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PLANTS INTERACTING WITH OTHER ORGANISMS

Architecture of Infection Thread Networks in Developing Root Nodules Induced by the Symbiotic Bacterium Sinorhizobium meliloti on Medicago truncatula^1,[W]

Department of Molecular and Cell Biology, University of Connecticut, Storrs, Connecticut 06269–3125

During the course of the development of nitrogen-fixing root nodules induced by Sinorhizobium meliloti on the model plant Medicago truncatula, tubules called infection threads are cooperatively constructed to deliver the bacterial symbiont from the root surface to cells in the interior of the root and developing nodule. Three-dimensional reconstructions of infection threads inside M. truncatula nodules showed that the threads formed relatively simple, tree-like networks. Some characteristics of thread networks, such as branch length, branch density, and branch surface-to-volume ratios, were remarkably constant across nodules in different stages of development. The overall direction of growth of the networks changed as nodules developed. In 5-d-old nodules, the overall growth of the network was directed inward toward the root. However, well-defined regions of these young networks displayed an outward growth bias, indicating that they were likely in the process of repolarizing their direction of development in response to the formation of the outward-growing nodule meristem. In 10- and 30-d-old nodules, the branches of the network grew outward toward the meristem and away from the roots on which the nodules developed.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Daniel J. Gage (daniel.gage{at}uconn.edu).

^[W] The online version of this article contains Web-only data.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.072876.

^* Corresponding author; e-mail daniel.gage{at}uconn.edu; fax 860–486–4331.

Received October 13, 2005; returned for revision November 22, 2005; accepted November 23, 2005.

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