Pre-penetration apparatus formation during AM infection is associated with a specific transcriptome response in epidermal cells, plant signaling & behavior.

Arbuscular mycorrhizal (AM) associations have strikingly constant structural and functional features, irrespectively of the organisms involved. This suggests the existence of common genetic and molecular determinants. One of the most important characteristics of AM is the coating of intracellular hyphae by a proliferation of the plant plasma membrane, which always segregates the fungus in an apoplastic interface. This process of intracellular accommodation causes a dramatic reorganization in the host cell cytoplasm, which reaches its peak with the development of the so-called prepenetration appartus (PPA), a specialised aggregation of organelles described in epidermal cells and predicting fungal development within the cell lumen. We have recently correlated PPA development with the significant regulation of 15 Medicago truncatula genes. Among these. a nodulin-like and an expansin-like sequence are good candidates as molecular markers of epidermal cell responses to AM contact. Our results also suggest a novel role for the kinase DMI3 in enhancing the up-regulation of these two genes and down-regulating defence-related genes such as the Avr9/Cf-9 rapidly elicited protein 264. We here comment on these recent findings and their possible outcomes.