Assembly structure of endophytes in northern wild berry fruits – special focus on bilberry Vaccinium myrtillus L.

Wild berries are important natural resources in northern Europe due to their economic and therapeutic value. The microorganisms (mainly bacteria and fungi) inhabiting the wild berry fruit tissues (so called endophytes) are largely unknown, although they likely have a close interaction with the host and can be important mutualists. In general, endophytes can provide several benefits to the host, such as promoting plant growth, enhancing tolerance against biotic or abiotic stress, improving fruit quality, and closely interacting with the host via metabolic crosstalk. Understanding the diversity and driving factors affecting the endophytic communities is crucial because a balanced endophytic community is mandatory to ensure plant health. A change in the endophytic composition could disrupt the balance and negatively impact the host plant. In this thesis, I assess the impacts of both internal (host species, fruit developmental stages and phenolic composition) and external (climatic regions, weather conditions and soil properties) factors on the endophytes of wild berry fruits, with a special focus on fungal endophytes and bilberry Vaccinium myrtillus L. Two other studied wild berry species, lingonberry (Vaccinium vitis-idaea L.) and crowberry (Empetrum nigrum L.), were included to test the impact of host species and host phenolic composition. I used marker gene and high-throughput sequencing to examine the endophytic composition, along with Liquid Chromatography-Mass Spectrometry to analyze the phenolic compositions of the berries. I found that wild berry fruits host a larger proportion of cultivable fungal than bacterial endophytes. The structure of fungal communities differed between host berry species and host developmental stages. Furthermore, there was a strong link between the host phenolic and fungal community composition. The climatic region affected the diversity of the fungal endophytic communities, specifically higher fungal diversity in bilberries was found in the south compared to the north of Finland. Among abiotic factors, weather conditions during the growing season rather than soil properties affected the diversity and structure of the fungal endophytic communities.