Sugar sensing in higher plants.
TL;DR: It is proposed that hexokinase may have dual functions and may act as a key sensor and signal transmitter of sugar repression in higher plants and the involvement of glycolysis and other metabolic pathways is eliminated.
Abstract: Sugar repression of photosynthetic genes is likely a central control mechanism mediating energy homeostasis in a wide range of algae and higher plants. It overrides light activation and is coupled to developmental and environmental regulations. How sugar signals are sensed and transduced to the nucleus remains unclear. To elucidate sugar-sensing mechanisms, we monitored the effects of a variety of sugars, glucose analogs, and metabolic intermediates on photosynthetic fusion genes in a sensitive and versatile maize protoplast transient expression system. The results show that sugars that are the substrates of hexokinase (HK) cause repression at a low concentration (1 to 10 mM), indicating a low degree of specificity and the irrelevance of osmotic change. Studies with various glucose analogs suggest that glucose transport across the plasma membrane is necessary but not sufficient to trigger repression, whereas subsequent phosphorylation by HK may be required. The effectiveness of 2-deoxyglucose, a nonmetabolizable glucose analog, and the ineffectiveness of various metabolic intermediates in eliciting repression eliminate the involvement of glycolysis and other metabolic pathways. Replenishing intracellular phosphate and ATP diminished by hexoses does not overcome repression. Because mannoheptulose, a specific HK inhibitor, blocks the severe repression triggered by 2-deoxyglucose and yet the phosphorylated products per se do not act as repression signals, we propose that HK may have dual functions and may act as a key sensor and signal transmitter of sugar repression in higher plants.
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TL;DR: Various factors pertaining to cold acclimation, promoter elements, and role of transcription factors in stress signaling pathway have been described, and the role of calcium as an important signaling molecule in response to various stress signals has been covered.
2,626 citations
Cites background from "Sugar sensing in higher plants."
...In response to a stress, the carbohydrate status of a leaf gets altered and this might serve as a metabolic signal in response to stress [111,112]....
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01 Jun 1997
TL;DR: The primary effect of plants response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency as discussed by the authors.
Abstract: ▪ Abstract The primary effect of the response of plants to rising atmospheric CO2 (Ca) is to increase resource use efficiency. Elevated Ca reduces stomatal conductance and transpiration and improves water use efficiency, and at the same time it stimulates higher rates of photosynthesis and increases light-use efficiency. Acclimation of photosynthesis during long-term exposure to elevated Ca reduces key enzymes of the photosynthetic carbon reduction cycle, and this increases nutrient use efficiency. Improved soil–water balance, increased carbon uptake in the shade, greater carbon to nitrogen ratio, and reduced nutrient quality for insect and animal grazers are all possibilities that have been observed in field studies of the effects of elevated Ca. These effects have major consequences for agriculture and native ecosystems in a world of rising atmospheric Ca and climate change.
1,906 citations
TL;DR: Differences among species that can be traced to different capacities for water acquisition, rather than to differences in metabolism at a given water status, are described.
1,838 citations
Cites background from "Sugar sensing in higher plants."
...It is recognized that sucrose and other sugars regulate the expression of many genes involved in photosynthesis, respiration, N and secondary metabolism, as well as defence processes, thus integrating cellular responses to stress (Koch, 1996; Jang and Sheen, 1997)....
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01 Jun 1996
TL;DR: The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes.
Abstract: Plant gene responses to changing carbohydrate status can vary markedly Some genes are induced, some are repressed, and others are minimally affected As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization Abundant sugar levels exert opposite effects through a combination of gene repression and induction Long-term changes in metabolic activity, resource partitioning, and plant form result Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes
1,727 citations
Cites background from "Sugar sensing in higher plants."
...Initial work on photosynthetic genes and their metabolic effectors is reviewed by Sheen (161) and discussed by Stitt et al (175)....
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...Microbial sugar-sensing mechanisms are an important resource for development of testable hypotheses in plants (50, 51, 69, 160)....
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...Sheen J. 1990....
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...Sheen J. 1994....
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...In at least one study, the effects of nonmetabolizable sugars were shown to be blocked by addition of mannoheptulose, an inhibitor of hexokinase (69)....
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TL;DR: AMP-activated protein kinase and SNF1-related protein kinases in higher plants are likely to be involved in the response of plant cells to environmental and/or nutritional stress.
Abstract: Mammalian AMP-activated protein kinase and yeast SNF1 protein kinase are the central components of kinase cascades that are highly conserved between animals, fungi, and plants. The AMP-activated protein kinase cascade acts as a metabolic sensor or "fuel gauge" that monitors cellular AMP and ATP levels because it is activated by increases in the AMP:ATP ratio. Once activated, the enzyme switches off ATP-consuming anabolic pathways and switches on ATP-producing catabolic pathways, such as fatty acid oxidation. The SNF1 complex in yeast is activated in response to the stress of glucose deprivation. In this case the intracellular signal or signals have not been identified; however, SNF1 activation is associated with depletion of ATP and elevation of AMP. The SNF1 complex acts primarily by inducing expression of genes required for catabolic pathways that generate glucose, probably by triggering phosphorylation of transcription factors. SNF1-related protein kinases in higher plants are likely to be involved in the response of plant cells to environmental and/or nutritional stress.
1,480 citations
References
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TL;DR: It is concluded that control is usually shared between Rubisco (which responds sensitively to CO2) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO2 concentration is doubled from 35 to 70 Pa.
Abstract: . In the first part of this review, I discuss how we can predict the direct short-term effect of enhanced CO2 on photosynthetic rate in C3 terrestrial plants. To do this, I consider: (1) to what extent enhanced CO2 will stimulate or relieve demand on partial processes like carboxylation, light harvesting and electron transport, the Calvin cycle, and end-product synthesis; and (2) the extent to which these various processes actually control the rate of photosynthesis. I conclude that control is usually shared between Rubisco (which responds sensitively to CO2) and other components (which respond less sensitively), and that photosynthesis will be stimulated by 25–75% when the CO2 concentration is doubled from 35 to 70 Pa. This is in good agreement with the published responses. In the next part of the review, I discuss the evidence that most plants undergo a gradual inhibition of photosynthesis during acclimation to enhanced CO2. I argue that this is related to an inadequate demand for carbohydrate in the remainder of the plant. Differences in the long-term response to CO2 may be explained by differences in the sink-source status of plants, depending upon the species, the developmental stage, and the developmental conditions. In the third part of the review, I consider the biochemical mechanisms which are involved in ‘sink’ regulation of photosynthesis. Accumulating carbohydrate could lead to a direct inhibition of photosynthesis, involving mechanical damage by large starch grains or Pi-limitation due to inhibition of sucrose synthesis. I argue that Pi is important in the short-term regulation of partitioning to sucrose and starch, but that its contribution to ‘sink’ regulation has not yet been conclusively demonstrated. Indirect or ‘adaptive’ regulation of photosynthesis is probably more important, involving decreases in amounts of key photosynthetic enzymes, including Rubisco. This decreases the rate of photosynthesis, and potentially would allow resources (e.g. amino acids) to be remobilized from the leaves and reinvested in sink growth to readjust the sink-source balance. In the final part of the review, I argue that similar changes of Rubisco and, possibly, other proteins are probably also involved during acclimation to high CO2.
1,031 citations
TL;DR: Cet article de synthese s'interesse au transport selectif, a la secretion de proteines, membranaires et autres, du reticulum endoplasmique vers l'appareil de Golgi.
Abstract: Cet article de synthese s'interesse au transport selectif, a la secretion de proteines, membranaires et autres, du reticulum endoplasmique vers l'appareil de Golgi. Ceci est possible grâce a des enzymes modifiant les chaines oligosaccharidiques des glycoproteines, des signaux de transport (mannose-6-phosphate et tetrapeptide KDEL
754 citations
"Sugar sensing in higher plants." refers methods in this paper
...To eliminate the possibility that the effect of 2-dG is due to a general inhibition of N-glycosylation, we treated transfected maize protoplasts with tunicamycin, another widely used inhibitor of N-glycosylation (Pelham, 1989)....
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TL;DR: It is shown that the transcriptional activity of seven maize photosynthetic gene promoters is specifically and coordinately repressed by the photosynthesis end products sucrose and glucose and by the exogenous carbon source acetate.
Abstract: Using freshly isolated maize mesophyll protoplasts and a transient expression method, I showed that the transcriptional activity of seven maize photosynthetic gene promoters is specifically and coordinately repressed by the photosynthetic end products sucrose and glucose and by the exogenous carbon source acetate. Analysis of deleted, mutated, and hybrid promoters showed that sugars and acetate inhibit the activity of distinct positive upstream regulatory elements without a common consensus. The metabolic repression of photosynthetic genes overrides other forms of regulation, e.g., light, tissue type, and developmental stage. Repression by sugars and repression by acetate are mediated by different mechanisms. The identification of conditions that avoid sugar repression overcomes a major obstacle to the study of photosynthetic gene regulation in higher plants.
714 citations
TL;DR: The techniques for electrical breakdown, electropermeabilization and electrofusion could be an important tool in this process, since the high electrical fields occurring naturally in the membrane play an important role in the selective transport of substances across the membrane as well as in natural regulatory processes.
Abstract: The considerable amount of activity in the field of electrofusion and electropermeabilization is very promising from the point of view of new insights into biomembranes and new technologies in the future for the production of new compounds and modification of cell systems for nutrition, energy production and the removal of waste products. It is particularly gratifying to see how basic science has provided the foundation for a useful technology, although in some cases the time needed to develop an application is very long. In other cases, it is necessary to overcome the difficulties posed by existing schools of thought which have been shown to be wrong. It is fascinating to observe the many developments and discoveries in the areas of physics, material science, space technology and electronics which are just waiting to be applied to biological systems. An increased interdisciplinary collaboration between physicists and biologists could provide considerable impetus to biology and its application in technology. However, this can only be achieved if basic research into biological membranes is accelerated. The techniques for electrical breakdown, electropermeabilization and electrofusion could be an important tool in this process, since we cannot rule out the possibility that the high electrical fields occurring naturally in the membrane play an important role in the selective transport of substances across the membrane as well as in natural regulatory processes.
680 citations
TL;DR: The chapter describes the silicone oil centrifugation that allows chloroplasts to be separated from the remainder of the protoplast and to be quenched within 2–3 sec of disrupting the protiplast.
Abstract: Publisher Summary This chapter focuses on the metabolite levels in specific cells and the subcellular compartments of plant leaves. Leaf material is difficult to fractionate because a typical plant cell contains several different, mechanically fragile subcellular organelles and is surrounded by a mechanically strong plant cell wall. Rather than attempting to isolate whole organelles or cells, leaves are frozen in liquid N 2 and then broken to small fragments that are enriched in material from a given compartment. These fragments are physically separated under conditions when the metabolic activity or redistribution of metabolites is prevented and subsequently their metabolism is quenched. The exclusion of water, or the use of extremely low temperatures, prevents metabolic activity during the fractionation procedures. The chapter describes the silicone oil centrifugation that allows chloroplasts to be separated from the remainder of the protoplast and to be quenched within 2–3 sec of disrupting the protoplast.
655 citations
"Sugar sensing in higher plants." refers background in this paper
...In yeast, 2-dG also causes strong repression at a low concentration (Zimmermann and Scheel, 1977; Ma et al., 1989)....
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...Hexokinase (HK) PII is considered to be the major sensing molecule of catabolite repression triggered by glucose (Entian, 1980; Entian and Frolich, 1984; Ma and Bostein, 1986; Ma et al., 1989; Rose et al., 1991)....
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...Although the regulatory domains have not been defined physically, catalytic activity is required for gene repression (Ma and Bostein, 1986; Ma et al., 1989; Rose et al., 1991)....
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