Functional genomics of symbiotic nitrogen fixation in legumes
Noble Research Institute, Ardmore, OK, USA.
Discovery of the first plant gene required for legume nodule development and symbiotic nitrogen fixation (SNF), LjNIN from Lotus japonicus, occured in 1999 (Schauser et al, 1999). Today, over 150 genes in multiple legume species have been found to be required for nodule development and/or effective nitrogen fixation, via forward-genetics or by genomics-informed reverse-genetics (Roy, Liu, et al., unpublished). These genes have been implicated in signaling between rhizobia and legumes, infection and accommodation of the micro-symbiont in plant cells, nodule organogenesis, and plant metabolism in support of bacterial nitrogen fixation. Although some of these genes appear to be indispensable for SNF in several legumes species, not all are necessary in all species. This may reflect genetic and/or other functional redundancy within species related to either ancient genome duplications or more recent tandem duplications of genes. It also appears to reflect, to some extent, distinct co-evolution of legumes and specific rhizobia. Despite the large number of genes now known to be required for SNF in legumes, our knowledge of the molecular and cellular basis of nodule development and metabolism remains fragmentary. This talk will summarize what is known about the genetics of the various processes that lead to and support SNF, including our work on Medicago truncatula, and highlight some of the gaps in our knowledge in these areas. Finally, we address the question: how can SNF be improved in legumes with so many genes implicated? We are using natural variation in SNF in approximately 200 ecotypes of M. truncatula to identify genes that contribute to effectiveness in this species, via genome-wide association studies, with a view to developing plant breeding strategies to improve SNF in crop legumes. Examples of putative SNF effectiveness genes will be presented.