Functional-genetic studies of arbucular mycorrhizal fungi and rhizobial infection in legume roots

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Legumes have a special place in agriculture and ecology because they form a symbiotic association with soil bacteria called rhizobia that can fix atmospheric nitrogen which is provided to the plant. This symbiosis, called nodulation, involves infection of the legume roots by rhizobia and the colonization of specialized root structures called nodules where the nitrogen fixation takes place. Legumes and most other land plants also form a root symbiosis with arbuscular mycorrhizal fungi which provides phosphate to the host plant. It is believed that the legume-rhizobia symbiosis evolved from the more ancient mycorrhizal symbiosis, and several genes have been discovered, so called ‘common symbiotic’ genes, that are required for both symbioses. My research uses transcriptomic and functional genetic studies to better understand the infection process controlled by these common symbiotic genes. Rhizobia and mycorrhiza both infect legume roots through epidermal cells, and rhizobial specifically infect through root hair tips. To study the early stages of infection we have developed a method to isolate roots hairs infected by rhizobial using the model legume Medicago truncatula. We are using microrrays and deep sequencing (RNAseq) technologies to compare the gene expression profiles of root hairs isolated from rhizobially infected roots and roots infected by mycorrhiza to identify common elements. In addition, using the transcriptomic data generated from these studies we are carrying out gene network analysis which helps identify regulatory elements that control the infection process. Using these approaches we have discovered many new genes with potential roles in these symbioses. The genes identified are being studied using corresponding mutants identified by reverse-genetic screens. A complementary forward genetic approach has lead to the identification of several mutants with defects in the various stages of rhizobial and mycorrhizal infection.

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