Regulation of mycorrhizal symbiosis: development of a structured nutritional dual model

A structured modeling approach was developed to assess and describe the role of nutrition in mycorrhizal symbiosis. Agrobacterium rhizogenes transformed Daucus carota hairy roots and the arbuscular mycorrhizal (AM) fungus Glomus intraradices were used as a biological model in Petri dish liquid cultures. A minimal medium was used, varying the initial phosphate (Pi) concentration (0.02, 0.04, 0.12 and 1.00 mM KH2PO4). Uptake of sugar and macronutrients was monitored during culture. Pi ion was the limiting nutrient at 0.02 and 0.04 mM Pi. The specific growth rate (dry mass) of non-mycorrhizal roots was significantly reduced under low Pi from 0.068±0.012 (≥0.12 mM Pi) to 0.04±0.006 (0.04 mM Pi) and 0.022±0.009 per day (0.02 mM Pi). Values for mycorrhizal roots were even lower. The specific spore production rate for G. intraradices was maximal at 0.042 per day for 0.12 mM Pi. A growth behavior dual model is presented for root dry mass and fungus spore number based on extra- and intracellular concentrations of Pi and sugar, and extracellular nitrate. Pi translocation between symbionts and roots’ sugar uptake by the fungus has also been described in the model. Calibration of the model with experimental data suggests that intracellular Pi storage in roots acts as a ‘decision switch’ for G. intraradices spore production. A significant AM fungus competition for soluble root sugars was also revealed.