Gas Exchange of Legume Nodules and the Regulation of Nitrogenase Activity

Hunt, S.

doi:10.1146/annurev.arplant.44.1.483

Cited by 88 publications

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“…Bacteroids within indeterminate nodules increase their DNA content and cell size, which might allow them to reach a higher metabolic rate to support nitrogen fixation 65,68 . The intensive DNA synthesis that is required for endoreduplication within bacteroids requires a large quantity of dNTPs that must be supplied by ribonucleotide reductase (RNR) 89 . For many bacterial species, performing DNA synthesis in both an oxygen-rich environment, such as the infection thread, and an oxygen-depleted environment, such as the symbiosome, would present a lethal problem.…”

Section: Box 3 Host Invasion Parallels Between Rhizobia and Animal Pamentioning

confidence: 99%

“…An oxygen-sensing bacterial regulatory cascade controls the expression of the nitrogenase complex and the microaerobic respiratory enzymes that are required to provide energy and reductant to nitrogenase 98 . This cascade is induced by the presence of low oxygen tension within the differentiating bacteroid 89,99,100 . The bacterial regulators include the oxygen-sensing two-component regulatory system FixL and FixJ, NifA, σ 54 and FixK 98 .…”

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How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

et al. 2007

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Nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which the bacteria can enter the plant root. Once the bacteria have been endocytosed within a host-membrane-bound compartment by root cells, the bacteria differentiate into a new form that can convert atmospheric nitrogen into ammonia. Bacterial differentiation and nitrogen fixation are dependent on the microaerobic environment and other support factors provided by the plant. In return, the plant receives nitrogen from the bacteria, which allows it to grow in the absence of an external nitrogen source. Here, we review recent discoveries about the mutual recognition process that allows the model rhizobial symbiont Sinorhizobium meliloti to invade and differentiate inside its host plant alfalfa (Medicago sativa) and the model host plant barrel medic (Medicago truncatula).The recent completion of the Sinorhizobium meliloti genome sequence, and the progress towards the completion of the Medicago truncatula genome sequence, have led to a surge in the molecular characterization of the determinants that are involved in the development of the symbiosis between rhizobial bacteria and leguminous plants. Aromatic compounds from legumes called flavonoids first signal the rhizobial bacteria to produce lipochitooligosaccharide compounds called Nod factors 1 . Nod factors that are secreted by the bacteria activate multiple responses in the host plant that prepare the plant to receive the invading bacteria. Nod factors and symbiotic exopolysaccharides induce the plant to form infection threads, which are thin tubules filled with bacteria that penetrate into the plant cortical tissue and deliver the bacteria to their target cells. Plant cells in the inner cortex internalize the invading bacteria in host-membrane-bound compartments that mature into structures known Correspondence to G.C.W. gwalker@mit.edu. Competing interests statementThe authors declare no competing financial interests. DATABASES Invasion of plant rootsAlthough plant roots are exposed to various micro-organisms in the soil, their cell walls form a strong protective barrier against most harmful species. The early steps in the invasion of barrel medic (M. truncatula) and alfalfa (Medicago sativa) roots by S. meliloti are characterized by the reciprocal exchange of signals that allow the bacteria to use the plant root hair cells as a means of entry. Initial signal exchangeFlavonoid compounds (2-phenyl-1,4-benzopyrone derivatives) produced by leguminous plants are the first signals to be exchanged by host-rhizobial symbiont pairs 1 (FIG. 1). Flavonoids bind bacterial NodD proteins, which are members of the LysR family of transcriptional regulators, and activate these proteins to induce the transcription of rhizobial genes 1,2 . For example, the M. sativa-derived flavonoid luteolin stimulates binding of an active form of NodD1 to an S. meliloti 'nod-box' p...

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Section: Box 3 Host Invasion Parallels Between Rhizobia and Animal Pamentioning

confidence: 99%

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confidence: 99%

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

et al. 2007

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“…For example, nitrogen is a primary component of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the enzyme that catalyzes photosynthetic reduction of CO 2 to carbohydrate, while photosynthesis supplies organic carbon to nodules, where it is used in the nitrogenase enzyme system as a source of energy and reducing power to fix N 2 . Because of this coupling, nitrogenase activity in plants is regulated by aspects of both carbon and N processes (Vance and Heichel 1991;Hunt and Layzell 1993;Hartwig and Nosberger 1994), including photosynthesis (rate of carbon supply), nitrogen availability (N source strength), and nitrogen demand (N sink strength). It has been argued that the demand for symbiotically fixed N directly governs nitrogenase activity (Hartwig and Nosberger 1994) and the supply rate of carbon determines the degree to which the energy requirements of N 2 -fixation are met (Vance and Heichel 1991;Hunt and Layzell 1993).…”

Section: Introductionmentioning

confidence: 99%

Nitrogenase activity and N 2 fixation are stimulated by elevated CO 2 in a tropical N 2 -fixing tree

1997

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Seeds of Gliricidia sepium, a fast-growing woody legume native to seasonal tropical forests of Central America, were inoculated with N 2 -fixing Rhizobium bacteria and grown in environmentally controlled glasshouses for 67-71 days under ambient CO 2 (35 Pa) and elevated CO 2 (70 Pa) conditions. Seedlings were watered with an N-free, but otherwise complete, nutrient solution such that bacterial N 2 fixation was the only source of N available to the plant. The primary objective of our study was to quantify the effect of CO 2 enrichment on the kinetics of photosynthate transport to nodules and determine its subsequent effect on N 2 fixation. Photosynthetic rates and carbon storage in leaves were higher in elevated CO 2 plants indicating that more carbon was available for transport to nodules. A 14 CO 2 pulse-chase experiment demonstrated that photosynthetically fixed carbon was supplied by leaves to nodules at a faster rate when plants were grown in elevated CO 2 . Greater rates of carbon supply to nodules did not affect nodule mass per plant, but did increase specific nitrogenase activity (SNA) and total nitrogenase activity (TNA) resulting in greater N 2 fixation. In fact, a 23% increase in the rate of carbon supplied to nodules coincided with a 23% increase in SNA for plants grown in elevated CO 2 , suggesting a direct correlation between carbon supply and nitrogenase activity. The improvement in plant N status produced much larger plants when grown in elevated CO 2 . These results suggest that Gliricidia, and possibly other N 2 -fixing trees, may show an early and positive growth response to elevated CO 2 , even in severely N-deficient soils, due to increased nitrogenase activity.

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“…The clear accumulation of carbohydrates in leaves supports its role as osmoprotectants in both species. However, total soluble sugars only increased in nodules of L. japonicus under salinity, suggesting the decline in nitrogenase activity observed in this specie was not caused directly by a lack of photosynthates (Hunt and Layzell 1993). This sugar accumulation may be due to the substantial decline in sucrose synthase activity, which controls the flux of carbon and energy for the bacteroids (Arrese-Igor et al 1999), leading to the decrease in nitrogen fixation.…”

Section: Lo´pez and C Lluchmentioning

confidence: 94%

“…Salinity is known to boost the nodular carbohydrate content and sucrose is the predominant carbohydrate in legume root nodules (Fouge`re et al 1991;Gordon et al 1997). It has been argued that the accumulation of sucrose under salinity, associated with inhibition of the sucrose hydrolytic enzyme sucrose synthase (Arrese-Igor et al 1999), induces a deficiency of carbon for bacteroids with the subsequent inhibition of nitrogenase activity (Hunt and Layzell 1993). A strong correlation between sugar accumulation and osmotic stress tolerance has been widely reported, including transgenic experiments (Gilmour et al 2000;Streeter et al 2001;Taji et al 2002).…”

Section: Introductionmentioning

confidence: 99%

Nitrogen fixation is synchronized with carbon metabolism inLotus japonicusandMedicago truncatulanodules under salt stress

López

Lluch

2008

Journal of Plant Interactions

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In the present work, the response to NaCl applied at the vegetative stage to Medicago truncatula and Lotus japonicus has been evaluated in order to ascertain whether the effect of salt stress on nitrogen fixation is due to a limitation on nodular carbon metabolism. Results show maximum sucrose synthase (SS) and alkaline invertase (AI) activities were obtained at the vegetative stage, when maximum nitrogenase activity was detected in both species. SS activity decreased with the salt treatment, providing evidence of the regulatory role of this enzyme for the carbon supply to the bacteroids. Phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (MDH) activities could account for higher nitrogen fixation efficiency detected in L. japonicus nodules and isocitrate dehydrogenase (ICDH) activity compensated for the carbon limitations that occur under salt stress. These results support that nitrogenase inhibition in nodules experiencing salt stress is doubt to a carbon flux shortage, as result of carbon metabolism enzymes activities down-regulation.

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Gas Exchange of Legume Nodules and the Regulation of Nitrogenase Activity

Cited by 88 publications

References 0 publications

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

Nitrogenase activity and N 2 fixation are stimulated by elevated CO 2 in a tropical N 2 -fixing tree

Nitrogen fixation is synchronized with carbon metabolism inLotus japonicusandMedicago truncatulanodules under salt stress

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