Associate Professor The Ohio State University Columbus, Ohio, United States
Optimal Foraging Theory predicts how an organism forages for resources in patchy environments, and has been widely used to study animal foraging behaviors. However, Optimal Foraging Theory has seldom been applied to microbes foraging in soil like arbuscular mycorrhizal (AM) fungi despite the evidence that AM fungi can perceive signals and respond to their environment. Thus, we know little about the foraging behaviors of AM fungi. To help address this gap we apply Optimal Foraging Theory to AM fungi foraging for nitrogen. We focus on nitrogen because AM fungi need more nitrogen than their plant hosts and soil nitrogen availability can shift the relationship between AM fungi and their hosts from mutualism to parasitism.
To address this gap, we set up an in vitro experiment using carrot (Daucus carota L.) hairy root inoculated with AM fungi (Rhizophagus irregularis) in bi-compartment plates (with root and hyphal compartments) to study AM fungal hyphal development in response to different nitrogen concentrations and forms. Inoculated roots were cultured in the root compartment on minimal media without nitrogen but in the hyphal compartment we added three concentrations (0 mM, 5mM and 10 mM) of three nitrogen forms (ammonium chloride, calcium nitrate or glycine) to minimal media. We focused on two external hyphal types: (1) branched absorbing structures (BAS), which are responsible for nutrient uptake and (2) runner hyphae (RH), which explore the environment. We imaged plates daily and recorded the hyphal extension rate, branching rate (of BAS and RH), branching angle (of BAS and RH), and the ratio of the length of BAS to the length of RH (BAS/RH) in the hyphal compartment.
Results showed variation in both BAS and RH structures and frequency as nitrogen concentration and form changed. We measured the daily change ratio of BAS/RH reflected to provide information on how AM fungi handled patches over time.
This work provides a foundation for studying AM fungal foraging activity and fills an important gap regarding how AM fungi forage for nutrients by applying the framework of Optimal Foraging Theory.