Trait complementarity and functional trade-offs in plant-mycorrhizal symbioses

The criteria by which plants select symbiotic partners are largely unknown, but functional complementarity of partner traits could be important to symbioses such as arbuscular mycorrhizas. Specifically, coarse-rooted plants are more likely to be limited by nutrient diffusion compared with fine-rooted plants, and therefore more reliant on fungi that can compensate with traits that maximize soil exploration. However there remains no evidence directly linking plant root traits and fungal functional traits. We tested the hypothesis that coarse-rooted plants associate with fungal partners that provide nutrient benefit by increased hyphal exploration. We collected roots from 30 plant species ranging from fine to coarse roots and used Next Generation Sequencing to identify the arbuscular mycorrhizal fungal community associated with each plant species. We found that plant root diameter was correlated with arbuscular mycorrhizal fungal community composition and diversity. We quantified exploration using root fragments from the same 30 species inoculated onto a common host plant grown in multi-compartment pots. Root fragments from coarse-rooted plants were associated with an increase in hyphal exploration away from the roots. Our results provide the first direct evidence of functional complementarity between plants and their arbuscular mycorrhizal fungal partners with important implications for fungal diversity and coexistence.