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PLANTS INTERACTING WITH OTHER ORGANISMS

Recruitment of Novel Calcium-Binding Proteins for Root Nodule Symbiosis in Medicago truncatula^1,[W],[OA]

Department of Agronomy and Plant Genetics (J.L., M.F., P.M.) and Department of Plant Biology (M.G., S.I., J.S.G.), University of Minnesota, St. Paul, Minnesota 55108; United States Department of Agriculture, Agricultural Research Service, St. Paul, Minnesota 55108 (S.S.M., B.B., D.A.S., C.P.V.); and Department of Plant and Soil Sciences, Delaware Biotechnology Institute, University of Delaware, Newark, Delaware 19711 (C.M.C., D.J.S.)

Legume rhizobia symbiotic nitrogen (N₂) fixation plays a critical role in sustainable nitrogen management in agriculture and in the Earth's nitrogen cycle. Signaling between rhizobia and legumes initiates development of a unique plant organ, the root nodule, where bacteria undergo endocytosis and become surrounded by a plant membrane to form a symbiosome. Between this membrane and the encased bacteria exists a matrix-filled space (the symbiosome space) that is thought to contain a mixture of plant- and bacteria-derived proteins. Maintenance of the symbiosis state requires continuous communication between the plant and bacterial partners. Here, we show in the model legume Medicago truncatula that a novel family of six calmodulin-like proteins (CaMLs), expressed specifically in root nodules, are localized within the symbiosome space. All six nodule-specific CaML genes are clustered in the M. truncatula genome, along with two other nodule-specific genes, nodulin-22 and nodulin-25. Sequence comparisons and phylogenetic analysis suggest that an unequal recombination event occurred between nodulin-25 and a nearby calmodulin, which gave rise to the first CaML, and the gene family evolved by tandem duplication and divergence. The data provide striking evidence for the recruitment of a ubiquitous Ca²⁺-binding gene for symbiotic purposes.

² These authors contributed equally to the paper.

³ Present address: USDA-ARS CICGR Unit, Ames, IA 50011.

⁴ Present address: DuPont Agriculture and Nutrition, Johnston, IA 50131.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Carroll P. Vance (vance004{at}umn.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.076711.

^* Corresponding author; e-mail vance004{at}umn.edu; fax 651–649–5058.

Received January 12, 2006; returned for revision January 12, 2006; accepted February 16, 2006.

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