Reassessment of major products of N2 fixation by bacteroids from soybean root nodules

Li, Youzhong; Parsons, Ruth; Day, David A.; Bergersen, F. J.

doi:10.1099/00221287-148-6-1959

Cited by 34 publications

(26 citation statements)

References 18 publications

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“…In a recent report using 15 N 2 labeling of soybeans it was reported that Ala rather than ammonium is the sole secretion product (Waters et al, 1998). However, others failed to repeat this experiment under similar conditions using the same bacterial strains inoculated onto soybean (Li et al, 2002). In a direct attempt to address this contradiction, pea plants were incubated in 15 N 2 and found to secrete predominantly ammonium but also significant amounts of Ala (Allaway et al, 2000), with the ratio depending on the concentration of ammonium that had built up in the medium.…”

Section: Export Of Nitrogen To the Plant And Its Assimilationmentioning

confidence: 87%

Nutrient Sharing between Symbionts

White

Prell²,

James³

et al. 2007

Plant Physiology

210

158

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In this review, we consider the exchange of nutrients between the host plant and the bacterial microsymbiont in nitrogen-fixing legume root nodules. During nodule formation, the host tissues and the bacterial microsymbiont develop in response to each other to form a specialized tissue that maintains an environment where nitrogen fixation can occur (Brewin, 2004;Mergaert et al., 2006;Prell and Poole, 2006). This complex development will not be considered here but, at the end of the process, specialized, nitrogen-fixing forms of the bacteria, known as bacteroids, reside in the plant cytosol, enclosed within plant-derived membranes. These organelle-like structures are known as symbiosomes; the plant-derived membrane that surrounds the bacteroid is the symbiosome (or peribacteroid) membrane and the space between the two is the symbiosome (or peribacteroid) space. An infected plant cell may be packed with thousands of symbiosomes. The exchange of nutrients that is fundamental to N 2 fixation therefore involves metabolism in the plant to provide carbon and nitrogen compounds to the bacteroids and to assimilate the metabolites that bacteroids release. Nutrients transferred between the symbionts must traverse both the symbiosome and the bacteroid membranes. It is clear that there is more than one pattern whereby successful nutrient exchange can take place: There are two basic types of legume nodules, determinate and indeterminate, and there are fundamental differences between the two in how they develop and in their carbon and nitrogen metabolism. This review focuses on the metabolism of carbon and nitrogen compounds in the symbionts and on the exchange of nutrients across the bacteroid and symbiosome membranes. Particular attention is paid to the movement of primary carbon and nitrogen sources and how they are utilized by both bacteroids and the plant.

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Section: Export Of Nitrogen To the Plant And Its Assimilationmentioning

confidence: 87%

Nutrient Sharing between Symbionts

White

Prell²,

James³

et al. 2007

Plant Physiology

210

158

View full text Add to dashboard Cite

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“…In the case of soybean bacteroids, it has been reported that alanine is the sole secretion product of nitrogen fixation (58), although this has been challenged (30). Nevertheless, these findings have questioned the established view that ammonium is the sole secretion product of N 2 fixation from the bacteroid.…”

mentioning

confidence: 77%

A Monocarboxylate Permease of Rhizobium leguminosarum Is the First Member of a New Subfamily of Transporters

2002

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Amino acid transport by Rhizobium leguminosarum is dominated by two ABC transporters, the general amino acid permease (Aap) and the branched-chain amino acid permease (Bra). However, mutation of these transporters does not prevent this organism from utilizing alanine for growth. An R. leguminosarum permease (MctP) has been identified which is required for optimal growth on alanine as a sole carbon and nitrogen source. Characterization of MctP confirmed that it transports alanine (K m ‫؍‬ 0.56 mM) and other monocarboxylates such as lactate and pyruvate (K m ‫؍‬ 4.4 and 3.8 M, respectively). Uptake inhibition studies indicate that propionate, butyrate, ␣-hydroxybutyrate, and acetate are also transported by MctP, with the apparent affinity for solutes demonstrating a preference for C 3 -monocarboxylates. MctP has significant sequence similarity to members of the sodium/solute symporter family. However, sequence comparisons suggest that it is the first characterized permease of a new subfamily of transporters. While transport via MctP was inhibited by CCCP, it was not apparently affected by the concentration of sodium. In contrast, glutamate uptake in R. leguminosarum by the Escherichia coli GltS system did require sodium, which suggests that MctP may be proton coupled. Uncharacterized members of this new subfamily have been identified in a broad taxonomic range of species, including proteobacteria of the ␤-subdivision, gram-positive bacteria, and archaea. A two-component sensor-regulator (MctSR), encoded by genes adjacent to mctP, is required for activation of mctP expression.Rhizobium leguminosarum is a member of a group of ␣-subdivision of the proteobacteria (the rhizobia), which form a species-specific symbiotic relationship with leguminous plants in order to fix atmospheric nitrogen. The plant supplies the bacteroid (symbiotic bacteria) with a carbon source (C 4 -dicarboxylic acid), while the plant receives reduced nitrogen from the bacteroid. Other rhizobia include Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium species (8,33,53). Prior to establishing a Rhizobium-legume symbiosis, bacteria must thrive in the soil environment, competing with many organisms for nutrients. Transporters of key nutrients, such as amino acids, may give a competitive advantage to rhizobia, allowing them to better colonize roots. This may account for the large number of transporters that have been revealed by the completed genome sequences of Sinorhizobium meliloti, Mesorhizobium loti, and Agrobacterium tumefaciens (13,26,59).Amino acid transport by R. leguminosarum is dominated by two permeases of the ABC transporter superfamily, the general amino acid permease (Aap) and the branched-chain amino acid permease (Bra). Aap and Bra are members of the polar amino acid transporter (transport classification [T.C.] number 3.A.1.3) and hydrophobic amino acid transporter (T.C. number 3.A.1.4) families, respectively (21, 44). However, both are atypical as they transport a broad range of amino acids (19,56). Indeed, Br...

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“…However, the recent demonstration that only alanine is secreted by soybean bacteroids suggests a radically different mechanism of nitrogen transport from the bacteroid to the plant (36). This is controversial and has been disputed by others who could not demonstrate alanine secretion by soybean bacteroids (18). Using the R. leguminosarumpea symbiosis as a tractable genetic and biochemical system, Allaway et al demonstrated that both alanine and ammonia are secreted by isolated bacteroids (1).…”

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confidence: 99%

Regulation ofl-Alanine Dehydrogenase inRhizobium leguminosarumbv.viciaeand Its Role in Pea Nodules

et al. 2004

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Alanine dehydrogenase (AldA) is the principal enzyme with which pea bacteroids synthesize alanine de novo. In free-living culture, AldA activity is induced by carboxylic acids (succinate, malate, and pyruvate), although the best inducer is alanine. Measurement of the intracellular concentration of alanine showed that AldA contributes to net alanine synthesis in laboratory cultures. Divergently transcribed from aldA is an AsnC type regulator, aldR. Mutation of aldR prevents induction of AldA activity. Plasmid-borne gusA fusions showed that aldR is required for transcription of both aldA and aldR; hence, AldR is autoregulatory. However, plasmid fusions containing the aldA-aldR intergenic region could apparently titrate out AldR, sometimes resulting in a complete loss of AldA enzyme activity. Therefore, integrated aldR::gusA and aldA::gusA fusions, as well as Northern blotting, were used to confirm the induction of aldA activity. Both aldA and aldR were expressed in the II/III interzone and zone III of pea nodules. Overexpression of aldA in bacteroids did not alter the ability of pea plants to fix nitrogen, as measured by acetylene reduction, but caused a large reduction in the size and dry weight of plants. This suggests that overexpression of aldA impairs the ability of bacteroids to donate fixed nitrogen that the plant can productively assimilate. We propose that the role of AldA may be to balance the alanine level for optimal functioning of bacteroid metabolism rather than to synthesize alanine as the sole product of N 2 reduction.

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Reassessment of major products of N2 fixation by bacteroids from soybean root nodules

Cited by 34 publications

References 18 publications

Nutrient Sharing between Symbionts

Nutrient Sharing between Symbionts

A Monocarboxylate Permease of Rhizobium leguminosarum Is the First Member of a New Subfamily of Transporters

Regulation ofl-Alanine Dehydrogenase inRhizobium leguminosarumbv.viciaeand Its Role in Pea Nodules

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