Plant Cell Advance Online Publication
Published on November 11, 2005; 10.1105/tpc.105.035410
Received June 21, 2005
Returned for revision September 25, 2005
Accepted October 17, 2005
Arbuscular Mycorrhizal Fungi Elicit a Novel Intracellular Apparatus in Medicago truncatula Root Epidermal Cells before Infection
Andrea Genre 1, Mireille Chabaud 2, Ton Timmers 2, Paola Bonfante 1, and David G. Barker 2*
1 Department of Plant Biology, University of Turin and Istituto per la Protezione delle Piante-Consiglio Nazionale delle Richerche, 10125 Turin, Italy
2 Laboratory of Plant-Microbe Interactions, Unité Mixte de Recherche Institut National de la Recherche Agronomique-Centre National de la Recherche Scientifique, 31326 Castanet Tolosan Cedex, France
* To whom correspondence should be addressed. E-mail: barker{at}toulouse.inra.fr.
The penetration of arbuscular mycorrhizal (AM) fungi through the outermost root tissues of the host plant is a critical step in root colonization, ultimately leading to the establishment of this ecologically important endosymbiotic association. To evaluate the role played by the host plant during AM infection, we have studied in vivo cellular dynamics within Medicago truncatula root epidermal cells using green fluorescent protein labeling of both the plant cytoskeleton and the endoplasmic reticulum. Targeting roots with Gigaspora hyphae has revealed that, before infection, the epidermal cell assembles a transient intracellular structure with a novel cytoskeletal organization. Real-time monitoring suggests that this structure, designated the prepenetration apparatus (PPA), plays a central role in the elaboration of the apoplastic interface compartment through which the fungus grows when it penetrates the cell lumen. The importance of the PPA is underlined by the fact that M. truncatula dmi (for doesn't make infections) mutants fail to assemble this structure. Furthermore, PPA formation in the epidermis can be correlated with DMI-dependent transcriptional activation of the Medicago early nodulin gene ENOD11. These findings demonstrate how the host plant prepares and organizes AM infection of the root, and both the plant-fungal signaling mechanisms involved and the mechanistic parallels with Rhizobium infection in legume root hairs are discussed.
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