A phylogenetic study of polyphenol oxidase (PPO) enzyme distribution within the plant family Poaceae (grass) and the invasive genus Bromus

Background/Question/Methods

Grasses (Family: Poaceae) include >11,000 species of great agricultural, economic, and environmental importance, but the evolution is not fully understood despite numerous attempts to rectify the phylogeny through both morphological and genetic means. Plant classification in phylogenies is a reflection of that species’ evolutionary history and the biological traits they possess; in contrast to whole genome sequencing, researchers sometimes just look at a few target sequences or traits to understand phylogenetic relationships. Previous work has shown that the enzyme polyphenol oxidase (PPO) is present in the roots of some Poaceae species but not others. Of particular interest is the genus Bromus, which includes a number of notoriously problematic highly invasive species. Here, we (1) conducted PPO enzyme assays of roots of 40 Poaceae species of bromes and non-bromes (combined with 24 published PPO values of previously assayed species for a total n=64) to test the hypothesis that invasive Bromusspecies have high levels of PPO, whereas non-invasive grasses have little or no PPO, (2) created a consensus tree by combining several existing phylogenetic trees, and (3) mapped PPO and invasiveness onto our consensus tree to investigate the phylogenetic history and evolution of PPO to understand those evolutionary relationships.

Results/Conclusions

Results showed that (1) roots of invasive grasses (n=27) do indeed have significantly higher PPO levels than roots of non-invasive (n=37) grasses ( P<0.01), in both Bromus and non-Bromus, and (2 & 3) phylogenetic distribution of phenetic polyphenol oxidase (PPO) of selected species in Poaceae is somewhat tractable, offering two interpretations: (a) PPO is not the ancestral condition, and some genera diverged prior to PPO acquisition, or (b) PPO is the ancestral condition and some genera lost the activity, as is suggested by the phylogeny. These new findings point to PPO as a trait correlated to invasiveness, and highlight ongoing taxonomic classifications that may shed light on evolutionary understanding of selection benefits of the PPO enzyme and grass evolution.