Sugars are found ubiquitously in plants and are regulated.

For sucrose it's pretty straightforward - it's basically kept at a low ish level, and put into storage or other intermediate compounds. These act to inhibit or promote other parts of the calvin cycle of sugar production (fructose 2,6 bisphosphate especially iirc).

Fructose is a bit more obscure. What are the regulatory feedback loops involved?



In short, fructose kinases (like Fru6P,2-kinase) and fructose phosphotases (such as Fru2,6bisP) regulate and are in turn regulated by fructose (specifically Fru6P) levels in plants. There are also several metabolites that regulate the rate at which those fructose enzymes function creating quite a complex fructose regulatory network. This phosphorylation can make fructose unavailable or available to proteins as well as providing energy in the process (and aparently the rate of this can be controlled effectively in vivo (Stitt, 1990)).

I haven't come across any cell uptake regulation, but remember plants are physiologically very different to humans and there may not be a like for like system in this case. Although I suspect there will be since in plants fructose can be concencentrated in fruiting bodies. Perhaps another answer, or a major edit to this answer can explain that.

Fructose is often referred to as a regulatory metabolite in plants i.e levels of fructose affect other pathways due to its phosphorylated state. However, there are lots of sugar regulating systems in plants. Although this article focuses more on glucose and galactose, it emphasises the idea of a regulatory web rather than a cycle when it comes to plants and sugar regulation. Below the studies are based on plants such as spinach and other leafy vegetation, not fruiting plants I'm afraid!

Kinase Control

3PGA, 2PGA, PEP, dihydroxyacetone P, and the 2-carbon intermediate glycollate-2-P are all metabolites that control phosphorylation that inhibit the kinase, whilst Pi activates the kinase. Interestingly PPi also inhibits the kinase by competing with ATP. As one can imagine, this causes sigmoidal ATP saturation kinetics and is clearly indicative of regulation.


Fru6P and Pi both inhibit Fru2,6bisP hydrolysis by FRU2,6PASE.

This article from 1996 has a figure showing just how complex sugar pathways can be. To expand on your question, we can see how fructose phosphorylation plays a role in regulating in Sucrose and Starch glycolysis (Plaxton, 1996).

Disclaimer: I haven't studied this since undergraduate level, so constructive comments are welcome and I encourage people to edit this answer to improve it and help it more aptly answer the question!


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