Compartmentalisation: A strategy for optimising symbiosis and tradeoff management.

Plant root architecture is developmentally plastic in response to fluctuating nutrient levels in the soil. Part of this developmental plasticity is the formation of dedicated root cells and organs to host mutualistic symbionts. Structures like nitrogen-fixing nodules serve as alternative nutrient acquisition strategies during starvation conditions. Some root systems can also form myconodules-globular root structures that can host mycorrhizal fungi. The myconodule association is different from the wide-spread arbuscular mycorrhization. This range of symbiotic associations provides different degrees of compartmentalisation, from the cellular to organ scale, which allows the plant host to regulate the entry and extent of symbiotic interactions. In this review, we discuss the degrees of symbiont compartmentalisation by the plant host as a developmental strategy and speculate how spatial confinement mitigates risks associated with root symbiosis.