Sucrose-feeding leads to increased rates of nitrate assimilation, increased rates of α-oxoglutarate synthesis, and increased synthesis of a wide spectrum of amino acids in tobacco leaves

Abstract

To investigate the importance of the sugar supply for the regulation of nitrogen and organic acid metabolism, various sugars and nitrogenous compounds were supplied for 8 h to detached tobacco leaves in low light. (i) In control leaves supplied with water, there was a large decrease of the Nia transcript level, a 50% decline of nitrate reductase (NR) activity, starch increased and sugars remained low, nitrate decreased by 50%, and amino acids increased only slightly during the 8 h incubation. About half of the nitrogen accumulating in amino acids was present in glutamine (Gln). (ii) When 25 mM sucrose was supplied, the in-vivo rate of nitrate assimilation (estimated from the accumulation of ammonium and amino acids) increased 2-fold. The Nia transcript level still decreased, but the decline of NR activity was less pronounced and NR activation was increased. The in-vivo net rate of ammonium assimilation (estimated from the accumulation of amino acids) also doubled after feeding sucrose. Ammonium and glutamate (Glu) decreased and Gln rose markedly, showing that in-vivo activity of glutamine synthetase had been stimulated. Glutamine still accounted for about half of the nitrogen, indicating that sucrose does not selectively stimulate glutamine synthase. Glutamate and aspartate decreased and all the minor amino acids increased, showing that the amino acid biosynthesis pathways are activated by sucrose. There was a decrease of 3-phosphoglycerate (3PGA) and phosphoenolpyruvate (PEP) and a large increase of a-oxoglutarate, showing that the¯ow of carbon from glycolysis into organic acids has been stimulated by sucrose. (iii) The changes of 3PGA, PEP, a-oxoglutarate, Glu, aspartate and the minor amino acids were smaller when 50 mM glucose was supplied, even though the internal levels of sugars at the end of the incubation resembled those found after feeding 25 mM sucrose. This indicates that the signals that regulate nitrogen and respiratory metabolism are derived from the uptake or metabolism of sucrose, rather than glucose. (iv) A dierent spectrum of changes was found when 20 mM nitrate was supplied. The estimated rate of nitrate assimilation increased 2-fold, and this was accompanied by an increase of NR activity but not of NR activation. Nitrate-feeding did not lead to a decrease of Glu, and the increase of minor amino acids was slightly smaller than with sucrose. There was a decrease of sugars, starch, and hexose phosphates, but 3PGA and PEP were not signi®cantly decreased and isocitrate increased instead of a-oxoglutarate. (v) A dierent spectrum of changes was also found when 10 mM Gln was supplied. The estimated rate of nitrate assimilation decreased, and this was accompanied by a decrease of NR activity and NR activation. Glutamate did not decrease, and the increase of minor amino acids was smaller than with sucrose. Starch and sugars remained high and, although hexose phosphates decreased, 3PGA and PEP were not signi®cantly decreased. Isocitrate remained unaltered and the increase of aoxoglutarate was smaller tha...

Influence of malate and 2-oxoglutarate on the NIA transcript level and nitrate reductase activity in tobacco leaves

“…3b) and NIA activity (Fig. 3c, see also Morcuende et al 1998), compared to leaves that were supplied with water. When malate was supplied there was a 2-to 2·5-fold increase of malate in the leaves (Fig.…”

The nitrate and ammonium nitrate supply have a major influence on the response of photosynthesis, carbon metabolism, nitrogen metabolism and growth to elevated carbon dioxide in tobacco

“…It is known that addition of ammonium leads to a rapid decrease of NR activity. This may be partly due to repression of Nia transcription by glutamine (Hoff et al 1994), and partly to increased post-translational inactivation and degradation of NR when glutamine is high (Scheible et al 1997c;Morcuende et al 1998). Despite these marked short-term effects of ammonium on nitrate assimilation, plants growing in ambient [CO 2 ] had similar NR activity on ammonium nitrate and on nitrate (see Fig.…”

“…In Plantago major, elevated [CO 2 ] led to a transient increase in NR activity, that was reversed after a few days (Fonseca, Bowsher & Stulen 1997). These reports of a decrease of NR activity in elevated [CO 2 ] are rather surprising, because exogenous sugars lead to increased expression of Nia (Cheng et al 1992;Vincentz et al 1993;Krapp et al 1993;Krapp & Stitt 1995;Morcuende et al 1998) and post-translational activation of NR (Kaiser & Huber 1994; Huber, Bachman & Huber 1996) in detached leaves. Recently Geiger et al (1998) showed that although elevated [CO 2 ] does not markedly increase the maximum NR activity in tobacco the diurnal regulation of NR is modified, allowing higher activity in the later part of the light period and during the night.…”

Metabolic Regulation of Leaf Senescence in Sunflower (Helianthus annuus L.) Plants

“…The high ammonium contents of young leaves are probably a result of strong photosynthetic nitrate reduction activity and photorespiration. In addition, young leaves have low contents in soluble carbohydrates, and sugar availability is known to be a limiting factor for ammonium assimilation (Morcuende et al, 1998). The high ammonium contents of senescent leaves are mainly due to protein degradation, amino acid deamination and nucleic acid catabolism, but also to photorespiration.…”