannins, and ratios of lignin and nitrogen in roots and/or leaves, which would lead to lower chemical recalcitrance of tissues. The decrease in root lignin abundance by either mycorrhizal type was associated with an expanded cortex, potentially facilitating symbiosis. Additionally, changes in lignin molecular composition by mycorrhizal symbiosis differed between plant phylogenetic lineages irrespective of mycorrhizal type.

Our results suggest that the mycorrhiza-associated changes in plant chemical traits that regulate litter decomposition may not be unique to AM or EcM associations; rather, both associations can reduce root and leaf chemical recalcitrance. Further, the differential modification in lignin composition by mycorrhizal symbiosis between plant phylogenetic groups highlights the influence of plant evolutionary history in plant-mycorrhizal interactions.

Methods
We characterized the chemistry of roots and leaves in non-colonized and mycorrhizal fungi-colonized seedlings in various combinations of plant hosts and mycorrhizal fungal species, forming either arbuscular or ectomycorrhizal associations. Specifically, the acid butanol assay was used to quantify the content of condensed tannins (CT) using cyanidin standards. Thioacidolysis was used to analyze the abundance and molecular composition of lignin. Lignin content (% w/w) was determined based on a standard curve of inputs of purified L. tulipifera lignin standard vs. their total yields of lignin monomers. For this analysis, individual seedlings were combined within each of the three trays due to limited sample amount, except for plant-fungus combinations present in only one tray, where individual seedlings were analyzed. The anatomy of root sections was analyzed using a Leica SPE inverted confocal microscope.