KSEF R&D Excellence: Towards Understanding the Function of Rice Orthologs of Legume Genes Required for Both Nodulation and Arbuscular Mycorrhizal Symbioses

The association between arbuscular mycorrhizal (AM) fungi and more than 80(~;;) of land plants and that between rhizobia and legumes represent the two most important symbiotic interactions between the plant root and microbes. In the AM symbiosis, the AM fungi assist the plant in assimilating phosphate and other mineral nutrients from the rhizosphere beyond the depletion zone formed by the root, whereas in the legume-rhizobia interaction, the symbiosis culminates in the formation of a novel plant organ, the root nodule, in which the bacteria fix atmospheric nitrogen. The two symbiotic associations are of critical importance in sustainable agriculture, since growth of land plants is often limited by the amounts of nitrogen and phosphate in the soil. Therefore, an in-depth understanding of the genetic and molecular mechanisms that underlie both symbioses will help to improve the efficiency of plants for acquisition of phosphorus and nitrogen from natural ecosystems. The finding that the two symbioses share overlapped signaling pathways in legumes suggests that the evolutionary younger nitrogen-fixing root nodule symbiosis has recruited functions from the ancient AM symbiosis. Most recently, a few of the legume genes required for both nodulation and AM symbioses were cloned from several legumes. Interestingly, some of these genes are highly conserved across multiple plant taxa, and their putative orthologs can be defined in non-legumes such as rice and Arabidopsis. The research proposed herein seeks to elucidate the function of non-legume orthologs of legume genes required for both nodulation and AM symbioses. Specific objectives are: (I) Development of RNAi silencing lines for rice orthologs of legume genes required for both nodulation and AM symbioses; (2) Molecular and phenotypic characterization of RNAi lines for AM symbioses; and (3) Complementation of the M truncatufa mutants that are defective in both nodulation and AM symbioses using the rice orthologs.