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Enhancing Photosynthesis with Sugar Signals

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Enhancing Photosynthesis with Sugar Signals Opinion TRENDS in Plant Science Vol.6 No.5 May 2001 197 to sophisticated fine control mechanisms represented Enhancing the best targets because they exerted the most control on flux. In the Calvin cycle, through which carbon enters the reactions of photosynthesis and photosynthesis with metabolism, there are four key irreversible enzymes: Rubisco, in the carboxylation phase of the cycle, and fructose-1,6-bisphosphatase (FBPase), sugar signals sedoheptulose-1,7-bisphosphatase (SBPase) and phosphoribulokinase (PRK), in the regenerative phase (Fig. 1). In experiments on transgenic tobacco and Matthew Paul, Till Pellny and Oscar Goddijn potato expressing antisense cDNA to each of these enzymes, a decrease of 50% maximum catalytic enzyme activity had minimal impact on the rate of Photosynthesis has long been a target in the quest to maximize crop photosynthesis under standard growth-room productivity to feed burgeoning populations. Recent evidence suggests that conditions1–4. The most extreme of these is PRK, 85% improved photosynthetic performance can be most easily achieved by of whose maximum catalytic activity can be removed modifying sugar-signalling mechanisms that control the expression of genes before there is any effect on photosynthesis4. This is for whole pathways and processes that determine photosynthetic capacity and because sophisticated fine control of PRK can source–sink balance, rather than by directly targeting individual ‘key’ enzymes. overcome a large genetic decrease in PRK protein4,5. Here, we highlight recent progress and support for the hypothesis that genetic The full story of control of Calvin cycle flux comes from modification of trehalose metabolism through its interaction with sugar- analysis under a range of conditions that plants are signalling pathways can enhance photosynthetic capacity. likely to encounter in the natural environment6,7. This emphasizes that control exerted on irreversible Calvin For a long time, one of the aims of plant scientists has cycle enzymes occurs through allosteric and post- been to improve photosynthesis to maximize crop translational control mechanisms, and that targeting productivity. The logic of the argument is clear: the these enzymes for genetic modification is problematic yield of crops broadly relates to the amount of light because of the counteracting fine control mechanisms. intercepted and the cumulative photosynthesis over a Another way in which control of enzyme activity is growing season. If photosynthesis could be increased exerted is through expression level. The catalytic then it would be possible to capitalize on and extend activity of reversible enzymes not subject to fine this relationship. Breeding and agronomic practice control is regulated in this way, and research using have effectively achieved this over the past century by transgenic plants has shown that reversible enzymes creating the conditions under which photosynthesis have the potential to exert a high degree of control. can flourish. New varieties have been bred with bigger This is true for aldolase, for example7. However, it is assimilate sinks and with better coordination of leaf not just the catalytic activity of reversible enzymes production with the amount of light available during that is controlled by gene regulation. A second the growing season. High fertilizer use has further important conclusion of plant science over the 1990s, facilitated a high photosynthetic rate and, in the which developed from several approaches and has horticulture industry, ‘fertilization’of tomato crops been confirmed by work on the response of plants to with CO2 is used routinely to maximize carbon fixation. elevated CO2, is that the control of photosynthesis at As our knowledge of plant biochemistry, the molecular level by sugar and nitrogen signals metabolism and, latterly, molecular biology has through changes in whole plant carbon–nitrogen increased, our sights have shifted towards more balance8–10 overrides the control of photosynthesis by targeted manipulation of metabolism itself. If we other mechanisms. This gives us clues to how to could find out which are the ‘key’ enzymes responsible proceed with the genetic modification of for controlling photosynthetic rate or those that lead photosynthesis. Direct genetic manipulation of to photorespiratory losses of the CO2, it would be photosynthesis by targeting key enzymes, even if it possible to target these enzymes to improve were possible to overcome post-translational control Matthew Paul* photosynthesis. Isolation of the genes and genetic mechanisms and the flexibility of metabolism and to Till Pellny transformation would enable photosynthetic carbon increase the photosynthetic rate, could be frustrated Crop Performance and acquisition to be increased directly. The reasoning by the overproduction of a sugar signal that repressed Improvement, IACR- behind this was good, as far as we knew at that time. expression of photosynthetic genes. To achieve Rothamsted, Harpenden, Hertfordshire, UK AL5 2JQ. significant positive changes in photosynthesis and *e-mail: The last will be first and the first last in metabolic resource allocation, it will be necessary to understand [email protected] regulation the signals and molecular mechanisms that control Oscar Goddijn Genetic modification of plant metabolism proceeded the expression not just of aldolase, PRK or Rubisco, Syngenta Mogen, apace during the 1990s and several strong messages but of whole pathways and suites of genes that Einsteinweg 97, 2333 CB have come from this. One is that our concept of ‘key’ determine photosynthetic capacity. These Leiden, PO Box 628, 2300 AP Leiden, The enzymes has changed drastically. It was thought that mechanisms underpin source–sink interactions and Netherlands. enzymes that were essentially irreversible and subject productivity in plants. http://plants.trends.com 1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)01920-3 198 Opinion TRENDS in Plant Science Vol.6 No.5 May 2001 homologies to microbial TPS genes can be found in Ru5P PRK CO2 the Arabidopsis genome, one of which has been cloned and shown to synthesize T6P (Ref. 12). This Isomerase 5 Ru1,5BP raises the question of what the function of these 5 R5P 5 Epimerase Rubisco genes and of trehalose metabolism in plants are. Xu5P In yeast, trehalose metabolism performs a signalling role13, although the precise mechanistic Transketolase 3PGA 3 5 details of this are still being unravelled, as are Glycerate-3-P kinase interspecific differences between yeast species and other microorganisms14. Evidence from our current S7P 7 Calvin cycle research suggests that a similar role has persisted 3 1,3BPGA 15 SBPase in plants . In experiments in which E. coli otsA and otsB genes, which encode TPS and TPP, Glyceraldehyde-phosphate S1,7BP 7 dehydrogenase respectively, were expressed in transgenic tobacco (Nicotiana tabacum), there were effects consistent E4P with an effect on sugar signalling. In plants Aldolase GAP 3 expressing E. coli TPS, rates of photosynthesis per 4 Transketolase Triose phosphate unit leaf area are increased by up to a third under isomerase saturating irradiance; in plants expressing TPP, Aldolase 15 F6P 6 photosynthesis is decreased by a similar amount . This shows that it is possible to increase the FBPase F1,6BP 6 DHAP Triose phosphate photosynthetic capacity of plants. Measurements are exported 3 continuing to determine the biochemical and TRENDS in Plant Science molecular basis of these results but the data are consistent with the hypothesis that genetic Fig. 1. The Calvin cycle of plants, the entry point of carbon in photosynthesis and metabolism and modification of trehalose metabolism has affected initial entry point of carbon into the food chain. The cycle is composed of 13 reactions catalysed by photosynthetic capacity through sugar signalling. 11 enzymes, and consists of three distinct phases. The first phase is carboxylation (blue arrows), catalysed by Rubisco. The second phase is the reductive part (red arrows), during which ATP and Our model to explain the findings centres on T6P. NADPH derived from the light reactions are used to convert 3-phosphoglyceric acid (3PGA) into In yeast, T6P has a role in controlling glycolysis and glyceraldehyde phosphate (GAP). The third phase (black arrows) regenerates ribulose-1,5- sugar signalling through its interaction with bisphosphate (Ru1,5BP) for the carboxylation reaction. Enzymes that are essentially irreversible hexokinase, a putative sensor in yeast and plants13. and that are traditionally classified as ‘key’ enzymes are highlighted in red [Rubisco, fructose-1,6- bisphosphatase (FBPase), sedoheptulose-1,7-bisphosphatase (SBPase) and phosphoribulokinase T6P has not been previously found in plant tissue, (PRK)]. Reversible enzymes are highlighted in blue. Abbreviations: 1,3BPGA, although this might be because researchers have not 1,3-bisphosphoglyceric acid; 3PGA, 3-phosphoglyceric acid; DHAP, dihydroxyacetone phosphate; been looking and because methods have not been E4P, erythrose-4-phosphate; F1,6BP, fructose-1,6-bisphosphate; F6P, fructose-6-phosphate; GAP, sensitive enough to detect it. We have developed a glyceraldehyde phosphate; R5P, ribose-5-phosphate; Ru1,5BP, ribulose-1,5-bisphosphate; Ru5P, ribulose-5-phosphate; S1,7BP, sedoheptulose-1,7-bisphosphate; S7P, sedoheptulose-7-phosphate; sensitive assay for T6P in plants, using a hexokinase Xu5P, xylulose-5-phosphate. Numbers highlighted
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