Relationships Between Nitrogen Metabolism and Photosynthesis

The effect of N-assimilation on the partitioning of carbon fixation between phosphoenolpyruvate carboxylase (PEPcase) and ribulose bisphosphate carboxylase/oxygenase (Rubisco) was determined by measuring stable carbon isotope discrimination during photosynthesis by an N-limited green alga, Selenastrum minutum (Naeg.) Collins. This was facilitated by a two process model accounting for simultaneous CO2 fixation and respiratory CO2 release. Discrimination by control cells was consistent with the majority of carbon being fixed by Rubisco. During nitrogen assimilation however, discrimination was greatiy reduced indicating an enhanced flux through PEPcase which accounted for upward of 70% of total carbon fixation. This shift toward anaplerotic metabolism supports a large increase in tricarboxylic acid cycle activity primarily between oxaloacetate and a-ketoglutarate thereby facilitating the provision of carbon skeletons for amino acid synthesis. This provides an example of a unique set of conditions under which anaplerotic carbon fixation by PEPcase exceeds photosynthetic carbon fixation by Rubisco in a C3 organism.

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confidence: 74%

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confidence: 80%

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confidence: 99%

See 1 more Smart Citation

Significance of Phosphoenolpyruvate Carboxylase during Ammonium Assimilation

Guy

Vanlerberghe²,

Turpin³

1989

“…In the proposed models, most of the carbon exported from the chloroplast is TP (see Table I for complete list of abbreviations). In the cytosol, Fru-2,6-P2 plays an important role in regulating the conversion of TP to sucrose through its effects on cytosolic It is well established that nitrogen exerts a major influence on respiratory carbon metabolism (1,9,15,16,31,(35)(36)(37)41). The assimilation of NH4' enhances starch breakdown and the flow of carbon into TCA cycle intermediates which are then used in amino acid biosynthesis.…”

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confidence: 99%

Regulation of Carbon Partitioning to Respiration during Dark Ammonium Assimilation by the Green Alga Selenastrum minutum

Turpin¹,

Botha²,

Smith³

et al. 1990

The assimilation of NH4 causes a rapid increase in respiration to provided carbon skeletons for amino acid synthesis. In this study we propose a model for the regulation of carbon partitioning from starch to respiration and N assimilation in the green alga Selenastrum minutum. We provide evidence for both a cytosolic and plastidic fructose-1,6-bisphosphatase. The cytosolic form is inhibited by AMP and fructose-1,6-bisphosphate and the plastidic form is inhibited by phosphate. There is only one ATP dependent phosphofructokinase which, based on immunological cross reactivity, has been identified as being localized in the plastid. It is inhibited by phosphoenolpyruvate and activated by phosphate. No pyrophosphate dependent phosphofructokinase was found. The initiation of dark ammonium assimilation resulted in a transient increase in ADP which releases pyruvate kinase from adenylate control. This activation of pyruvate kinase causes a rapid 80% drop in phosphoenolpyruvate and a 2.7-fold increase in pyruvate. The pyruvate kinase mediated decrease in phosphoonolpyruvate correlates with the activation of the ATP dependent phosphofructokinase increasing carbon flow through the upper half of glycolysis. This increased the concentration of triosephosphate and provided substrate for pyruvate kinase. It is suggested that this increase in triosephosphate coupled with the glutamine synthetase mediated decline in glutamate, serves to maintain pyruvate kinase activation once ADP levels recover. The initiation of NH4 assimilation causes a transient 60% increase in fructose-2,6-bisphosphate. Given the sensitivity of the cytosolic fructose-1,6-bisphosphatase to this regulator, its increase would serve to inhibit cytosolic gluconeogenesis and direct the triosephosphate exported from the plastid down glycolysis to amino acid biosynthesis.Over the last decade there has been a significant increase in our knowledge of the control of carbon partitioning from starch to sucrose in photosynthetic tissue (34). In the proposed models, most of the carbon exported from the chloroplast is TP (see Table I for complete list of abbreviations). In the cytosol, Fru-2,6-P2 plays an important role in regulating the conversion of TP to sucrose through its effects on cytosolic It is well established that nitrogen exerts a major influence on respiratory carbon metabolism (1,9,15,16,31,(35)(36)(37)41). The assimilation of NH4' enhances starch breakdown and the flow of carbon into TCA cycle intermediates which are then used in amino acid biosynthesis. At present there is no model which provides an integrative picture of the mechanism by which NH4' assimilation regulates this carbon partitioning. In the present study we report changes in cellular metabolites during the dark assimilation of NH4' by the Nlimited green alga Selenastrum minutum. We also report preliminary data on the localization and regulation of PFK, PFP, and FBPase in this organism. Together, these results allow us to propose an integrative model for the control of carbon partitionin...

“…This is one explanation for the accumulation of carbohydrates in the leaves of N2Fx plants. The question remained whether or not increased starch accumulation reflected an increase in activities of enzymes associated with the leaf starch synthesis pathway (1,3,4,16,23,25,33).…”

Section: Discussionmentioning

confidence: 99%

Photosynthate Metabolism in the Source Leaves of N₂-Fixing Soybean Plants

Veau¹,

Robinson²,

Warmbrodt³

et al. 1992

adaptation of these enzyme activities in the leaves of N2-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-i-phosphate (G 1 P), which had risen to chloroplast concentrations considerably in excess of the Km values for their respective target enzymes associated with starch synthesis, e.g. elevated GI P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N2-fixing as well as N-supplemented plants, and G1,6BP levels in N2-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N2-fixing and N-supplemented plants. This Soybean plants coexist symbiotically with Bradyrhizobium japonicum, the bacteria from which those plants can derive all of their reduced nitrogen through N2-fixation processes. Symbiotic soybeans often display very large accumulations of starch and/or sucrose in their source leaves, while concurrently, these same leaves exhibit much reduced soluble protein levels (3, 4). When N2-fixing plants are supplemented with N03-and NH4', or they are grown bacteria free from emergence with a sufficient supply of N,2 their source leaves exhibit a reallocation of carbon photosynthate skeletons to support amino acid and soluble leaf protein accompanied by a diminution of sucrose and starch (3, 4). Further, N2-fixing plants often possess foliar net CO2 assimilatory rates equivalent to those observed in leaves of plants supplemented with growth sufficient levels of N (4, 24). This has been interpreted to mean that approximately the same amount of newly synthesized carbon photosynthate (e.g. TP) that normally was channeled into the syntheses of amino acids and soluble leaf protein in N-supplemented plants, instead was directed to support the syntheses of starch and sucrose within N2-fixing soybean plant leaves (4). Excessive foliar starch accumulation in N2-fixing plants was similar to that observed in N-deficient plants, although N2-fixing soybeans were relatively healthy (compare refs. 3, 4, and 16 with 23 and 25). 2Abbreviations: N, inorganic nitrogen, i.e., N03-and/or NH4+; ADPG-PPiase, ADP-glucose pyrophosphorylase; FBPase, fructose-1,6-bisP phosphatase; PGM, phosphoglucomutase; PHI, phosphohexoisomerase; HK, hexokinase; ALD, aldolase; PFK, phosphofructokinase; SP, starch phosphorylase; AMY, amylase; TP, triose phosphate (glyceraldehyde-3-P and dihydroxyacetone-P); PVM, paraveinal mesophyll; TEM, transmission electron microscopy. NS plants, soybean plants propagated in vermiculite from seedling emergence through experiment termination (4 weeks), and supplied daily with nutrient solution containing 6 mm NH4NO3. Pri...