Seed fungal endophytes as promoters of Poa annua establishment and invasion in Maritime Antarctica

Invasive plants may cause a negative impact to native species due to competition for resources. This is the case of Poa annua, the only non-native plant species successfully established in Maritime Antarctica. Therefore, it is important to understand which factors could be driving the competitive success of P. annua, even despite the harsh environmental conditions. One of such factors is the establishment of novel mutualistic relationships with the resident soil fungi, which would improve plant growth and confer protection against stressful conditions. Additionally, plant-associated (endophytic) fungi could also participate in seedling development and establishment, by being vectored in seeds. Therefore, in this study we explored the composition and compared the diversity of fungal communities in different P. annua tissues (seeds, leaves and roots) with representative samples of Antarctic tundra soil by using ITS2 sequencing. Our results suggest that P. annua would have a strikingly different microorganism community compared with Antarctic soil, as only 6 amplicon sequence variants were shared among plants and soils; therefore, it is plausible that the invasive success of P. annua is mediated by its’ unique endophytes (rather than soil fungi). To test this hypothesis, we grew P. annua, Colobanthus quitensis and Deschampsia antarctica (Antarctica’s only two plant native species) from seeds with and without endophytes, and measured different parameters, including final biomass, germination rate, number of reproductive structures, and plant survival. Furthermore, an interspecific/intraspecific resource competition experiment based on the physical presence of native and invasive plants was conducted, where chemically-mediated plant-plant interference (allelopathy) production and plant growth rate were measured, and used in the calculation of a relative competition index. Our results showed that the unique host-associated fungal endophytes harbored within P. annua seeds enhanced several plants’ parameters, that these endophytes increased the competitive effects of P. annua against native plants and that one mechanism involved in plants’ competition is the production and release of phenolic allelochemicals. Yet, while the molecular mechanisms being modulated by endophytes are still unknown, our results suggest that endophytes have a role on traits related to the dispersal and adaptation process of the invasive species P. annua in Maritime Antarctica.