The role(s) played by plant hormones in the regulation of symbiotic structures

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I study the mutualistic interaction which exists between certain beneficial bacteria, the rhizobia, and legume plants.  As the result of this interaction, the rhizobia penetrate the plant root hairs and trigger the formation of a novel structure known as the nodule.  If the symbiosis is successful, the rhizobia, now hosted within the nodule, fix atmospheric nitrogen. When fixed in excess, the nitrogen becomes available to the plant. The early infection stages are thought to require two opposing chemical gradients across the cortex of the root; the successful integration of these gradients by the partners results in the initiation of the nodule. The first gradient is rhizobial in nature; complex molecules known as Nod factors are thought to be directed towards the root centre. This gradient triggers the first cell divisions of the inner cortex and the first morphological changes of the root hairs. The other gradient oriented towards the root surface is thought to involve plant hormones, specifically auxin and cytokinin (CK). In this application, I propose that, in response to the rhizobial infection, CKs coming from the vascular tissues located in the centre of the root move across several cell layers and accumulate into the inner cortical cells where they promote cell divisions. I postulate that the CKs pooled in these cells give positional information to the rhizobia, i.e., they act as a beacon, "attracting" the rhizobia towards the cells cluster.  To demonstrate that CKs are important signals for nodule initiation, I will (1.) create plant chimeras with altered CK levels; (2.) inoculate the chimeras and track the rhizobial infection at the cellular level; and (3.) follow the expression of molecular markers specific for certain steps of the infection. The methods used here will likely boost the use of pea as a model plant system as this plant is now amenable to molecular techniques. In the far future, results obtained in my research program could lead to the development of methods that could override mechanisms which prevent the proper establishment of the rhizobial symbiosis.  Agronomically, this development would be extremely worthwhile since successful infections of plants by rhizobia are a key step to reducing the use of chemical fertilizers on crop plants.