Symbiotic Nitrogen Fixation

Abstract

Nitrogen for all living organisms ultimately comes from atmospheric dinitrogen (N 2 ). Biological nitrogen fixation is a reaction whereby N 2 is reduced to ammonium (NH 3 ) by a special enzymatic system, of which the primary enzyme is the nitrogenase complex. The nitrogenase system is found in various prokaryotes (diazotrophic bacteria or diazotrophs). They can be grouped into two categories: one is the group of free-living nitrogen fixers which inhabit the soil, fresh water and seawater, and the other is a group of bacteria that exhibits a highly efficient nitrogen fixation system in endosymbiotic association with plants. Symbiotic nitrogen fixation in legumes is the prominent representative of the second group. Legume plants form root nodules by symbiosis with a group of soil bacteria collectively termed Rhizobium, in which endosymbiotic rhizobia are capable of fixing N 2 . Nitrogen fixation by the legume-Rhizobium symbiosis accounts for more than 50% of biological nitrogen fixation and has a critical importance in agriculture. Some kinds of symbiotic nitrogen fixers are able to fix nitrogen in their free-living state, and such free-living nitrogen fixing bacteria can reside (not intracellularly) inside the tissues of higher plants. For example, some kinds of endophytic bacteria, which inhabit the intercellular space and/or the vascular cylinders of plants, fix N 2 , and are thought to contribute nitrogen nutrition to these plants. In addition, various free-living nitrogen-fixing bacteria have been identified from the rhizosphere of many plants, and make a loose symbiotic association with those plants. Representatives of nitrogen-fixing microorganisms are shown in Table 3.1.Plant Metabolism and Biotechnology, First Edition. Edited by Hiroshi Ashihara, Alan Crozier, and Atsushi Komamine.

Are we there yet? The long walk towards the development of efficient symbiotic associations between nitrogen-fixing bacteria and non-leguminous crops

“…Root nodule symbioses are only found in plants of a monophyletic clade often referred to as “FaFaCuRo” ( Fabales , Fagales , Curcubitales , and Rosales ) but are incredibly diverse in modes of infection by rhizobia or Frankia , nodule anatomy, and metabolism [32–35]. Associations between legumes and rhizobia are so efficient that legumes are found in a wide range of environments across the globe and used in nearly all cropping systems [36].…”

Loss‐of‐function of ASPARTIC PEPTIDASE NODULE‐INDUCED 1 (APN1) in Lotus japonicus restricts efficient nitrogen‐fixing symbiosis with specific Mesorhizobium loti strains

“…In symbiosomes, they further differentiate into a symbiosis‐specific form, the bacteroid, and then develop N 2 ‐fixing ability. Housing within the nodule cells and differentiation into bacteroids are essential for rhizobial N 2 fixation; therefore, bacteroid differentiation and N 2 ‐fixing activity are under strict control through interactions between host plant cells and endosymbionts (reviewed in Kouchi, ). The host‐determined Fix – mutants, which are able to form morphologically normal nodules with endosymbiotic bacteria but are defective in nitrogen fixation, are the most useful tool with which to identify the legume genes involved in such interactions.…”

The Effect of Various Forms of Sulfur on Soil Organic Matter Fractions and Microorganisms in a Pot Experiment with Perennial Ryegrass (Lolium perenne L.)

“…Furthermore, it was observed that among the sulfate fertilizers tested, the applied ammonium sulfate significantly increased the number of copies of the nif H gene in soil compared to soils fertilized with potassium sulfate or magnesium sulfate only ( Table 3 ). This observation is difficult to explain because sulfur is an essential component of nitrogenase, and this enzyme’s biosynthesis is based on high levels of imported sulfate [ 88 , 89 ]. The aforementioned changes in the content of water-soluble organic carbon fractions, which were not related to changes in the number of molecular markers of bacteria, suggest that there were no significant changes in the number of bacteria, but the microbial utilization of labile carbon nutrients in soil fertilized especially with sulfates was more intensive.…”