Photosynthesis result in the synthesis of two carbohydrates in plants, sucrose and starch. In photosynthesis, light energises the carboxylation of carbon dioxide (CO2) in the presence of water to give carbohydrates (sugars) and oxygen (O2). Understanding the bio-synthesis pathways and also the way in which these carbohydrates are regulated can lead to plants bio-engineers being able to manipulate whole pathways in order to produce more sucrose, as will be the case in sugarcane research or to produce more starch as is the interest in potatoes. Sucrose and starch are products of two physically separated gluconeogenic pathways, sucrose in the cytosol and starch in the chloroplast. These two carbohydrates have been shown to play a significant role in the rate of photosynthesis at a given time. Sucrose and how it regulates photosynthesis will be discussed in the article.
How sucrose regulates photosynthesis
Sucrose is synthesised from triose phosphates imported from the chloroplast. Biosynthesis occurs via 11 reactions catalysed by different enzymes. Out of these eleven reactions, the effect of two enzymes fructose 1,6-bisphosphatase ( FBPase) and sucrose phosphate synthase (SPS), on photosynthesis have been well researched.
The key reaction that is the hydrolysation of fructose-1,6-bisphosphate to fructose-6-phosphate which is catalysed by FBPase has been found to involve a key cellular signalling molecule, fructose-2,6-bisphosphate (Nielsen etal.,2004).This molecule is regulated by a bifunctional enzyme , fructose-6-phosphate,2-kinase(F6P,2K)/fructose-2,6-bisphosphatase(F26BPase) (Nielsen etal.,2004).This regulating substance regulates partitioning of carbon in the cell towards either sucrose or starch (Nielsen et al., 2004, Taiz and Zeiger, 2006).Studies done on transgenic plants show that high levels of fructose-2,6-bisphosphate in leaves favours the portioning of carbon towards starch synthesis and low levels at low levels sucrose biosynthesis is favoured (Nielsen et al., 2004).This partitioning is due to a response to the hexose phosphate pool (i.e. fructose-6-phosphate) as high levels of fructose-6-phosphate have been shown to increase the fructose-2,6-bisphospahte concentration, which then limits the hydrolysation of fructose-1,6-bisphosphate to fructose-6-phosphate,by inhibiting the action of FBPase and thus limiting the production of fructose-6-phosphate and thus sucrose synthesis is decreased and starch synthesis is increased (Taiz and Zeiger, 2006)
SPS on the other hand, regulates sucrose synthesis by being activated or inhibited by glucose-6-phosphate and inorganic phosphates (Pi) respectively (Taiz and Zeiger, 2006). Glucose-6-phosphate deactivates the kinase SnRK1, which in turn stimulates the activation of sucrose phosphate phosphatise (SPP) resulting in increased sucrose synthesis whereas Pi activates SnRK1 which inhibits SPS resulting in decreased levels of sucrose synthesis (Taiz and Zeiger, 2006).
This is thus signal transduction showing that sugars can act as signaling molecules for their own synthesis. Three SPS gene families have been isolated in plants (Lunn and MacRae, 2003), and this in a way goes to show that SPS indeed is essential as plants even ensure its presence by having more than one gene family encoding it. Sucrose is transported to different parts of the plant to give carbon thus energy to these organs and this organs are regarded as sinks as they cannot synthesise sucrose and the leaves act as the source as photosynthesis takes place in the leaves. Thus sugar concentrations required by these sink organs also drives photosynthesis as more sucrose will be produced if necessary and less will be produced. Other organs of plants stores carbon in the form of starch and thus these organs need a lot of sucrose in their cytosol to be converted to starch and thus also drives photosynthesis or carbon metabolism.
Obviously this is not the whole story behind how carbohydrates regulate photosynthesis. It is apparent that no one enzyme involved is sucrose synthesis or rather to put it broadly, in the photosynthetic pathway, acts on its own to regulate the rate of photosynthesis, they rather are affected by the availability of products from other reactions. Thus different enzymes which catalyses these reactions are involved. This goes on to show that whole pathways have to be manipulated in order to increase carbon production by photosynthesis, and that the production level of this carbohydrates h goes back again to regulate the photosynthetic rate.
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