Using PhytoPET imaging to elucidate trade-offs in plant-fungal symbiont interactions

Background/Question/Methods: Although a number of ectomycorrhizal fungi (EMF) species have been shown to provide nutrients, promote drought resistance and, thus, enhance productivity in host plants, the specific costs and benefits of the mututalistic interactions are hard to establish.  To quantify the dynamic transport of carbohydrates and nutrients between the host plant and a fungal symbiont in different environments, we tested a scalable Positron Emission Tomography imaging system (PhytoPET) with flexible detector arrangement capability that was developed for plant research of the type described here.  We first compared the ¹¹C-carbohydrate transport in white oak (Quercus alba) seedlings grown in sterile sand, with or without a fungal inoculum.  Each oak seedling was placed in a dual-chambered cuvette where the aboveground portion was labeled with ¹¹CO₂ via photosynthesis. The transport and accumulation of assimilated ¹¹C-carbohydrate was tracked through the plant using the PhytoPET imager.  The PhytoPET system consists of a series of self-contained detector modules based on single Position Sensitive PhotoMultiplier Tubes (PSPMTs) with scintillator that allow for flexible individual stacking to accommodate the whole-plant geometry.  The radioactivity data was acquired for all regions of the plants for 90 min and the images were reconstructed using a Maximum Likelihood Expectation Maximization (MLEM) reconstruction code. 

Results/Conclusions: Significant ¹¹C radioactivity was detected in the leaves of the oak seedling after 10 min of labeling, indicating the relatively slow photosynthetic rate of white oak.  For a symbiont-infected seedling, significant radioactivity in the stem region indicated translocation from the leaf petiole down the stem after another 10 min and the 11C-carbohydrate was translocated to the root tips and fungal hyphae after 30 min.  The root-symbiont region showed high accumulation of ¹¹C-carbohydrate after 90 min.  In contrast, the seedlings grown in sterile soil showed a relatively greater translocation of carbohydrate into aboveground sinks than to the root tips.  These data suggest the high cost of symbiont to the plant host.  These data will be coupled with ¹³N-translocation to estimate the putative benefit of symbiont to the host plant.   The PhytoPET technique allows for high temporal and 3-D spatial resolution to determine the trade-offs in plant-fungal symbiont mutualism.