A New Substrate Cycle in Plants. Evidence for a High Glucose-Phosphate-to-Glucose Turnover from in Vivo Steady-State and Pulse-Labeling Experiments with [13C]Glucose and [14C]Glucose

Abstract

Substrate (futile) cycling involving carbohydrate turnover has been widely reported in plant tissues, although its extent, mechanisms, and functions are not well known. In this study, two complementary approaches, short and steady-state labeling experiments, were used to analyze glucose metabolism in maize (Zea mays) root tips. Unidirectional rates of synthesis for storage compounds (starch, Suc, and cell wall polysaccharides) were determined by short labeling experiments using [U-14 C]glucose and compared with net synthesis fluxes to determine the rate of glucose production from these storage compounds. Steady-state labeling with [1-13 C]glucose and [U-13 C]glucose showed that the redistribution of label between carbon C-1 and C-6 in glucose is close to that in cytosolic hexose-P. These results indicate a high resynthesis flux of glucose from hexose-P that is not accounted for by glucose recycling from storage compounds, thus suggesting the occurrence of a direct glucose-P-to-glucose conversion. An enzyme assay confirmed the presence of substantial glucose-6-phosphatase activity in maize root tips. This new glucose-P-to-glucose cycle was shown to consume around 40% of the ATP generated in the cell, whereas Suc cycling consumes at most 3% to 6% of the ATP produced. The rate of glucose-P cycling differs by a factor of 3 between a maize W22 line and the hybrid maize cv Dea, and is significantly decreased by a carbohydrate starvation pretreatment.The development of nonphotosynthetic tissues is closely related to Suc import. Suc is degraded by Suc synthase (SuSy) or invertase to provide UDP-Glc or hexoses for the biosynthesis of structural or storage compounds and for ATP production (Fig. 1). In 1988, Hargreaves and ap Rees used pulse-chase experiments to show the presence of a cycle of synthesis and degradation of Suc in pea roots. This cycle, consuming ATP without any apparent physiological function, was described as a futile cycle, according to Fell's definition (Fell, 1997), and, according to more recent terminology, which refers to a possible role for these processes, it can be described as a substrate cycle (the Suc cycle). Since then, the Suc cycle has been found in many other tissues: chenopodium cells, potato (Solanum tuberosum) tubers, ripening banana (Musa cavendishii), maize (Zea mays) root tips, tomato (Lycopersicon esculentum) cells, and tomato fruit (Hatzfeld and Stitt, 1990;Hill and ap Rees, 1994; DieuaideNoubhani et al., 1995 DieuaideNoubhani et al., , 1997N'tchobo et al., 1999;Rontein et al., 2002). One can estimate that, for the synthesis and degradation of one molecule of Suc, 1.5 to two molecules of ATP are consumed (Fig. 1). Depending on the tissue, this cycle could consume between 5% and 70% of the ATP produced by the cell (Hatzfeld and Stitt, 1990;Hill and ap Rees, 1994;Dieuaide-Noubhani et al., 1995;Fernie et al., 2002;Rontein et al., 2002).Two other substrate cycles related to central carbohydrate metabolism have been described in plants, hexose-P 4 triose-P cycling and the starch synthesi...

Carbon Cycling in Anabaena sp. PCC 7120. Sucrose Synthesis in the Heterocysts and Possible Role in Nitrogen Fixation

“…Substrate cycling involving carbohydrate turnover in plant tissues has been widely reported, but its mechanisms and functions remain poorly understood (Alonso et al, 2005). Our results suggest that a Succycling mechanism may be operating in heterocysts, allowing cell metabolism to shift easily from Suc production to degradation, through A/N-Inv, hexokinase, hexose-P mutase, hexose-P isomerase, AGPase, SPS-B, and SPP activity.…”

Sucrose synthase in unicellular cyanobacteria and its relationship with salt and hypoxic stress

“…These sugar cycles were proposed in plants to allow a pathway's net flux to respond to factors controlling respiration, maintaining osmotic potential, controlling sugar accumulation, and promoting sugar signaling (Rohwer and Botha 2001;Roby et al 2002). However, although it has been widely reported in plants, its mechanisms and functions remain poorly understood (Alonso et al 2005).…”

ADP-Glucose Pyrophosphorylase-Deficient Pea Embryos Reveal Specific Transcriptional and Metabolic Changes of Carbon-Nitrogen Metabolism and Stress Responses

“…In the cytoplasm, up-regulated expression of Suc synthase-1, SPS, SPP, UGP, PGM, and PGI suggest activated cycling of hexose-P and Suc. Such metabolic phenomena have been commonly observed in plant cells, where the consumption and/or production of ATP (Alonso et al, 2005) can be offset against the change of expression and/or activity of common respiratory enzymes. Vacuolar and neutral invertases were down-regulated, potentially to keep free hexoses low, which avoids hexose-mediated sugar sensing (Koch, 2004).…”