Fungal symbionts associated with an invasive grass decrease germination and litter degradation of native species, with distinctive metabolites as a likely mechanism

Background/Question/Methods

Plant invasion is a major component of global change, leading to environmental transformations that alter biogeochemical cycles and associated ecosystem services. As the ecological traits of many plants are influenced by their microbiome, microbial communities that are introduced with non-native plants, or are co-opted by invaders, likely play a role in invasion success. At the Santa Rita Experimental Range in southern Arizona, the non-native perennial grass Eragrostis lehmanniana (ERLE) was introduced to stabilize soil and provide forage for cattle. It now dominates the landscape and outcompetes diverse native grasses across the southwest. We examined how fungal symbionts of ERLE influence seed germination and litter degradation of both the invader and native grasses, and characterized fungal metabolomes to detect mechanisms of action. First, seeds of ERLE and three native grasses (Bouteloua gracilis, E. intermedia, and Leptochloa dubia) were inoculated by three common root-endophytic fungi isolated from ERLE, and evaluated for seed germination. Second, we used liquid chromatography-mass spectrometry to characterize metabolomes of fungi in isolation and paired with each plant species. Finally, because endophytes complete their life cycles via litter degradation, we used mass-loss experiments to quantify the capacity of ERLE endophytes to degrade its litter and that of co-occurring natives.

Results/Conclusions

Root symbionts of the invasive grass (ERLE) consistently decreased seed germination of three native grasses, yet typically increased germination of ERLE. Investigation of metabolomes yielded 1,630 characterized compounds, phenols and proteins were the most prevalent compound classes in all three fungi, followed by lipids, carbohydrates, and amino sugars. The metabolome of each fungal isolate differed, both in isolation and in response to native and non-native grass seeds. Metabolite expression for all fungi differed in response to seeds of ERLE and the related native E. intermedia, which differed markedly from fungal response to B. gracilis and L. dubia. Ongoing analyses are highlighting distinctive metabolites expressed by each fungal strain in response to seeds of native and non-native plants, and how such metabolites may be important in other interactions relevant to grassland ecology. Mass loss experiments revealed that endophytes of ERLE degrade ERLE litter more efficiently than litter of co-occurring natives, highlighting another axis of chemical exchange between symbionts of the invader and native species. Taken together, our results indicate that root-endophytic fungi associating with an invasive grass may impact plant communities via metabolic signals that shape both seed germination and litter degradation, thus impacting ecosystem services in a grassland system.