Choosing your partner: Trade-offs in dynamic carbon and nitrogen exchange between plants and their fungal symbionts

Background/Question/Methods:

Plant-fungal interactions span a broad spectrum of functional relationships between the extremes of mutualistic (beneficial) to pathogenic ones.  For survival, plants must be capable of distinguishing pathogens from mutualists, and of determining which fungal partner is most likely to provide a limiting nutrient resource most efficiently.  To determine the trade-offs between exchange of carbon from the plant and nitrogen from the symbiont, we compare short-term temporal dynamic exchanges of radiolabeled carbohydrates from a corn plant with labeled absorbed N from different fungal isolates.  Corn germlings were transplanted in split-root cuvettes with either one side inoculated with one fungal species, with both sides sterile (control), or both sides- inoculated with the same species.  They were grown in controlled environmental growth chambers at the Duke University phytotron under optimal conditions.   Once the fungal inoculate is well established, the plant is labeled with radiolabeled ¹¹CO₂ via photosynthesis, and the transport and accumulation of assimilated ¹¹C-carbohydrates through the plant and fungal hyphae is followed using a PhytoPET imager.  The plant is then labeled with ¹³NO₃ via the soil medium and the ¹³N transport is followed again with PhytoPET.  The pools and rate of radioisotope transport between the two root portions and among the plant organs are compared.

Results/Conclusions:

For seedlings with Leptodontidium inoculate on one side and one side fungal-free, corn seedlings labeled with ¹¹CO₂ via photosynthesis transferred 26% more ¹¹carbohydrate to inoculated roots than to fungal-free ones, and at a 32% greater velocity.  However, the proportion of assimilated ¹¹C that accumulated in fungal inoculated roots vs. sterile roots depended on time of day and on fungal symbionts (e.g. Mortierella).  Transport of radio-labeled ¹³N from roots to the leaf mid-vein was more rapid than carbohydrates. Given the renewed interest in using microbial symbionts to enhance agricultural productivity, gaining insights about how soil fungi might be used to enhance plant productivity is central to sustainable agriculture.