Nodulation and Root Development in the Model Legume Lotus Japonicus

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The nitrogen fixing symbiosis between members of the legume family (Fabaceae) and diverse Gram- negative soil bacteria, commonly known as rhizobia, is an agriculturally important phenomenon. Moreover, the process of nodule formation and intracellular reduction of di-nitrogen by the bacteria represents a very interesting developmental process for basic studies on plants. Nodule ontogeny, as well as the number of nitrogen fixing nodules formed on legume plant roots in response to rhizobial infection is predominantly controlled by a plant genetic program. The underlying molecular mechanism is not well understood and forms the topic of investigation in this proposal. Using genetic approach, the PIs laboratory identified a gene (Har1), from the diploid model legume Lotus japonicus, that is required for normal interaction with symbiotic bacteria. In the presence of rhizobia, the mutant plants show an altered symbiotic phenotype, characterized by the formation of excessive numbers of nodules which cover almost the entire root length. The latter coincides with nearly total inhibition of root- and stem growth and rapid deterioration of mutant plant vitality. Interestingly, the same mutation was found also to significant modify the root developmental program of uninoculated plants. Thus, it is likely that the Har1 gene product functions in both root morphogenesis and the mechanism by which the host plant regulates symbiotic development. The experiments proposed will utilize molecular genetic approaches to (1) clone and characterize the L. japonicus Har1 gene, and (2) identify additional genes in a pathway(s) for root and symbiotic development by genetic suppressor analysis. It is hoped that the results of these analyses will result in a better understanding of the molecular basis of symbiotic plant-microbe interactions and may eventually contribute to the pool of basic scientific knowledge that will be required for the extension of symbiotic nitrogen fixation to presently non-nodulated agronomicall important plants, such as cereals.