The complexity of phloem structural diversity and its implications for angiosperm evolution

Background/Question/Methods

The widespread diversification of angiosperms is often attributed to key innovations in xylem anatomy that facilitated both cell specialization and the development of hydraulically efficient vessels. However, the few studies that exist on phloem evolution suggest that the phloem evolved in parallel to the xylem resulting in more derived lineages having shorter elements with less oblique plates and simpler sieve areas. Whether this evolutionary trend is robust across a molecular phylogeny and whether xylem and phloem conducting cells exhibit correlated evolution remains relatively unexplored.  In this study, we investigated patterns of phloem evolution and tested whether there is correlated evolution between the two parts of the vascular system in angiosperms. We compiled published data on phloem and xylem anatomy along with collecting detailed anatomical information on sieve tube and vessel elements in a subset of forty species using a combination of light, confocal and electron microscopy.

Results/Conclusions

Similar to the xylem, the lengths of conduits in the phloem are not constrained by the phylogeny (no phylogenetic signal) and long sieve tube elements occur in many of the major clades including Magnoliidae, Campanulidae, Malvidae and Fabidae. In woody species, there is a strong link between the length of elements in both the xylem and phloem because these conducting cells arise from the same cambial initials. Traits critical in determining sieve tube conductivity, such as sieve plate length and the number of sieve areas, exhibit a greater phylogenetic signal than expected from a Brownian motion model of evolution with the species containing the shortest plates and fewest sieve areas occurring in the Rosales. However, there is significant variation within individuals in the angle and size of sieve plates that could have implications for long distance transport. Taken together, this research demonstrates concurrent changes in xylem and phloem anatomy across the phylogeny that could cause variation both in plant water relations and carbon transport efficiency and may thus be critical in understanding the diversification of angiosperms into different habitats.