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Plant Physiology Preview Published on December 29, 2005; 10.1104/pp.105.072876
Received October 13, 2005 Architecture of infection thread networks in developing root nodules induced by the symbiotic bacterium Sinorhizobium meliloti on Medicago truncatula
University of Connecticut, Department of Molecular and Cell Biology, 91 N. Eagleville Rd., U-3125 Storrs, CT 06269-3125 * Corresponding author; email: daniel.gage{at}uconn.edu.
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-day 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-day old and 30-day old nodules, the branches of the network grew outward toward the meristem and away from the roots on which the nodules developed.
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