The symbiotic relation between plants and rhizobial bacteria begins with the development of root nodules, sites of nutrient exchange where bacteria reside and provide nitrogen fixation. Initiation of nodule formation requires bidirectional communication between host and guest, and it is thought that the earliest steps involve the bacterial production of a lipo-chitooligosaccharide called Nod factor in response to flavonoid molecules secreted by the plant. This exchange of signals activates development of nodule primordia. Mitra et al. investigated why a mutant strain of Medicago truncatula called dmi3 exhibited a limited response to Nod factor, in which calcium oscillations in root hair cells in response to Nod factor were observed, yet induction of specific genes and nodule development were blocked. The authors determined, by microarray analysis, that the expression level of a particular transcript was reduced in the dim3 mutant compared with wild-type plants. The corresponding gene encodes a protein with high sequence similarity to mammalian calcium/calmodulin-dependent protein kinase II (CaMKII) and contains calcium-binding domains, a serine-threonine kinase domain, and a CaM-binding domain. Such plant calcium/calmodulin-dependent protein kinases (CCaMKs) are poised to respond to both free calcium and calcium that is bound to calmodulin. Although CCaMKs have been identified in a number of plant species, their functions have not yet been assigned. The mutated gene in the dim3 strain had a premature termination site, resulting in a nonfunctional kinase. Because mammalian CaMKII activity is modulated by calcium oscillations, it is likely that the DIM3 CCaMK transduces the Nod factor-induced calcium spike signal to downstream effectors that activate the nodule developmental program.
R. M. Mitra, C. A. Gleason, A. Edwards, J. Hadfield, J. A. Downie, G. E. D. Oldroyd, S. R. Long, A Ca2+/calmodulin-dependent protein kinase required for symbiotic nodule development: Gene identification by transcript-based cloning. Proc. Natl. Acad. Sci. U.S.A. 101, 4701-4705 (2004). [Abstract] [Full Text]