Results
Our results showed a clear shift in root morphology over time in seed plants, with thinner root systems present in more derived lineages, whereas Gymnosperms and Magnoliids showed thicker roots and lower SRL values. RTD was relatively similar across groups (Fig 1), although it decreased from woody to non-woody plants across phylogenetic groups (Fig S1). The transition to ECM state had different effects on root morphology depending on the phylogenetic group (Table 1, Figure S1). Gymnosperms showed a substantial decrease in root diameter after mycorrhizal change (Table S2) but this result is strongly dependent on the sole divergence between the AM Cupressales and the ECM Pinales, so the association between root diameter and mycorrhizal state was not significant after phylogenetic correction (Fig. 2, Table 1). There was evidence of an increase in RTD from AM to ECM species, but only for the analysis on seed plants as a whole. On the other hand, there was no evidence that the transition to associations with Dikarya fungi was associated with root morphology in Rosids, suggesting that this group already had thin roots, high SRL and relatively high RTD by the time they acquired ECM capabilities. (Table 1, Fig 2). The loss of the mycorrhizal symbiosis was significantly associated with root morphology at the level of the seed plants and within some seed plant clades (Table 1). There were significant decreases in root diameter after switching to NM state but mostly when comparing all plants (Table 1, Table S2, Figure S1). SRL increased significantly for non-mycorrhizal groups when compared to basal groups (Table 1) and within Asterids. Non-mycorrhizal species also showed a decrease in RTD at the seed plant level and within Monocots and Asterids. Finally, mycorrhizal changes were not associated with leaf vein density, which corresponds with previous studies reporting similar leaf traits among mycorrhizal groups (Koele et al. 2012, Table S3)
We found a significant negative correlation of vein density with both diameter and RTD but a positive relationship with SRL among current species (Fig. 3). Moreover, leaf vein density mycorrhizal state and growth habits were still significant once they were included in the model (Table S4) but the significance of the association was lost after phylogenetic corrections (Table 2), suggesting that the trend is strongly influenced by the morphological difference between plant clades. Habit was the only significantly associated factor with root morphology within clades, suggesting that there are important changes in root morphology associated with ecological strategies and lifespan. In support of the “integrated” scenario, we found no significant effect of mycorrhizal state in root traits after correction for habit and vein density. The same result was obtained after removing Ericoid mycorrhizal plants (data not shown), and after removing ECM species and testing only for the transition from AM to NM (F = 0.13, p =0.8, F = 0.15, p =0.86 and F=0.97, p=0.38 for Diameter, SRL and RTD respectively). On the other hand, habit remained highly significant (F = 22.9, p <0.0001, F = 10.56, p <0.0001 and F=6.33, p=0.003, for diameter SRL and RTD respectively). This suggests that the transition to herbaceous habit seems to have meaningful effect on root morphology in Angiosperms (Table 2).
Finally, the ancestral reconstructions illustrate a shift in leaf venation along with increases in SRL and thinner roots but not with RTD trends (Fig 4). The reconstruction suggests that the ancestral seed plants possessed relatively low SRL, intermediate root diameter and RTD, in addition to low vein density. Root diameter in the Magnoliids, some Eudicots and Monocots increased during time, suggesting that the coarse distal roots represent a specialization rather than the ancestral stage. During the diversification of Angiosperms in the mid-Creteaceous emergent groups of Asterids and Rosids showed a substantial increase in SRL, and reduction in root diameter as vein densities increased, supporting the idea of a coordinated lift in plant hydraulics. In addition, we detected a previously unreported evolutionary divergence in RTD between Rosids and Asterids, with the former evolving to higher values than other plant groups, whereas Asterids showed similar RTD trends to those in the Monocots and Magnoliids clades.