|Miller, Susan - Sue|
|Samac, Deborah - Debby|
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2006
Publication Date: 5/1/2006
Citation: Liu, J., Miller, S.S., Graham, M., Bucciarelli, B., Catalano, C.M., Sherrier, D.J., Samac, D.A., Ivashuta, S., Fedorova, M., Matsumoto, P., Gantt, J.S., Vance, C.P. 2006. Recruitment of novel calcium-binding proteins for root nodule symbiosis in Medicago truncatula. Plant Physiology. 141:167-177. Interpretive Summary: Legume plants in symbiosis with the appropriate soil bacteria can convert atmospheric nitrogen gas to nitrogen fertilizer. This process, known as symbiotic nitrogen fixation, occurs in small wart-like structures on roots called nodules. For symbiotic nitrogen to occur the legume plant and the bacteria must be in communication by signaling to each other. Calcium is an important signal molecule in all biological systems. In studies reported here we investigated whether calcium-binding proteins were important in root nodule symbiosis. We demonstrated that the legume plant Medicago produces a group of unusual calcium-binding proteins which appear to surround the bacteria in symbiotic root nodules. In Medicago plants which are unable to make these calcium-binding proteins, symbiotic root nodules appear to be inefficient in nitrogen fixation. Upon analysis of Medicago DNA we found six genes that encode differing variants of the calcium-binding proteins, and the genes were located in close proximity to each other. These results are important because they are the first to document a new class of calcium-binding proteins that appear to be required for legume root nodule symbiosis. The findings show that the plant has developed proteins important in communicating with bacteria. Modification of the genes encoding these calcium-binding proteins may help improve symbiotic nitrogen fixation.
Technical Abstract: Legume-rhizobia symbiotic nitrogen (N2 ) fixation plays a critical role in sustainable N management in agriculture and in Earth's N cycle. Signaling between rhizobia and legumes initiates development of a unique plant organ, the root nodule, where the bacteria undergo endocytosis and become surrounded by a plant membrane to form a symbiosome (Sym). Between this membrane and the encased bacteria there exists a matrix-filled space (the symbiosome space, SymS) that is thought to contain a mixture of plant- and bacterially-derived proteins and about which little is known. Maintenance of the symbiosis state requires continuous communication between the plant and bacterial partners. Here we show in the model legume Medicago truncatula (Mt) that a novel family of six calmodulin-like proteins (CaMLs) are expressed specifically in root nodules, are localized within the SymS, and are required for effective symbiosis. All six nodule-specific CaML genes are clustered in the Mt genome along with two other nodule-specific genes, nodulin-22 and nodulin-25. Sequence comparisons and phylogenetic analysis suggests that an unequal recombination event occurred between nodulin-25 and a nearby calmodulin (CaM) 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 ubiquitious Ca2+-binding gene for symbiotic purposes.