| The arbuscular mycorrhizal fungi (AMF) are obligate symbionts that colonize the roots of the majority of land plants and take up photosynthetically fixed carbon. In return, the host plant benefits with increased nutrient uptake, resistance to pests, improved water relations and drought resistance, increased growth and yield. The extraradical mycelium of the fungus acts as an extension of the root system enabling more thorough exploration of the soil for nutrients such as phosphorous (P), and nitrogen (N), which are then transported to the roots.; The mechanisms and regulation of N assimilation and transport in AM fungi is not well understood. The proposed mechanism predicts that the AMF Glomus intraradices assimilates inorganic N into arginine (Arg) in extraradical mycelia (ERM) and transports that Arg to the intraradical mycelia (IRM) where ammonia is released for transfer to the host plant via catabolic reactions of the urea cycle. To test hypotheses about the uptake, transformation and transfer of N, a dual approach of stable isotope labeling, and gene expression analysis were used. Using sterile in vitro carrot co-cultures, fungal hyphae were exposed to different forms of N, and the resultant pattern of fungal gene expression in the ERM and IRM were analyzed by quantitative real-time PCR. The results of stable isotope labeling experiments indicated that N was transferred to the host plant in large quantities without carbon. The genes of primary N metabolism were expressed in the ERM while the genes associated with Arg breakdown were more highly expressed in the IRM. Conversely, the expression of fungal genes involved in N assimilation, the urea cycle and ammonium transporter (AMT) were down-regulated relative to controls as sucrose was depleted from the growth medium. This was reversed by supplemental glucose demonstrating that transcriptional regulation of these genes is dependent on carbon availability. The same pattern of down regulation of these fungal genes was observed when pot-grown Medicago sativa plants colonized by Glomus intraradices were placed in the dark to limit carbon availability. These results showed that nutrient exchange in the symbiosis is tightly controlled and suggests a mechanism for regulation of coupled nutrient exchanges between the symbionts. |
