Nitrogen Fixing Bacteria - Rhizobia
Symbiotic nitrogen fixation is part of a mutualistic relationship in which plants Interactions between plants and associative nitrogen-fixing bacteria, which are .. in legume and nonlegume nitrogen fixation provide new insight into ways of. A Darwinian perspective on improving nitrogen-fixation efficiency of legume crops and Nitrogen fixation could be increased in various ways, but most of these This might explain how the rhizobium-legume relationship originally evolved. The bacteria then invade root cells and transform into bacteroids; they swell and. Guide A Other plants benefit from nitrogen fixing bacteria when the bacteria die and release nitrogen to the environment, or when the bacteria live in Nitrogen fixation by legumes is a partnership between a bacterium and a plant.
The deepest divisions occur between the Bradyrhizobium group, which is closely related to Blastobacter, Rhodopseudomonas, etc. All the nodule-forming bacteria of legumes are loosely referred to as rhizobia.
Currently, seven genera and at least 28 species of rhizobia are recognized Wang and Martinez-Romero, ; Sy et al. Physiology and molecular biology of N-fixation Due to the experimental tractability and agricultural importance of the legume-rhizobium symbiosis, molecular signaling between plants and bacteria and the ensuing development of symbiotic nodules have been intensively studied in a few species, producing a wealth of information.
Legume roots secrete a variety of iso flavonoids which induce symbiotic genes in homologous bacteria reviewed in Spaink, ; Long, ; Cohn et al. The ability of the bacteria to perceive a particular flavonoid signal is mediated in part by the transcriptional regulator NodD, which varies functionally among rhizobial strains.Nitrogen Fixation
Bacterial nod factors are composed of four to five beta 1—4 linked N-acetyl glucosamine units a chitin backbone and a fatty acid. Nod factors can vary in their fatty acids, the lengths of their sugar backbones, and the saturation of the acyl unit and decorations glycosylation, sulfation, methylation of the reducing and non-reducing ends of the backbone Perret et al. Hence, the diversity of nod factors produced by rhizobia, and discrimination of these factors by plants, contribute the second level of specificity to the interaction and create an opportunity for partner choice by the plant Perret et al.
In a compatible interaction, the infection thread expands from the root hair to subtending cortical cells and fills with a glycoprotein matrix.
Compatibility at this stage depends, in part, on recognition by the plant of particular polysaccharides on the bacterial cell wall. Compatible bacteria multiply and move into the root cortex as the nodule structure develops around them. Certain plant cells within the developing nodule then engulf rhizobial cells and surround them with the peribacteroid membrane.
Within this structure, bacteria differentiate into bacteroids, change shape, and upregulate nitrogenase and the auxiliary enzymes required for dinitrogen reduction. However, bacterial release from the infection thread can be blocked in incompatible interactions, suggesting that the plant requires appropriate signals from the bacteria at this stage as well Perret et al.
Host plant cells then provide energy in the form of dicarboxylic acids malate and succinate and ensure a low but steady oxygen flux via leghaemoglobin regulation Waters and Emerich, In an effective symbiosis, the bacteroids carry out nitrogen fixation and ship ammonia possibly via an alanine shuttle to the plant across the peribacteroid membrane Waters and Emerich, The fantastic advances in our understanding of plant-rhizobium signal transduction, cooperative nodule development, and the biochemistry of nitrogen fixation reveal a complex and elegant coevolutionary tapestry.
If the fitnesses of host and symbiont, or host and parasite, correlate strongly from one generation to the next i.
Spatio-temporal control of mutualism in legumes helps spread symbiotic nitrogen fixation
However, strict vertical transmission has rarely been detected, while horizontal transmission occurs widely, in the major plant-microbe mutualisms—mycorrhizae Smith and Read, and root-nodule N fixation a vertically transmitted above-ground N-fixation symbiosis is described by Perkins and Peters, The degree to which dispersal of host and symbiont diaspores may be spatially co-constrained, and thus lead to genetic associations across multiple generations imperfect vertical transmissionhas not been adequately studied Wilkinson, Lotka-Volterra type models without vertical transmission, which posit reciprocal benefits with no cost, predict positive frequency-dependent selection favoring the most common host and symbiont genotypes Law and Koptur,and lead to infinite population expansion.
Incorporating costs produces more realistic outcomes Boucher et al. However, if benefiting a partner entails a cost, then cheating, i.
These models fall into two major classes Bull and Rice, Partner fidelity Partner-fidelity models, including those based on the prisoner's dilemma, show that mutualism can be stable if pairs of individuals interact repeatedly and adjust their behavior in response to their partner's behavior in the previous interaction Axelrod and Hamilton, Thus, stability depends upon the ability of an individual to recognize the individual with which it interacted previously, maintain fidelity to that partner, remember the outcome of the previous interaction, and modify its behavior in accord with that outcome.
Recent extensions of the model show that stability disappears when individuals are allowed to reactively adjust their level of investment, but that adding spatial structure can restore stability although levels of altruism continually fluctuate Eshel and Cavalli-Sforza, ; Doebeli and Knowlton, These models may explain cooperation in certain interactions e.
First, individual plants normally associate with many symbionts. Second, plant and microbe usually disperse independently Duggar, ; Cass-Smith and Pittman, ; Erdman, ; Moodie and Vandacevye, ; Vincent, ; Alexander, ; Gallon and Chaplin, ; Bottomley, ; Genkai-Kato and Yamamura,so plant-microbe interactions are not iterated but must be renewed every generation. Partner choice Partner-choice models, in contrast, do not depend upon repeated interactions. Choice may be based either on receipt of honest signals indicating the relative qualities of potential partners or on active evaluation of partner quality through some sort of trial interaction.
The quality of the commodity a partner offers in trade contributes to setting the value the other trader is willing to offer in exchange, and hence influences choice.
Email Facebook LinkedIn Legumes form a unique symbiotic relationship with bacteria known as rhizobia, which they allow to infect their roots. This leads to root nodule formation where bacteria are accommodated to convert nitrogen from the air into ammonia that the plant can use for growth.
This symbiotic nitrogen fixation allows legumes to thrive in habitats with limited nitrogen availability. Researchers from Denmark and Germany have now found that an intact nitrogen-fixing symbiosis in Lotus japonicus is needed for the establishment of taxonomically diverse and distinctive bacterial communities.
Nitrogen Fixation by Legumes
Integrating these highly specific binary interactions into an ecological community context is critical for understanding the evolution of symbiosis and efficient use of rhizobia inoculum in agricultural systems. Legumes are known as pioneer plants colonising marginal soils, and as enhancers of the nutritional status in cultivated soils. This beneficial activity has been explained by their capacity to engage in symbiotic relationship with nitrogen-fixing rhizobia.
The beneficial effect of this symbiosis is not limited to legume hosts, but extends to subsequent or concurrent plantings with non-legumes as exemplified by ancient agricultural practices with legume cropping sequences or intercropping systems. This symbiosis likely involves a beneficial activity of legumes on the nutritional status of the soil as well as the soil biome.
However, the mechanisms underpinning these symbiotic interactions in a community context and their impact on the complex microbial assemblages associated with roots remain largely unknown.