Factors shaping community structure of endophytic fungi–evidence from the Pinus-Viscum-system

Abstract

Endophytic fungal communities have been shown to be highly diverse in almost every host plant species analyzed so far. However, the factors shaping their compositions are largely unknown. To elucidate the impact of various factors, 10 independent replicates of DNA extracts from each of 17 surface-sterilized leaf and stem samples were analyzed by pyrosequencing of fungal ITS1 rRNA gene amplicons. Altogether, 154 fungal OTUs (operational taxonomic units), represented by 953,385 sequences, were found in at least 2 samples from Viscum album ssp. austriacum and/or its host Pinus sylvestris. Deviating from earlier, cultivation-based assessments, the communities were dominated by OTUs related to the genus Mortierella and OTUs not assignable to a certain fungal phylum. However, Ascomycota were still the most diverse group in terms of OTU richness and already hypothesized organ and host preferences of certain endophytic Xylariaceae isolated from the Pinus-Viscum-system could be confirmed. Host species and organ type were also the major factors shaping the detected fungal communities. The two plant species clearly differed according to the endophytic fungal communities, but only stems and needles of Pinus were inhabited by significantly different fungal assemblages. Interestingly, only the 1 and 3 year old stem sections differed according to the endophytic fungal community, while differently aged leaves of both plants were indistinguishable in this regard. Size of the organs had no impact on fungal communities in Pinus, but shorter internodes and smaller leaves showed at least a tendency to differ from the corresponding larger organs in Viscum. Fungal communities also differed slightly between the two sampling sites, lying 200 km apart, and between the three sampling campaigns. Because the samples were drawn within 15 days, this finding indicates that seasonal shifts clearly outweigh aging effects in host plant with perennial leaves. The results therefore provide strong evidence against a linear development of the endophytic fungal communities in Pinus sylvestris and Viscum album over the years. The communities seem to establish themselves already in the year the respective organs emerge. Further study is required to clarify whether they predominantly establish anew each year, or if the core community persists throughout subsequent years. The most abundant endophytic OTUs are known from soil and/or dead plant material and are supposed to represent latent decomposers. The study reveals for the first time that host and/or organ preferences of putatively saprotrophic fungi are predominantly responsible for compositional differences in the endophytic fungal communities between host plants and organs. While the analyses are shown to provide rather robust results, the significance of genetic abundance, as revealed by high-throughput sequencing analyses, remains an unsettled issue. This is discussed in detail, as well as the challenges in assigning taxonomic names to OTUs.