Résumé:
More than 80% of plant species are able to develop arbuscular mycorrhizal (AM) symbiosis in association with glomeromycete fungi. In contrast, only some species of the Eurosid I clade, confined to four orders and ten Angiosperm families, are able to form nitrogen-fixing root nodule symbioses with soil bacteria. This concerns plants of the Legume family (Fabaceae) and Parasponia associated with Rhizobium bacteria and actinorhizal plants associated with the actinomycete Frankia. Similarly to Legumes, the actinorhizal symbiosis results in the formation of nitrogen-fixing root nodules. However, unlike Legume nodule, the actinorhizal nodule has a same origin and structure than a lateral root. Thus, the study of actinorhizal nodules is of particular interest not only for investigating its specific properties but also, for determining common characteristics shared with Legume nodules.
We have studied the role of CgCCaMK gene during the symbiotic process and nodule organogenesis in the actinorhizal tree Casuarina glauca. CCaMK encodes a calcium and calmodulin dependent protein kinase. In the signalisation cascade leading to both nodulation and mycorrhization in Legumes, this gene is acting downstream the calcium oscillations (« calcium spiking ») that occur during the early steps of the symbiotic interaction. It has been suggested that these calcium oscillations are decoded and transduced by the CCaMK protein.
We have monitored the spatio-temporal expression of a PromCgCCaMK::GUS fusion during actinorhizal nodulation and have shown that reporter gene expression was correlated with the presence of Frankia along the symbiotic process. This data highlights the role of CgCCaMK during Frankia infection. In addition, we have investigated the role of the CCaMK autoinhibitory/CAM domain in actinorhizal nodule organogenesis. To achieve this goal, we have obtained truncated versions of CgCCaMK lacking the autoinhibitory/CAM domain, and then expressed them into C. glauca. We have also used truncated forms of MtCCaMK from Medicago truncatula. The expression of these CCaMK constructs from C. glauca and M. truncatula was found to induce spontaneous nodulation in the absence of Frankia bacteria. These results suggest that deregulation of the calcium and calmodulin dependent protein kinase is able to reactivate the symbiotic signalling pathway and genes acting downstream CCaMK that are needed for nodule
organogenesis.
