Rhizobium and Other N ‐fixing Symbioses

Abstract

Nitrogen‐fixing symbioses between bacteria and plants are major nitrogen contributors to the terrestrial biosphere. The Rhizobium –legume interaction is the best known and is agronomically the most important one. Several alpha‐ and betaproteobacterial species (rhizobia) can infect legume roots via root hair or crack invasion, preceded by signal exchange in the rhizosphere and followed by signal transduction in the root, resulting in the construction of a nitrogen‐fixing root nodule. Actinorhizal nodule formation by members of the actinobacterial genus Frankia follows the same pattern but is less known in detail. The nitrogen‐fixing root symbioses have the same evolutionary origin as mycorrhizal symbioses. Cyanobacterial nitrogen‐fixing symbioses are more diversified and formed mainly with representatives of genus Nostoc , and occur in diverse plant groups, the best known being Azolla , Cycas and Gunnera . Whole‐genome sequencing of the microbial symbionts has revealed interesting features related to the genetics of signalling, genome architecture and biogeography. Key Concepts: Symbiotic nitrogen fixation in plants contributes significantly to terrestrial ecosystems and sustainable agriculture. Symbiotic nitrogen‐fixing interactions arose several times during evolution. The Rhizobium –legume symbiosis serves as a model and paradigm for other symbiotic interactions between bacteria and plants. Nitrogen‐fixing root symbioses have an origin common with mycorrhizal symbioses. Most rhizobia have well‐conserved nodulation genes which encode signals, nodulation factors, which are perceived by the plant. Cyanobacterial symbioses with plants are diversified, but involve mainly one microbial genus, Nostoc .

Isolation and Molecular Characterization of Elite Indigenous <i>Rhizobia</i> Nodulating <i>Phaseolus</i> bean (<i>Phaseolus vulgaris</i> L.)

“…Rhizobia are soil bacteria that are well known for their symbiotic relationship with legumes even though they are also found in soil devoid of legumes [23] [47]- [49]. Rhizobia are grouped into two major groups which are fast grower and slow growers [50].…”

“…Through symbiotic relationship between rhizobia and legume, rhizobia provide combined form of nitrogen to plant while plant provides shelter and energy to rhizobia [23] [47]- [49]. Some findings explained that, the drivers of rhizobia-legume interaction are dryness and infertility of the soils [51].…”

“…That means the rhizobia nodulate legume in order to get shelter from hostile environment and legume accepts rhizobia in order to get access to combined nitrogen hence be able to survive and colonize the area which are dry with low nitrogen (infertile areas). But the real origin of rhizobia-legume symbiosis is not known up to moment [23]. What is a known is that, symbiosis between rhizobia and legumes is genetically controlled.…”

“…What is a known is that, symbiosis between rhizobia and legumes is genetically controlled. Rhizobia have a nodulation gene (nodABC) which encode enzyme responsible for core structure of the signal molecule (Nod factor, NF) needed to induce nodule formation in host plant [23]. Initial interaction consists of stimulation of biochemical activity in the rhizobial strains by flavonoid and isoflavonoid molecules in the plant root exudates [53].…”

“…Among alternatives thought, it is the use of Biological Nitrogen Fixation (BNF). BNF in large percent is done by a gram negative soil bacteria called rhizobia which are capable of fixing free atmospheric nitrogen (N 2 ) into ammonia (NH 3 ), a form which can be utilized by plants [22] [23].…”