Tit for tat? A mycorrhizal fungus accumulates phosphorus under low plant carbon availability.
The exchange of carbohydrates and mineral nutrients in the arbuscular mycorrhizal symbiosis must be controlled by both partners in order to sustain an evolutionarily stable mutualism. Plants down-regulate their carbon flow to the fungus when nutrient levels are sufficient, while the mechanism controlling fungal nutrient transfer is unknown. Here we show that the fungus accumulates nutrients when c