Nitrate Activation of Cytosolic Protein Kinases Diverts Photosynthetic Carbon from Sucrose to Amino Acid Biosynthesis

Abstract

The regulation of carbon partitioning between carbohydrates (principally sucrose) and amino acids has been only poorly characterized in higher plants. The For many years, extensive research has been devoted to the regulation of the synthesis of sucrose, the exported form of photoassimilate in most leaves (7,11,17,43). This has provided the basic elements for the description of the fine control network that regulates sucrose synthesis and photosynthetic C partitioning between the cytosol (sucrose synthesis) and the chloroplasts (starch synthesis). In leaves, NO3-assimilation takes place in the same compartments, i.e. cytosol (NO3-reduction to NO2-) and chloroplasts (reduction of NO2-to NH4' and assimilation of the latter into glutamate) as sucrose and starch synthesis, respectively. The reduction of N02-to NH4+ uses photochemically generated reducing power, as does the reduction of CO2 to carbohydrate, and is considered to be an important sink for the products of photosynthetic electron flow (31). In consideration of the common requirements of NO3-and CO2 metabolism in terms of reducing power, ATP, and carbon skeletons, important questions have been raised concerning the impact of NO3-on photosynthesis and the partitioning of photosynthetic products to amino acids at the expense of carbohydrate synthesis (44).The effect of nitrate on the rate of sucrose synthesis was studied with mature leaves detached from wheat seedlings (Triticum aestivum) (49). A linear inverse relationship was shown between the rate of sucrose synthesis and the rate of uptake and assimilation of NO3-, with no effect on insoluble carbohydrate synthesis. The effect of NO3-on sucrose synthesis appeared within the first hour after the leaves were fed NO3-. It slowed the rate of synthesis without altering the sucrose storage capacity of leaves. The rate of CO2 fixation was only slightly reduced (5).The diversion of 14C-labeled photosynthetic carbon away from carbohydrate synthesis toward organic acid and amino acid synthesis in leaves fed with NO3-provided evidence that sucrose synthesis is regulated at the level of partitioning of C between the two pathways in the leaves of higher plants (5). The increased carbon flux to amino acids is always associated with a decrease in the PEP' content and the activation of PEPcase in the leaves. The activation is not impaired by cycloheximide (48). The increased demand for carbon skeletons created by high rates of NO3-or NH4' assimilation was thus met by the diversion of fixed carbon to the anapleurotic pathway. Concomitantly, the rate of sucrose synthesis was restricted by a decrease in SPS activity. These results are in agreement with the suggestions that (a) PEPcase is the protein whose activity is most affected by N availability in leaves (45) and (b) SPS activity is a major component that controls the flux of carbon into sucrose (29,43

Developmental regulation of photosynthate distribution in leaves of rice

“…4, Nakamura et al 1997). These findings were in accord with the proposal that the alternative allocation of DHAP-derived C to the synthesis of amino acids or sucrose is regulated by the relative strengths of PEPC and SPS (Champigny and Foyer 1992).…”

“…It has been proposed that the distribution of photosynthetically fixed carbon (C) between amino acids and saccharides is regulated by the relative 2 activities of sucrose phosphate synthase (SPS) and phosphoenolpyruvate carboxylase (PEPC; Champigny and Foyer 1992, Foyer et al 1994, Huber et al 1994). In C 3 -plants, PEPC acts to replenish TCA cycle intermediates which serve as source material for the synthesis of amino acids (Stitt 1999).…”

Effect of fertilization on ozone‐induced changes in the metabolism of birch (Betula pendula) leaves

“…There is ample evidence that nitrogen supply is a potent activator of PEPC (Wingler et al, 1994), simultaneously causing a decrease in the sucrose phosphate synthase (SPS). Thus the C flow might be directed away from the synthesis of sucrose towards that of amino acid (Champigny & Foyer, 1992). Because SPS activity was also enhanced in the HF plants (Einig et al, unpublished) and malate, but not aspartate, was the main product of PEPC activity, its stimulation cannot be explained by N alone.…”

“…One primary role of PEPC in C3 plants appears to be the anapleurotic synthesis of C4 acids (Melzer & O'Leary, 1987;Champigny & Foyer, 1992). The first product of this reaction chain is oxaloacetate, which is either transaminated to aspartate or rapidly reduced to malate.…”

Expression of an Arabidopsis Aspartate Kinase/Homoserine Dehydrogenase Gene Is Metabolically Regulated by Photosynthesis-Related Signals but Not by Nitrogenous Compounds1

“…The regulation of carbon partition between carbohydrates and amino acids is subject to a complex metabolic regulation by light, photosynthesis-related metabolites, and nitrogenous compounds (Champigny and Foyer, 1992;Staswick, 1994;Lam et al, 1995). Under conditions of high sugar but limiting nitrogen levels, carbohydrates accumulate preferentially, whereas under conditions of high carbon and high nitrogen levels, a larger proportion of the carbon is shifted toward production of amino acids and proteins (Champigny and Foyer, 1992;Sadka et al, 1994;Staswick, 1994).…”