Plant Physiology Preview
Published on October 17, 2008; 10.1104/pp.108.125674
OPEN ACCESS ARTICLE
Received July 2, 2008
Accepted October 10, 2008
Mechanism of infection thread elongation in root hairs of Medicago truncatula and dynamic interplay with associated rhizobial colonization
Joelle Fournier , Antonius C. J. Timmers , Bjorn J. Sieberer , Alain Jauneau , Mireille Chabaud , and David G. Barker *
Laboratoire des Interactions Plantes Micro-organismes (LIPM), UMR CNRS-INRA 2594/441, F-31320 Castanet Tolosan, France, Institut Federatif de Recherche 40, Pole de Biotechnologie Vegetale, BP17 Auzeville, F-31326 Castanet-Tolosan, France
* Corresponding author; email: David.Barker{at}toulouse.inra.fr.
In temperate legumes, endosymbiotic N-fixing rhizobia gain access to inner root tissues via a specialized transcellular apoplastic compartment known as the infection thread (IT). In order to study IT development in living root hairs a protocol has been established for Medicago truncatula which allows confocal microscopic observations of the intracellular dynamics associated with IT growth. Fluorescent labeling of both the IT envelope (AtPIP2;1-GFP) and the host endoplasmic reticulum (GFP-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 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 (i) IT growth involves a host-driven cellular mechanism analogous to that described for intracellular infection by arbuscular mycorrhizal fungi (Genre et al., 2005), (ii) the non-regular growth of the thread is a consequence of the rate-limiting colonization by the infecting rhizobia, and (iii) bacterial colonization involves a combination of bacterial cell division and sliding movement within the extracellular matrix of the apoplastic compartment.
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