A Model of Nutrient Exchange in the Rhizobium-Legume Symbiosis

Kahn, Michael L.; Kraus, J.; Somerville, John

doi:10.1007/978-94-009-5175-4_26

Cited by 103 publications

(53 citation statements)

References 23 publications

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“…10), whereas at 20% 02 there is a lag of only 2 to 3 min (Figs. lA, 5 [25°C], 6 [20% 02], 6, and 12 [initial]). …”

Section: Effect Of Light Deprivationmentioning

confidence: 99%

“…It is likely that the introduction of C2H2 would cause a major decrease in the glutamate and aspartate necessary for this shuttle. There are also other amino acids that may be involved in the transfer of carbon and reductant from the host cytoplasm to the nitrogenfixing microsymbiont (5,6,11,16 ase activity during the decline by making more reductant available.…”

Section: Effect Of Light Deprivationmentioning

confidence: 99%

See 1 more Smart Citation

Factors Affecting the Acetylene-Induced Decline during Nitrogenase Assays in Root Nodules of Myrica gale L.

Tjepkema

Schwintzer²

1992

Plant Physiol.

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Our goal was to determine why the rate of acetylene reduction by nodules of actinorhizal plants declines after an initial peak value. The decline was eliminated by pretreatment with argon, indicating that the decline is initiated by cessation of ammonia synthesis. When 02 concentration was decreased during the decline, the rate of acetylene reduction increased. This shows that during the decline there is either 02 toxicity or competition between respiration and nitrogenase for reductant. The decline was not eliminated when uptake hydrogenase was inactivated by pretreatment with acetylene, showing that cessation of H2 oxidation is not the primary cause of the decline. The effects of a variety of other treatments on the decline were also studied. Overall, we conclude that the cessation of ammonia formation is the primary cause of the acetylene-induced decline. We hypothesize that the supply of reductant for nitrogenase depends on amino acids that are depleted following cessation of ammonia formation. We also conclude that the initial peak rate of acetylene reduction provides the best measure of nitrogenase activity.The acetylene reduction assay for nitrogenase activity has a number of desirable characteristics: it is fast and low in cost and allows repeated assays of the same material. Unfortunately, the rate of C2H2 reduction is often not constant as a function of time, with an initial peak rate being followed by a decline. In legume nodules, there is typically little or no recovery from this decline (7, 21), whereas in actinorhizal plants there is a variable recovery of activity, with the recovered rate approaching the initial peak rate in some instances (9,10,13,15,17,20). We found an especially large decline followed by a strong recovery in water-cultured plants of Myrica gale L. and used this as a model system to study the C2H2-induced decline. Our goal was to determine the causes of this phenomenon and the conditions under which the acetylene reduction assay most accurately measures nitrogenase activity. We examined the effects of cessation of ammonia production, temperature, 02 concentration, inactivation of uptake hydrogenase, and light deprivation. Our results show that nitrogenase activity is affected by changes in metabolism caused by the cessation of ammonia production in the presence of C2H2. We also conclude that the initial peak rate of C2H2 reduction is the best measure of nitrogenase activity. ' This work was supported by U.S. Department of Agriculture grants 87-CRCR-1-2431 and 87-CRCR-1-2440. 1451 MATERIALS AND METHODSMyrica gale L. was grown from seeds, inoculated, and grown as described by Monz and Schwintzer (9). Inoculation was with Frankia strain LLR16 1101, and seedlings were grown in individual water cultures (1/4 strength, nitrogen-free Hoagland solution, 250-mL plastic bottles) in a growth chamber with a 16-h photoperiod (450 umol m-2 s-' PPFD, 70 to 80% humidity, 22°C day, 20°C night). The solution level in the bottles was kept below the nodulated portion of the roots, and the ...

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“…10), whereas at 20% 02 there is a lag of only 2 to 3 min (Figs. lA, 5 [25°C], 6 [20% 02], 6, and 12 [initial]). …”

Section: Effect Of Light Deprivationmentioning

confidence: 99%

Section: Effect Of Light Deprivationmentioning

confidence: 99%

Factors Affecting the Acetylene-Induced Decline during Nitrogenase Assays in Root Nodules of Myrica gale L.

Tjepkema

Schwintzer²

1992

Plant Physiol.

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“…A mutant of R. meEiZoti lacking this aminotransferase activity was symbiotically defective, implying that this pathway, or its reverse, is in some way necessary for the nitrogen fixation process. It has also been proposed that in bacteroids aspartate is exported during the operation of an aspartate-malate shuttle which would generate reductant, NADH, and thus provide energy for nitrogen fixation (Kahn et al, 1985;Appels & Haaker, 1991;Kouchi et al, 1991). To study the metabolic interactions between the host plant and the bacteria, it is first necessary to have a basic understanding of the transport systems involved in the uptake of compounds which may be transferred between them.…”

Section: Introductionmentioning

confidence: 99%

Aspartate transport in Rhizobium meliloti

Watson¹,

Rastogi²,

Chan³

1993

Journal of General Microbiology

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Aspartate transport in Rhizubium meliluti was found to be mediated by at least two transport systems. High rates of aspartate uptake, necessary for growth on aspartate as a carbon source, required the dicarboxylate transport (Dct) system, which also transports succinate, fumarate and malate. The apparent K,,, for aspartate transport by this system was about 10 mM, compared to 15 p~ for succinate. This difference in affinity was also apparent in competitive inhibition studies, which showed that succinate effectively inhibits aspartate transport. Although aspartate was not a preferred substrate, it was a very efficient inducer of the Dct system. Both the Dct system and a second aspartate transport system were capable of supplying aspartate for use as a nitrogen source. The second system had a lower apparent K, for aspartate transport (1.5 mM), and was competitively inhibited by glutamate. This aspartateglutamate system was regulated independently from the Dct system, since it functioned in mutants lacking the Dct system regulatory genes dctB and dctD, and its induction did not coactivate the Dct system. Uptake kinetics in cultures growing on aspartate as nitrogen source showed rapid substrate exchange between extracellular and internal aspartate. R. meliloti was shown to be able to selectively activate the two uptake systems, and also regulated its metabolism as required to utilize aspartate as either carbon or nitrogen source.

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“…The NH + % ion would not diffuse across biological membranes, giving rise to concerns about the mechanism by which fixed N # left bacteroids (e.g. Kahn et al, 1985). Subsequently, an NH + % channel was identified on the peribacteroid membrane (PBM) (Tyerman et al, 1995) and the accepted interpretation persisted.…”

Section: Introductionmentioning

confidence: 99%