The Making of Symbiotic Cells in Arbuscular Mycorrhizal Roots

The establishment of arbuscular mycorrhizae (AM) requires the host root cells to undergo significant structural and functional modifications leading eventually to reciprocal beneficial effects. We here discuss some of the responses that are induced in root cells during the different steps of AM colonization. A combination of genetics, molecular and cellular approaches reveals in fact that the establishment of a functional symbiosis appears to be at the end of a series of plant-controlled checkpoints, each of which is required but non-sufficient to achieve the next one. The recognition of AM fungi as symbionts takes place in the rhizosphere, and switches the host plant to an alert status, by activating specific signalling pathways and responses on a cell to organism level. Direct cellular contact on the root surface is a central event in the interaction that mainly impacts on the contacted cell, where prepenetration responses open the way to the intracellular accommodation of the fungal symbiont. This constructive phase extends to epidermal and cortical cells and culminates with the establishment of functional arbuscules, which accomplish the key symbiotic functions related to nutrient exchanges. The modulation of arbuscule development and functioning represent the core process of the symbiosis and its study keeps bringing novel information on the molecular, cellular and metabolic mechanisms that rule this ancient interaction.