| Legumes have the unique ability to live in symbiosis with nitrogen fixing Rhizobium bacteria. This symbiosis leads to the formation of root nodules. These nodules are organs specifically formed to host the bacteria, allowing them to differentiate into a symbiotic form that can fix atmospheric nitrogen, which is used by the plant. Nodule formation requires developmental reprogramming of root cells as well as infection by the bacterium. Both processes are triggered by specific lipochito-oligosaccharides -named Nod factors- that are secreted by Rhizobium when colonizing the root. Nod factors induce many symbiotic responses that are essential for root nodule formation and in some species even the formation of complete nodules can be triggered (lacking bacteria). This underlines the importance of Nod factor signalling in root nodule development. Since long it has been questioned why legumes are so special that they can have a nitrogen fixing Rhizobium symbiosis? Since Nod factor perception and signalling is in the heart of this symbiosis, it can be argued that the underlying signalling pathway could be unique for legumes. The recent cloning of 7 key-components of the legume Nod factor signalling network, as well as the availability of comprehensive genome sequence information of different plant species provides now the opportunity to study in non-legumes the functioning of a symbiotic signalling network that sets in motion Rhizobium root nodule formation. Here we aim to do so by a modular approach by determining whether a Nod factor signalling network activating a reporter gene can be created in non-legumes. The proposed studies will also reveal to what extend morphological responses associated with nodule formation can be triggered on non-legumes and will provide first insights in how legumes evolved their unique ability to establish this symbiosis. |
