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Most legume plants can form nodules, specialized lateral organs that form on roots, and house nitrogen-fixing bacteria collectively called rhizobia. The uptake of the phytohormone auxin into cells is known to be crucial for development of lateral roots. To test the role of auxin influx in nodulation we used the auxin influx inhibitors 1-naphthoxyacetic acid (1-NOA) and 2-NOA, which we found reduced nodulation of

Legumes form mutualistic associations with nitrogen-fixing bacteria and arbuscular mycorrhizal (AM) fungi which increases nutrient availability to the plant. Nodulation is generally restricted to legumes and has co-opted genes required for the AM symbiosis. As a result, both associations share common genes and have analogous infection strategies, including the early lipochitooligosaccharide (LCO) signals that plants perceive from the symbionts. In order to discover novel genes required for both these symbioses, I used both forward and reverse genetic approaches. A forward genetic screen in Medicago truncatula revealed an H+-ATPase mutant Mtha1 with a defective arbuscule phenotype. The reverse screen investigated several candidates previously identified as having enhanced expression in the nodule inception mutant during nodulation that were also expressed in AM-colonized roots. Four of these genes were chosen for further study. The first, a novel AM-specific gene, BiFunctional Protein (BFP) has a predicted a role in lipid modification. Phylogenetic analysis revealed that BFP was restricted to AM host plants including a member of the liverworts, Marchantia paleacea. BFP expression during AM interactions in M. truncatula was found to be dependent on the common symbiotic gene DOES NOT MAKE INFECTIONS 3. AM colonization assays using mutants for BFP in M. truncatula and Oryza sativa were inconclusive. Three CCAAT-Binding Factor transcription factors were also studied (CBF1, CBF2 and CBF3). Promoter-reporter studies revealed that these genes are also expressed during rhizobial infection of wild type plants. While the M. truncatula cbf1 mutant did not exhibit a consistent nodulation phenotype, mutant analysis for CBF3, showed it is required for normal infection during root hair colonisation of rhizobia in M. truncatula. Based on these results and a further analysis of expression data and published work, I propose a model in which CBF3 forms a complex with NF-YA1 and NF-YC2 during early responses to LCOs during nodulation.