Submitted to: Australian Journal of Agricultural Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 10, 2000
Publication Date: N/A
Interpretive Summary: Alfalfa in symbiosis with soil rhizobia bacteria can convert atmospheric nitrogen gas to nitrogen fertilizer. This process, known as symbiotic nitrogen fixation, occurs in small wart-like structures on roots called nodules. Improvement of legume nitrogen fixation is an important goal for agricultural research because it: 1) improves agricultural sustainability; 2) reduces the need for expensive nitrogen fertilizer thus improving farme profit margin; and 3) increases soil health. Several strategies have been used to enhance symbiotic nitrogen fixation in legumes. Research reported in this article describes how biochemical and genetic approaches have been used in alfalfa to improve nitrogen fixation. Selection efforts have been successful in modifying root nodule activity for assimilation of symbiotically fixed nitrogen. In addition, the genes controlling root nodule nitrogen and carbon assimilation have been isolated and characterized. New genotypes of alfalfa have been produced that have strikingly altered nitrogen fixation and root morphology. Lastly, molecular studies have revealed the rate-limiting steps in plant control of nitrogen assimilation. These findings are important because they show that a multidisciplinary approach is required to make progress in selection for traits to improve nitrogen fixation. The results also show that numerous plant and bacterial genes are necessary for good nitrogen fixation. The data are important because they identify probable targets for improving nitrogen fixation and give insight as to what approaches have or have not been successful. The results can be used by plant breeders, geneticists, and physiologists to design rational strategies for plant breeding programs aimed at improving nitrogen fixation.
Technical Abstract: Improvement of symbiotic nitrogen fixation involves selection for a very complex trait encompassing a large number of genes from both the host plant and microsymbiont. Selection of a single biochemical or morphological trait is unlikely to lead to improvement of nitrogen fixation. Selection for higher root nodule enzyme activity can be successful, but to achieve improved plant performance these enzyme traits will need to be stacked. Selection for biochemical traits laid the foundation for isolating and elucidating the role of several plant genes involved in root nodule nitrogen and carbon assimilation. Access to these genes offers new tools that may prove useful in altering plant nitrogen nutrition. The development of legume plant genotypes incapable of nitrogen fixation (ineffective) are requisite for biochemical and genetic studies of factors controlling nitrogen fixation. Ineffective plants have proven useful for both applied and fundamental studies. An important goal is to develop a wide range of ineffective or non-nodulating legume genotypes. Remaining practical questions regarding biochemical regulation of legume plant-rhizobial nitrogen fixation include: 1) can overexpression of specific plant genes enhance nitrogen nutrition; 2) can model legume systems such as Medicago truncatula and Lotus japonicus be utilized to improve nitrogen fixation in their agriculturally important relatives; 3) will selection for early nitrogen fixation and delayed nodule senescence contribute significantly more biologically fixed nitrogen to legume growth; and 4) is the development of legumes that store more fixed nitrogen in roots a viable approach for enhancing the use of symbiotically fixed nitrogen in agriculture?