Fungal root endophytes and their role in carbon to nitrogen exchange with plants

Root fungal endophytes are present in most plants and co-occur with other mycorrhizal fungi. Their intraradical colonization suggests a special, differentiated relationship with host plants and increases opportunities for close interactions between hosts and fungal symbionts (e.g., carbon to nutrient exchange or hormone signalling). During the symbiosis between plant and arbuscular mycorrhizal fungi (AMF), specific trading features are established. These features have been incorporated into the biological market hypothesis, where dynamics of carbon to nutrient trading in the plant‐mycorrhizal fungal mutualism are compared to trades in a market economy. Multiple examples of similar dynamics have been shown for root endophytic fungi: soil nutrients are transported to the plant in exchange for carbon. However, plants have been shown to be able to reward AMF that exchange larger amount of nutrients (and vice versa), while at least some root fungal endophytes have been described as “by-product mutualists”, where the fungal symbiont enhances the performance and fitness of their host plant by providing benefits, but not requiring major investments from the host. Whereas AMF have received large attention, the role of fungal endophytes in carbon to nutrient exchange with plants remains largely uninvestigated.

In this study we aimed at developing a controlled system to evaluate effects of multiple fungal endophytes (Colletotrichum tofieldiae and Cladophialophora chaetospira) on a model plant species (Lotus japonicus) and their role in the carbon to nitrogen exchange in the presence of different nitrogen sources (organic and inorganic). We further developed this controlled system to include plants colonized by AMF. Two-compartment petri dishes were used to achieve plant root colonization in a nutrient limited compartment and allow separation of nutrient sources only accessible by the fungal endophytes. We performed dual ¹⁵N and ¹³CO₂ pulse-labelling experiments to trace the fate of plant carbon into fungal biomass and of different nitrogen sources into plant aboveground tissues. We analysed root for RNA sequencing to gain insights into the genetic controls over the observed dynamics.

We successfully established a controlled system and found that C. tofieldiae can elicit positive effects on plant growth and nitrogen acquisition. These effects are dependent on the nutrient source to which the fungus has access to, with positive effects displayed in the presence of organic nitrogen. Plants exchange relatively less carbon to C. tofieldiae accessing organic nitrogen. Root transcriptome shows specific changes in response to root fungal colonization which are dependent on the nitrogen source available to the fungal endophyte. Furthermore, the presence of AMF did not modify the observed carbon to nitrogen exchange dynamics.

In conclusion we show that the root fungal endophyte C. tofieldiae can play an important role for plant nutrient acquisition in the presence of organic nitrogen. The trade of nitrogen for plant carbon displays different features from the AMF symbiosis (i.e., higher amount of nitrogen is not rewarded with plant carbon investment) and different gene regulations are involved. Our results indicate complementarity between C. tofieldiae and AMF during root colonization, offering mechanistic explanations for the concomitant presence of AMF and fungal endophytes in terrestrial ecosystems.