Understanding plant-fungal nutritional strategies using stable isotopes

Abstract

The mycorrhizal symbiosis is widely accepted as a relationship for mutual carbon-for-nutrient trading. In contrast, hundreds of mycoheterotrophic plant species are identified subverting the usually mutual mycorrhizal symbiosis to utilize their fungal partner as an organic carbon and nitrogen source. Additionally, the focus on nutrient trading in plant-mycorrhizal fungal relationships underrates other common root fungi, such as dark septate endophytes (DSE) and fine root endophytes (FRE). The thesis hypothesizes (i) the existence of far more mycoheterotrophic plant species than currently estimated and (ii) a mycorrhiza-like nutritional role for DSE and FRE in plant-fungi relationships (Figure 1). Isotope applications of the elements carbon (C), nitrogen (N) and hydrogen (H) have proven to be a valuable tool to elucidate organic and inorganic nutrient fluxes between plants and fungi. Thus, the utilization on a fungal source is evident for achlorophyllous plant species belonging to 17 plant families on either arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) fungi or on litter-decomposing and wood-decomposing fungi. The presence of fully autotrophic plant species on the one hand and achlorophyllous, fully mycoheterotrophic plant species on the other hand obliges us to postulate an intermediate strategy for chlorophyllous plant species which obtain their C energy by means of photosynthesis (autotrophy) while simultaneously channeling off organic C and N from a fungal source (mycoheterotrophy). Evidence of partial mycoheterotrophy is commonly found but yet mostly restricted to only two plant families associated with EcM fungi (Orchidaceae and Ericaceae). Full mycoheterotrophy, indeed, appears most often with AM fungi while evidence of partial mycoheterotrophy on AM fungi is very scarce. Interestingly, the AM symbiosis appears with a continuum of different fungal morphotypes with intercellular Arum-morphotype AM and intracellular Paris-morphotype AM at the ends of the continuum. So far analyzed, all fully mycoheterotrophic AM plant species appear with intracellular Paris-morphotype AM, thus chlorophyllous Paris-morphotype AM plant species are candidates for partially mycoheterotrophic nutrition. Due to the overwhelming distribution of AM plant species, the presence of partial mycoheterotrophy on AM fungi could have far-reaching implications for our understanding of plant community functioning which we might have overlooked until now. Thus, a stable isotope natural abundance approach was realized to evaluate the extent of partial mycoheterotrophy on AM fungi. Furthermore, it is evident that the elusive DSE and FRE fungi are also commonly distributed across all plant clades while little is known about their nutritional role in plant-fungi relationships. Interestingly, DSE and FRE inhabit both mycorrhizal and non-mycorrhizal plant species. The latter might provide an opportunity to shed light onto the nutritional functions of DSE and FRE fungi in plant-fungi symbioses. In this thesis isotope applications were…