Root endophytes along a primary succession gradient in northern Finland

Abstract

Primary successional vegetation gradients are characterized by changes in the soil microbial communities. However, information on possible shifts of the root endophytes along these gradients is scarce. The objective of the current study was to identify root endophytic fungi from a primary successional gradient on land uplift seashore of a geographically isolated island area. We applied a sequencing approach by amplifying the ITS region with fungal specific primers. We used mainly an isolate-based method, and to compare the abundance of culturable and unculturable endophytes, direct sequencing of one representative plant specimen Deschampsia flexuosa was also carried out. A total of 38 cultured endophytic strains were sequenced from Empetrum nigrum (Empetraceae), Vaccinium vitis-idaea (Ericaceae) and Deschampsia flexuosa (Poaceae). Out of these, 27 were identified as Phialocephala fortinii, three as Mollisia minutella, four as Phialophora sp., one as Ascomycetes sp. and three remained unidentified. The strains clustered into five clades in the phylogram, mostly irrespective of the successional stages and hosts from which they had been isolated. The early successional seashore dune ridge plants however, seemed to host a distinct fungal taxon, Phialophora sp. Culture-independent methods were applied on a root sample of a mid-successional Deschampsia flexuosa specimen and a total of 16 clones were randomly selected and sequenced. Out of 16 sequences, 13 were identified as unculturable strains and three showed closest similarity to a basidiomycete Cortinarius callisteus. The unculturable sequences were grouped into two main clades and were different from any culturable isolate in this study. Our results suggest that (i) P. fortinii dominates the isolate data at mid to late successional stages, (ii) roots of the ericaceous plants and the grass Deschampsia flexuosa are colonized by the same endophytic fungi in this ecosystem, and (iii) unculturable endophytes are common and potentially more abundant than the culturables. To our knowledge, this is the first report of the molecular phylogenies of the DSE in the mid-boreal zone and also the first report of the unculturable root endophytes of D. flexuosa.