Architecture of Infection Thread Networks in Nitrogen-Fixing Root Nodules
During the development of nitrogen-fixing root nodules, symbiotic bacteria are often delivered to the nodule interior by a network of tubes formed by the invagination of plant cell wall and plasma membrane. These tubes, called infection threads, are coope
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Abstract During the development of nitrogen-fixing root nodules, symbiotic bacteria are often delivered to the nodule interior by a network of tubes formed by the invagination of plant cell wall and plasma membrane. These tubes, called infection threads, are cooperatively constructed by both the plant host and its symbiotic bacteria. This chapter outlines how infection threads develop in root hairs and in root cortical cells, and how the three-dimensional architecture of infection thread networks in nodules change during the course of nodule development. Threedimensional reconstructions of infection thread networks inside M. truncatula nodules infected with Sinorhizobium meliloti show that the infection threads form relatively simple, treelike networks that exhibit changes in growth orientation as nodules mature. Questions concerning possible mechanisms that determine the direction of infection thread development in nodule tissue, and whether or not the mechanisms of infection thread construction in nodule tissue differ from those in root hairs, are discussed.
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Introduction
Sinorhizobium meliloti, similar to other rhizobial α-proteobacteria of the genera Rhizobium, Mesorhizobium, Azorhizobium, and Bradyrhizobium, is able to form nitrogen-fixing root nodules when it infects compatible host plants from the Leguminosae family (Batut et al. 2004; Brewin 2002; Gage 2004). Such symbioses are usually specific in that a particular bacterial species is able to infect a narrow range of host plants, though there are exceptions to this rule (Perret et al. 2000). All known interactions between rhizobial bacteria and their legume hosts require that the bacteria synthesize a lipooligosaccharide molecule called Nod factor. Nod factor is required for many of the changes seen in roots upon inoculation with compatible bacteria (see Limpens and Bisseling 2008). Another feature common to D.J. Gage Department of Molecular and Cell Biology, University of Connecticut, 91 N. Eagleville Road, U-3125, Storrs, CT 06269-3125, USA e-mail: [email protected]
Plant Cell Monogr, doi:10.1007/7089_2008_5 © Springer-Verlag Berlin Heidelberg 2008
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these symbioses is that bacteria must enter the root before they begin to fix nitrogen in conjunction with their host. There is substantial variation in how such entry can occur. In some cases, bacteria enter root systems through cracks between epidermal cells, or at sites where lateral roots are emerging from developed roots. Bacteria then induce underlying cortical cells to form infection structures, through which the bacteria gain access to nodule tissue and they eventually enter host cell cytoplasm (Chandler 1978, 1982; De Faria et al. 1988; Gonzalez-Sama et al. 2004; Lotocka et al. 2000; Ndoye et al. 1994; Rana and Krishnan 1995). This form of infection is often considered to be more primitive than other forms because the the host root hairs do not seem to undergo any particularly sophisticated cellular differentiation during the process. There are instances in which such a
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