In natural ecosystems, many plants are able to grow on nutrient-poor soils by living together for mutual benefit with microorganisms, an example of a process called symbiosis. The symbiosis between arbuscular mycorrhizal (AM) fungi and >80% of land plants and that between rhizobia and legumes represent the two most important symbiotic interactions between plant roots and microbes. In the AM symbiosis, the AM fungi assist the plant in assimilating phosphate, whereas in the legume-rhizobia symbiosis, the rhizobia fix atmospheric nitrogen for use by the plant. The finding that the two symbioses share overlapping signaling pathways in legumes suggests that the evolutionary younger nitrogen-fixing symbiosis has acquired functions from the ancient AM symbiosis. Most recently, a number of legume genes required for nodulation and AM symbiosis were cloned from several legumes. Interestingly, nearly all these genes are highly conserved across multiple plant taxa, and their putative orthologs can be defined in non-legumes such as rice. This research seeks to elucidate the function of non-legume orthologs of legume genes required for nodulation and AM symbiosis. Specific objectives are: (1) Characterization of the function of the putative rice orthologs of legume genes required for nodulation and AM symbiosis using reverse genetics tools; (2) Complementation of the legume mutants that are defective in nodulation and/or AM symbiosis using the putative rice orthologs; and (3) Comparative transcript profiling of wild-type and mutant rice lines under the infection of AM fungi using the Rice Affymetrix GeneChip. This research will shed light on the genetic and molecular mechanisms underlying evolution of root symbioses in plants. Broader impacts: The students and postdocs working on this project will be trained in the areas of plant genomics, molecular biology of plant-microbe interactions, evolution, and bioinformatics. Outreach to high school students and teachers will be carried out through a program at the University of Kentucky.
