Angiosperm Root Evolution: The Relationship Between Root Anatomy and Mycorrhizal Colonization Among Angiosperm Trees.

The association between plants and arbuscular mycorrhizal fungi (AMF) is possibly the most successful and extensive symbiotic relationship in nature. There are relatively few studies explaining how this association has evolved over time, and to what extent root anatomy defines the level of colonization of AMF in roots. Since AMF habitat is restricted to the root cortex, we predicted that evolutionarily basal Angiosperms will have larger total cortical area and a lower root:stele diameter ratio than species from more derived groups. Moreover, if roots are evolving to decrease their dependency on AMF by limiting the amount of habitat offered to AMF, we would expect that the decrease in root diameter across species is primarily associated with a decrease in cortex rather than stele diameter.  Finally, for the first root order (most distal roots), we predicted that mycorrhizal colonization will correlate positively with root diameter across species. To test our hypotheses we sampled root systems from 34 species of trees representing three main clades of Angiosperms (Magnoliids, Asterids and Rosids). Roots were dissected by root order, dyed with Trypan blue, and mounted on slides. Then we quantified mycorrhizal colonization and assessed stele and entire root diameter for each order. Our results showed that Magnoliids have a lower root:stele ratio (p=0.0001) than other groups. We also found that root diameter decreased proportionally faster than stele diameter (slope = 0.45, p=0.001) across species, suggesting a stronger reduction in cortex than stele area as root diameter decreased. However, we did not find a significant relationship between root diameter and mycorrhizal colonization (p>0.05). Our results support the idea that roots are evolving to decrease the area dedicated to mycorrhizal habitat, but this does not translate to lower levels of AMF colonization.