Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: May 3, 2010
Publication Date: June 14, 2010
Citation: Vance, C.P. 2010. Legume genomics: promise versus reality [abstract]. XIII National Meeting of the Spanish Society of Nitrogen Fixation and II Portuguese-Spanish Congress on Nitrogen Fixation, June 15-18, 2010, Zaragoza Spain. p. 5-6. Available: http://digital.csic.es/bitstream/10261/25138/1/Book_Contributions_%20XIIISEFIN2010.pdf. Technical Abstract: Legume root nodules, the specialized organs in which symbiotic nitrogen fixation (SNF) occurs, are structurally and metabolically complex organs. Their development and function depends upon coordinated gene expression between the host plant and rhizobial partner. Depending upon the symbiosis, nodule growth can be either determinate or indeterminate giving rise to elongate, corralloid, or spherical nodules, respectively. Within nodule cells the bacterial partner is enclosed by a host plant derived membrane termed the symbiosome membrane. The symbiosome membrane separates the host plant cytosol from the bacterial partner but yet allows the selective transport of molecules between the two organisms. Root nodules also differ in principal nitrogen (N) assimilation and transport products. Legume species with indeterminate root nodules, such as Medicago truncatula and pea, generally assimilate and transport symbiotically fixed N as amides. By contrast, most legume species having determinate nodules, such as common bean and soybean, assimilate and transport N as ureides. Adding to the complexity is that most metabolic processes in nodules related to SNF take place at a reduced oxygen concentration of 10 microM which is needed to protect the rhizobial enzyme nitrogenase from inactivation. Basically, N-fixing symbiotic root nodules of legumes are model factories for carbon (C) and N metabolism housed in an oxygen-limited structure that is adapted for rapid transport of small molecules. Although nodules comprise less than 5% of legume total biomass, they can catalyze the reduction of more than 100 kg N per hectare each year. Globally fixing 45-60 Tg N2 each year is astounding because only a few kgs of rhizobial nitrogenase are involved in catalysis. Although root nodules comprise only a small proportion of plant weight, they consume between 13-28% of legume total photosynthate. It is, therefore, strikingly apparent that integration of metabolism, transport of nutrients, and protection of the rhizobial partner from oxygen create a labyrinth of biochemical and genetic interactions.