A novel defense? Understanding the role of the enzyme polyphenol oxidase in the invasive genus Bromus in plant competition

Background/Question/Methods

Biological invasions have caused hundreds of species extinctions. Much theory and research focus on which factors promote biological invasions. One proposed mechanism focuses on chemical interactions among plants. The term “novel weapons” has been used to suggest that invasive plants exude allelochemicals that are unknown to the native populations. The roots of Bromus species have been shown to produce the enzyme polyphenol oxidase (PPO), the function of which is unknown. We hypothesized PPO may be used as a “novel defense” against allelopathic invading species such as Centaurea sp. and Artemisia sp. because many of the allelochemical compounds serve as substrates for PPO. To evaluate this hypothesis, PPO-producing bromes were grown together with allelochemical-producing species collected in New Jersey to test for competitive effects. Because ecosystems are increasingly susceptible to plant invasions, this research is of particular importance for conservation of biodiversity and prevention of future invasions. Accordingly, we examined ecological impacts of a PPO-producing grass Bromus versus a non-PPO-producing grass Festucagrown with and without allelopathic forbs by collecting quantitative data on height, number of leaves, leaf lengths, and harvest biomass on both target and competitor plants.

Results/Conclusions

Results indicate that (a) the PPO-producing grass Bromus inermis produced a greater number of leaves as well as longer leaves when grown with allelopathic forb Centaurea stoebe than conspecifically; (b) when Bromus inermis was grown with the competitor C. stoebe, B. inermis produced more roots than when grown conspecifically; and (c) root growth was suppressed in non-PPO producer F. rubra when grown with C. stoebe. This supports our hypothesis and is suggestive of a benefit of growing PPO-equipped plants with allelopathic C. stoebe, a benefit absent in non-PPO plants. When B. inermis was grown with allelopathic Artemisia vulgaris, however, both shoot and root growth was suppressed; F. rubra produced similar results. This may be due to differences in plant growth form, allelochemical compounds, or method of delivery of compounds. Future plans include growing B. inermis and F. rubra with partially decomposed biomass of C. stoebe and A. vulgaris and additional observational field studies, as preliminary field surveys indicate forb and grass growth together.