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PLANTS INTERACTING WITH OTHER ORGANISMS

Mechanism of Infection Thread Elongation in Root Hairs of Medicago truncatula and Dynamic Interplay with Associated Rhizobial Colonization^1,[W],[OA]

Laboratoire des Interactions Plantes Micro-Organismes, UMR CNRS-INRA 2594/441, F–31320 Castanet-Tolosan, France (J.F., A.C.J.T., B.J.S., M.C., D.G.B.); and Institut Fédératif de Recherche 40, Pôle de Biotechnologie Végétale, F–31326 Castanet-Tolosan, France (A.J.)

In temperate legumes, endosymbiotic nitrogen-fixing rhizobia gain access to inner root tissues via a specialized transcellular apoplastic compartment known as the infection thread (IT). To study IT development in living root hairs, a protocol has been established for Medicago truncatula that allows confocal microscopic observations of the intracellular dynamics associated with IT growth. Fluorescent labeling of both the IT envelope (AtPIP2;1-green fluorescent protein) and the host endoplasmic reticulum (green fluorescent protein-HDEL) has revealed that IT growth is a fundamentally discontinuous process and that the variable rate of root hair invagination is reflected in changes in the host cell cytoarchitecture. The concomitant use of fluorescently labeled Sinorhizobium meliloti has further revealed that a bacteria-free zone is frequently present at the growing tip of the IT, thus indicating that bacterial contact is not essential for thread progression. Finally, these in vivo studies have shown that gaps within the bacterial file are a common feature during the early stages of IT development, and that segments of the file are able to slide collectively down the thread. Taken together, these observations lead us to propose that (1) IT growth involves a host-driven cellular mechanism analogous to that described for intracellular infection by arbuscular mycorrhizal fungi; (2) the non-regular growth of the thread is a consequence of the rate-limiting colonization by the infecting rhizobia; and (3) bacterial colonization involves a combination of bacterial cell division and sliding movement within the extracellular matrix of the apoplastic compartment.

The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.or) is: Joëlle Fournier (joelle.fournier{at}toulouse.inra.fr).

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.125674

^* Corresponding author; e-mail david.barker{at}toulouse.inra.fr.

Received July 2, 2008; accepted October 10, 2008; published October 17, 2008.

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