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Genetic dissection of Nod factor signalling in Medicago truncatula

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Genetic dissection of Nod factor signalling in Medicago truncatula
Besma Ben Amor1, Françoise de Billy1, Fabienne Maillet1, Varma Penmetsa2, Giles Oldroyd3, Sydney Shaw3, Charles Rosenberg1, Sharon Long3, Doug Cook2, Clare Gough1
1LBMRPM, CNRS-INRA, France. 2University of California, USA. 3Stanford University, USA.

Using plant (Medicago truncatula) and bacterial (Sinorhizobium meliloti) genetics, we are studying the mechanisms controlling Nod factor perception and signalling leading to nodulation. Characterisation of M. truncatula Nod- mutants, led to the identification of five genes (dmi1, dmi2, dmi3, NSP1 and nsp2), mutants in which are pleiotropically affected in Nod factor responses, indicating that these genes are required for a Nod factor-activated signal transduction pathway leading to symbiotic responses such as root hair deformation, nodulin gene expression and cortical cell division (1, 2). Characterisation of a new M. truncatula mutant led to the identification of a sixth gene, called NFP, that also controls Nod factor signal transduction. The nfp mutant is defective in all Nod factor responses tested, and, unlike dmi mutants, the nfp mutant shows a wild-type mycorrhization phenotype. These results suggest that NFP either encodes a Nod factor receptor or controls an early step of signal transduction specific to nodulation (3). Analysis of other M. truncatula Nod- mutants enabled us to identify the HCL gene, involved in signalling during rhizobial infection. The HCL gene controls Rhizobium-induced root hair curling and hcl mutants are defective in the formation of signalling centres that normally provide positional information for reorganisation of the microtubular cytoskeleton in root hairs and outer cortical cells in response to Rhizobium (4). By screening populations of M. truncatula and by mutant analysis, genes have been identified that appear to control infection thread formation and studies are underway to determine whether they also play a role in Nod factor recognition.

1 Catoira et al. Plant Cell (2000) 12, 1647.

2 Wais et al. Proc. Natl. Acad. Sci. USA. (2000) 97, 13407.

3 Benamor et al. Plant J. (2003) 34, 495.

4 Catoira et al. Development. (2001) 128, 1507.

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