1. Department of Disease and Stress Biology, John Innes Centre, Norwich NR4 7UH, UK
2. Center for Integrated Biotechnology and Department of Horticulture, Washington State University, Pullman, Washington 99164-6414, USA

Correspondence to: Giles E. D. Oldroyd¹ Correspondence and requests for materials should be addressed to G.E.D.O. (Email: giles.oldroyd@bbsrc.ac.uk).

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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