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Formation of organelle-like N2- fixing symbiosomes in legume root nodules is controlled by DMI2. Proc Natl Acad Sci USA

Laboratory of Molecular Biology, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands.
Proceedings of the National Academy of Sciences (Impact Factor: 9.81). 08/2005; 102(29):10375-80. DOI: 10.1073/pnas.0504284102
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ABSTRACT In most legume nodules, the N2-fixing rhizobia are present as organelle-like structures inside their host cells. These structures, named symbiosomes, contain one or a few rhizobia surrounded by a plant membrane. Symbiosome formation requires the release of bacteria from cell-wall-bound infection threads. In primitive legumes, rhizobia are hosted in intracellular infection threads that, in contrast to symbiosomes, are bound by a cell wall. The formation of symbiosomes is presumed to represent a major step in the evolution of legume-nodule symbiosis, because symbiosomes facilitate the exchange of metabolites between the two symbionts. Here, we show that the genes, which are essential for initiating nodule formation, are also actively transcribed in mature Medicago truncatula nodules in the region where symbiosome formation occurs. At least one of these genes, encoding the receptor kinase DOES NOT MAKE INFECTIONS 2 (DMI2) is essential for symbiosome formation. The protein locates to the host cell plasma membrane and to the membrane surrounding the infection threads. A partial reduction of DMI2 expression causes a phenotype that resembles the infection structures found in primitive legume nodules, because infected cells are occupied by large intracellular infection threads instead of by organelle-like symbiosomes.

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    • "qPCR was then carried out on Light Cycler 480 using the LightCycler 480 SYBER Green I (Roche) device as previously described (Gonzalez- Rizzo et al., 2006). Samples were normalized using the constitutive MtACTIN2 as a reference gene (Limpens et al., 2005). Progeny of mutant lines altered in DNF1 was genotyped using primers listed in Supplementary Table S1 following the method described in Ratet et al. (2010). "
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    ABSTRACT: Medicago truncatula belongs to the legume family and forms symbiotic associations with nitrogen fixing bacteria, the rhizobia. During these interactions, the plants develop root nodules in which bacteria invade the plant cells and fix nitrogen for the benefit of the plant. Despite massive infection, legume nodules do not develop visible defence reactions, suggesting a special immune status of these organs. Some factors influencing rhizobium maintenance within the plant cells have been previously identified, such as the M. truncatula NCR peptides whose toxic effects are reduced by the bacterial protein BacA. In addition, DNF2, SymCRK, and RSD are M. truncatula genes required to avoid rhizobial death within the symbiotic cells. DNF2 and SymCRK are essential to prevent defence-like reactions in nodules after bacteria internalization into the symbiotic cells. Herein, we used a combination of genetics, histology and molecular biology approaches to investigate the relationship between the factors preventing bacterial death in the nodule cells. We show that the RSD gene is also required to repress plant defences in nodules. Upon inoculation with the bacA mutant, defence responses are observed only in the dnf2 mutant and not in the symCRK and rsd mutants. In addition, our data suggest that lack of nitrogen fixation by the bacterial partner triggers bacterial death in nodule cells after bacteroid differentiation. Together our data indicate that, after internalization, at least four independent mechanisms prevent bacterial death in the plant cell. These mechanisms involve successively: DNF2, BacA, SymCRK/RSD and bacterial ability to fix nitrogen. © The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology.
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    • "During early stages of symbiotic interaction , Nod factor re - ceptors activate a conserved common symbiotic signaling cas - cade ( Oldroyd , 2013 ) . Components of this pathway also play a role in the release of rhizobia from infection threads ( Limpens et al . , 2005 ; Ovchinnikova et al . , 2011 ) . Our results support the notion that the Nod factor receptor complex triggers symbiotic interface formation via a signaling cascade that , at least in part , is similar to the common symbiotic signaling pathway . The role of Nod factor signaling in the formation of the symbiotic in - terface could provid"
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    ABSTRACT: Rhizobial Nod factors are the key signaling molecules in the legume-rhizobium nodule symbiosis. In this study, the role of the Nod factor receptors NOD FACTOR PERCEPTION (NFP) and LYSIN MOTIF RECEPTOR-LIKE KINASE3 (LYK3) in establishing the symbiotic interface in root nodules was investigated. It was found that inside Medicago truncatula nodules, NFP and LYK3 localize at the cell periphery in a narrow zone of about two cell layers at the nodule apex. This restricted accumulation is narrower than the region of promoter activity/mRNA accumulation and might serve to prevent the induction of defense-like responses and/or to restrict the rhizobium release to precise cell layers. The distal cell layer where the receptors accumulate at the cell periphery is part of the meristem, and the proximal layer is part of the infection zone. In these layers, the receptors can most likely perceive the bacterial Nod factors to regulate the formation of symbiotic interface. Furthermore, our Förster resonance energy transfer-fluorescence lifetime imaging microscopy analysis indicates that NFP and LYK3 form heteromeric complexes at the cell periphery in M. truncatula nodules.
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    • "Obviously, the joint system of nitrogen-fixing nodules development, based on coordinated infection and nodule organogenesis programs, is an evolutionary achievement of most advanced legumes. Another achievement of advanced RLS is symbiosome, which is highly differentiated and autonomic structure in contrast to the other beneficial plant-microbial symbioses (Brewin 2004, Limpens et al. 2005), and is literally a tiny factory of nitrogen fixation for the host. "
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