Nitrogen-Fixing Plant-Microbe Symbioses

Abstract

The symbiotic conversion of atmospheric nitrogen into nitrogenous compounds assimilated by higher organisms is an essential part of the global N cycle and a supporting pillar of agricultural practices. The nitrogen fixing symbiosis is not confined only to leguminous plants but also found some other plant families. The symbiotic relationship between plants and rhizobia is initiated by the release of flavonoids into the rhizosphere by the plant root. The release of flavonoids are sensed by the rhizobial transcriptional regulator NodD, which leads to symbioses-specific responses such as the release of Nod factors. The enzymes involved in the synthesis of basic Nod factor structure are encoded by the nodABC genes, conserved in almost all types of rhizobia. The perception of Nod factor by the plant is mediated by a receptor-like kinase, which induces intracellular calcium oscillations and leads to the deformation of root hairs through the restructuring of the cytoskeleton, leading to the formation of an infection thread. Although rhizobia are capable of synthesizing their own amino acids in the free living stage, within the infection thread rhizobia are dependent on the plant host for amino acids and other compounds.

Rhizobia were initially classified on the basis of their morphological, physiological characteristics and dominantly on host plant that they nodulate but after the invention of molecular techniques their molecular characteristics were taken into consideration. Thus, rhizobial taxonomy was repeatedly revised and refined. Currently, about 145 species of rhizobia have described from the genera Azorhizobium, Allorhizobium, Agrobacterium, Bradyrhizobium, Ensifer and Rhizobium and the taxonomic status of some genera and species has been revised. Root nodulating beta-rhizobia from different legumes have only been described recently, but the molecular evidence showed that they are existed as legume symbionts for 50 million years. Different species of beta-rhizobia contain common nodulation genes like nodABC, nodD, nifH, and these genes are very similar to the symbiotic genes of traditional rhizobia.

Phylogenetic analyses are the basis to understanding the evolutionary history of individual genes or entire genomes of microorganisms. Initially, the bacterial phylogenies were reconstructed on the basis of morphological and physiological characteristics of the cell. Later, sequence analysis and chemical content analysis were introduced to further improve phylogenies. Sequencing of 16S rRNA genes not only contribute significantly to the elimination of plasmid-borne characteristics from rhizobial taxonomy, but also helped in the identification of beta and gamma rhizobia. Nowadays, the sequencing of housekeeping genes, DNA profiling and the application of DNA arrays have become standard methods in bacterial taxonomy.

The legume-rhizobia association contributes significantly to the symbiotic biological nitrogen-fixing process, but other microbes such as cyanobacteria, endophytic bacteria, or Frankia sp. also form nitrogen-fixing associations of various degrees of intimacy with plants. Of course, the main benefit of legume cultivation comes from their ability to fix nitrogen in symbiosis with rhizobia but others are also beneficial for plants growth and survival in different environments. Aside from its biological benefit, biological nitrogen fixation is also critical for agriculture due to its impact on farming cost, sustainable land use, soil quality, and mitigation of greenhouse gas emissions. The present review covers the mechanism of legume-rhizobium symbiosis, rhizobial taxonomy, and non-rhizobial symbiotic nitrogen fixation processes by cyanobacteria, endophytic diazotrophs and Frankia sp.