Abstract
Legumes, such as Medicago truncatula, form mutualistic symbiotic relationships with nitrogen-fixing rhizobial bacteria. This occurs within specialized root organs—nodules—that provide the conditions required for nitrogen fixation. A rhizobium-derived signalling molecule, Nod factor, is required to establish the symbiosis. Perception of Nod factor in the plant leads to the induction of Ca2+ oscillations1, and the transduction of this Ca2+ signal requires DMI3 (refs 2, 3), which encodes the protein kinase Ca2+/calmodulin-dependent protein kinase (CCaMK). Central to the regulation of CCaMK is an autoinhibitory domain that negatively regulates kinase activity. Here we show that the specific removal of the autoinhibition domain leads to the autoactivation of the nodulation signalling pathway in the plant, with the resultant induction of nodules and nodulation gene expression in the absence of bacterial elicitation. This autoactivation requires nodulation-specific transcriptional regulators in the GRAS family. This work demonstrates that the release of autoinhibition from CCaMK after calmodulin binding is a central switch that is sufficient to activate nodule morphogenesis. The fact that a single regulation event is sufficient to induce nodulation highlights the possibility of transferring this process to non-legumes.
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Acknowledgements
We thank D. Barker for providing the ENOD11-GUS lines; A. Downie for his critical review of the manuscript and for discussions; H. Miwa for assistance with the confocal microscope; K. Findlay and S. Bunnewell for assistance with the histological studies; and S. Long for efforts in facilitating this collaborative work. This study was supported by the BBSRC as a David Philips Fellowship to G.O. and a grant, by the Royal Society as a Wolfson Merit award to G.O., by Washington State University Agricultural Research Center and by the US National Science Foundation.Author Contributions A.M. defined conditions and generated constructs for protein expression; S.C. and T.Y. did the in-vitro biochemistry; C.G. completed all the rest of the experimental work and wrote the paper with G.O.
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Supplementary information
Supplementary Methods
A complete list of the primers used to generate the DMI3 deletion constructs and a more detailed description of protocols. (DOC 29 kb)
Supplementary Table 1
ENOD11-GUS induction in dmi1, dmi2, nsp1, and nsp2 transformed with the DMI3 deletion constructs. (DOC 26 kb)
Supplementary Figure 1
Localisation of DMI3 and DMI3 deletion constructs fused to GFP (pdf 2.8 MB). (PDF 555 kb)
Supplementary Figure 2
Activation of ENOD11-GUS by DMI3-T271A and DMI3-T271D in dmi3-1 and DMI3 1-326 in dmi1 and nsp1 lines (pdf 556 KB). (PDF 2816 kb)
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Gleason, C., Chaudhuri, S., Yang, T. et al. Nodulation independent of rhizobia induced by a calcium-activated kinase lacking autoinhibition. Nature 441, 1149–1152 (2006). https://doi.org/10.1038/nature04812
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DOI: https://doi.org/10.1038/nature04812
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