title: Understanding plant-fungal nutritional strategies using stable isotopes
Creators: name: Giesemann, Philipp
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: arbuscular mycorrhiza
subject: Arum-type
subject: endogonales
subject: dark septate endophyte
subject: fine root endophyte
subject: Mucoromycotina
subject: mycoheterotrophy
subject: Paris-type
subject: plant-fungus symbiosis
subject: stable isotope natural abundance
subject: 500
subject: 570
subject: 580

description: 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 used to decipher a functional role in nutrient trading for the elusive DSE and FRE. Isotope tracer applications were performed to evaluate a carbon-for-nutrient trading while natural abundances were used to decipher whether either organic or inorganic soil compounds might be exchanged in the field. In brief, the thesis consists of four manuscripts. Manuscript 1, 2 and 3 found plant species that were isotopically distinguished in 13C, 2H and frequently 15N appeared either with intercellular Arum-morphotype AM or intracellular Paris-morphotype AM. The stable isotope enrichment is most likely explained by a partially mycoheterotrophic nutrition on Paris-morphotype AM fungi. Thus, the Paris-morphotype appears to be a necessary prerequisite for partial mycoheterotrophy on AM fungi. Furthermore, Manuscript 3 and 4 provide evidence for a functional role of the ubiquitous DSE and FRE fungi in terms of plant-fungi nutrient trading. For the dual symbiosis of DSE and Paris-morphotype AM fungi in Equisetaceae, isotopic evidence supports an organic soil N transfer from the former and organic C transfer from the latter. Thus, organic nutrient transfer in plant-fungi symbioses may not be limited to mycoheterotrophs. This suggests DSE and FRE fungi may occupy a previously under-recognized but ecologically relevant role similar to mycorrhizas. Summarizing, isotope natural abundance compositions supported a partially mycoheterotrophic nutrition for the Paris-morphotype AM-forming forest herbaceous species Paris quadrifolia (true lover’s knot, Melanthiaceae) and Anemone nemorosa (‘wood anemone’, Ranunculaceae) (Manuscript 1). A literature survey of isotope natural abundance compositions resulted in 135 plant species being either achlorophyllous forming Paris-morphotype (13 species), chlorophyllous forming Paris-morphotype (63 species) or chlorophyllous forming Arum morphotype (59 species). Partial mycoheterotrophy appeared frequently among the chlorophyllous Paris morphotype AM plant species (31 out of 63 species under study), especially herbaceous forest seed plants and pteridophytes (Manuscript 2 and 3). Isotope natural abundance compositions of non-mycorrhizal plant species belonging to the plant families Equisetaceae (horsetails), Cyperaceae (sedges) and Caryophyllaceae (carnation family) supported active or passive acquisition of organic soil N compounds via DSE fungi (Manuscript 3). Carbon-for-nutrient trading was deciphered for a non-mycorrhizal Lycopodiaceae plant species (club moss) (Manuscript 4). Lab-provided inorganic nutrient tracers applied to FRE fungi were transferred towards the plant partner while the plant partner provided C in exchange. Furthermore, the isotope natural abundance of the club moss supports a transfer of organic soil N compounds in field sites.