Dark septate endophytes and arbuscular mycorrhizal fungi (Paris-morphotype) affect the stable isotope composition of ‘classically’ non-mycorrhizal plants

The vast majority of terrestrial plants exchange nutrients with fungal
partners forming different mycorrhizal types. The minority of plants
considered as non-mycorrhizal, however, are not necessarily free of any
fungi, but are frequently colonized by elusive fungal endophytes, such as
dark septate endophytes (DSE) or fine root endophytes (FRE). While a
functional role of FRE in improvement of nutrient gain was recently
elucidated, the function of DSE is still in discussion and was here
addressed for 36 plant species belonging to the families Equisetaceae,
Cypereaceae and Caryophyllaceae. Molecular and microscopic staining
approaches were conducted to verify the presence of DSE in the
investigated species. Stable isotope natural abundances of the elements
carbon, nitrogen, hydrogen and oxygen and total nitrogen concentrations
were analyzed for the respective species of the target plant families and
accompanying mycorrhizal and non-mycorrhizal (Brassicaceae) plant species.
Staining approaches confirmed the presence of DSE in all investigated
species within the families Equisetaceae, Cyperaceae and Caryophyllaceae.
A co-colonization with Paris-type arbuscular mycorrhiza (AM) was
occasionally found by staining and molecular approaches in species of the
Equisetaceae. Species of the Equisetaceae, Cyperaceae and Caryophyllaceae
were significantly 15N-enriched in comparison to accompanying plants. In
addition, a significant 13C and 2H enrichment and increased total nitrogen
concentrations were found for representatives of the Equisetaceae. The
15N-enrichment found here for representatives of Equisetaceae, Cyperaceae
and Caryophyllaceae provides evidence for a functional role of the
ubiquitous DSE fungi. DSE fungi obviously provide access to 15N-enriched
soil organic compounds probably in exchange for organic carbon compounds
from plant photosynthesis. As indicated by additional 13C- and
2H-enrichments, representatives of the Equisetaceae apparently gain
simultaneously organic carbon compounds from their AM fungi of the
Paris-morphotype. Thus, species of the Equisetaceae have to be considered
as partially, or in case of the achlorophyllous fertile Equisetum arvense,
as fully mycoheterotrophic at least in some stages of their life cycle. So
far mostly underappreciated fungi classified as DSE are suggested to
occupy an ecologically relevant role similar to mycorrhizae and the
occurrence of simultaneous functions of DSE and AM fungi in Equisetaceae
is proposed.