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How is Sucrose Synthesis in leaves coordinated with the supply of photosynthate and the rate of export to other tissues?

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Introduction

How is Sucrose Synthesis in leaves coordinated with the supply of photosynthate and the rate of export to other tissues? The instant light touches a plant it must undergo changes in its metabolism to accommodate for the influx of CO2 that enters into the Calvin cycle. This complicated set of adjustments requires very fine regulation in order to respond quickly and accurately to maintain the constant flow and net fixation of Carbon. In this essay I shall describe how the synthesis of the major products of Carbon fixation, sucrose and starch, are regulated in response to the supply of photosynthate and the rate of export to other tissues. I shall outline the biochemical steps resulting in their synthesis and how the elements within these pathways contribute to the overall regulation of the system; starting with the kinetic regulation provided by the Triose Phosphate Transporter (TPT) and moving onto the enzymic regulation provided by Fructose-1,6-bisphosphatase (FBPase) and Sucrose-phosphate Synthase (SPS). Background The Calvin cycle is responsible for taking 3CO2 and one inorganic phosphate (Pi) and creating a triose phosphate (C3-P either GA-3P or DHAP). In order to maintain the cycle 5/6th of the carbon in the triose phosphates must be immediately recycled to regenerate Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco). The remaining 1/6th is selectively outputted into one of two main photosynthate pathways resulting in the creation of a molecule that can be broken down for use in respiration; starch, a large carbohydrate (CHO) ...read more.

Middle

However it is important to note that we are making a distinction between the effects an enzyme has on the flux of a pathway (it's individual control) and how that pathway is regulated (the model of interactions). There is a world of difference between the in vitro kinetics of an enzyme and the way in which it behaves in vivo. Merely isolating the enzymes is a hugely difficult task but this is compounded by the huge rates of turnover seen in the intermediates of the pathways investigated. Stitt (1987) discovered that within 2-3 seconds of darkening a leafs metabolites have changed so much that they resemble those found in the leaf at the middle of the night rather than one that is photosynthesising. Initially techniques such as freeze clamping (Stitt 1990) were used but this did not provide the split second cessation of metabolism required to perform studies in vivo. Stitt proceeded to develop techniques involving centrifugation quenching that allowed for metabolism to be studied in protoplasts in very small time steps allowing for analysis of regulation. This was not the answer to everything though since the enzymes in question will undoubtedly be modulated by a wide range of effectors, experiments had to be carefully planned and controlled to obtain a response to only one factor. Once the enzymes that made up the sucrose synthesis pathway had been elucidated through in vitro analysis, focus moved over to determining the regulation of the pathway. Studies of the in vitro kinetics of the individual enzymes isolated two enzymes that were good candidates for regulation. ...read more.

Conclusion

There is a degree of amplification seen in the system where not only are the allosteric effectors acting on the "fine" modulation of SPS but also they are allosteric effectors of the "coarse" control components. This allows the system to respond very sensitively to the changes in metabolic intermediates. To my knowledge no transgenic experiments have been performed using SPS modification and so the information here is still only model and has yet been proved conclusively. Discussion. This essay has only briefly described some of the known control components of sucrose synthesis and how it is coordinated with photosynthesis and the levels of photosynthate. But from the information given two major concepts of regulation have arisen. Firstly that there is a "feed forward" regulation in the system where the intermediates of one step act as the allosteric activators of enzymes further downstream. This coordinates sucrose synthesis with the rate of photosynthesis. The next component is a "feed back" regulation which responds to the levels of free Pi in the system. Due to the kinetics of the TPT the levels of triose phosphates in the chloroplast will increase if the sucrose synthesis pathway is not able to liberate enough Pi and this cross-talk between the sucrose synthesis pathway and the Calvin cycle allows for coordinated repartitioning of triose phosphates into starch. The system provides for a lot of very fine control and is able to respond to a wide range of changes in the environment and its use of amplification in certain scenarios allows for a rapid and sensitive response. ...read more.

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