CHARACTERIZING NOVEL SYMBIOSIS MUTATIONS BETWEEN BARREL MEDIC AND SINORHIZOBIUM MELILOTI
Hector Trujillo, Kathrin Wippel, Sharon Long.
Stanford University, Stanford, CA.
Plants require nitrogen for growth and survival. Although nitrogen is freely available in the atmosphere as N2, plants can only use nitrogen as NH3. Nitrogen fixation is the conversion of N2 to NH3, which is catalyzed by soil bacteria known as rhizobia. Certain plants, known as legumes, form an important symbiotic relationship with these rhizobia via the formation of root nodules, where nitrogen fixation takes place. Nodule formation depends on complex chemical interactions between the plant and bacteria that are not fully understood. To investigate the underlying mechanisms of nodule formation, we performed a forward genetics screen where we tried to link Fix- phenotypes (nodules that don’t fix nitrogen) to the underlying genotypes. To isolate Fix- nodules, Medicago truncatula seeds were randomly mutated with high-energy neutron bombardment. Viable seeds were grown and inoculated with their symbiont, Sinorhizobium meliloti, which carried 3 reporter constructs. These reporter constructs each tracked distinct stages of nodule formation and were used to chronologically assess when in the nodulation scheme nitrogen fixation was interrupted. The plants, however, were grown under media that stressed them to the point where the roots died in the process. Thus, we optimized the growth conditions to obtain the most meaningful data. For the future, finding and characterizing these novel Fix- phenotypes will lead to further mechanistic understanding of the general nodulation process.