How Sugar Beet Roots Import Sugar and Glucose Efficiently

Jim Crocker
12th April, 2024

How Sugar Beet Roots Import Sugar and Glucose Efficiently

Image Source: Natural Science News, 2024

Key Findings

  • Study at Julius-Maximilians-Universität Würzburg identifies key sugar transporters in sugar beets
  • STP13 is a high-affinity transporter, sensitive to sugar levels and can move both glucose and sucrose
  • STP13's flexibility and cold tolerance may help improve sugar beet cultivation and efficiency
Sugar beets, the root vegetable that is a key source of sugar in Europe and North America, have long fascinated scientists due to their ability to store high concentrations of sucrose—about 20% by weight in their taproots. Understanding how these plants accumulate sucrose is not just a matter of academic curiosity but has significant implications for agriculture and bioenergy production. A recent study by researchers at Julius-Maximilians-Universität Würzburg[1] has shed light on the mechanisms sugar beets use to transport sucrose into their cells, a discovery that could lead to more efficient sugar production. Previous research had identified a transporter, known as TST, which is responsible for moving sucrose into the vacuoles of the taproot where it is stored[2]. However, the transporters involved in getting sucrose across the plant cell's outer membrane—the plasma membrane—remained a mystery. This step is crucial because it is the first point of entry for sucrose into the taproot cells where it will eventually be stockpiled. The Würzburg team tackled this problem by first looking at how sugar beet taproot cells respond to sucrose. They found that adding sucrose to these cells triggered a flow of protons into the cell and a subsequent change in electrical charge across the plasma membrane, suggesting that sucrose uptake was linked to a proton, which is a positively charged particle. This is consistent with a transport mechanism known as a sugar/proton symport, where the movement of sucrose into a cell is coupled with the movement of protons. To pinpoint the specific transporters involved, the researchers conducted a comprehensive analysis of the taproot's gene activity, which led them to two promising candidates: PMT5a and STP13. Further experiments were needed to determine the roles of these proteins, so the team turned to a technique where the transporters are expressed in the oocytes (egg cells) of the African clawed frog, Xenopus laevis, a common method for studying the function of plant transport proteins. Their findings were intriguing. PMT5a turned out to be a low-affinity glucose transporter, which means it prefers to move glucose over sucrose and is less sensitive to changes in sugar concentration. It also relies on both voltage and protons to function but does not transport sucrose. On the other hand, STP13 was identified as a high-affinity transporter, which means it is very sensitive to sugar concentration and can transport both glucose and sucrose, though it has a preference for glucose. STP13 also proved to be more tolerant to cold than PMT5a, which could have implications for sugar beet cultivation in colder climates. The study's use of structural modeling to visualize how STP13 interacts with different sugars was particularly innovative. This approach suggested that the protein has a flexible binding site that can accommodate both simple sugars like glucose and larger molecules like sucrose. This flexibility could explain how STP13 is able to transport different types of sugars into the cell. The identification of these transporters is a significant step forward in our understanding of sugar transport in plants. It builds on earlier work that had characterized other aspects of sugar transport, such as the sucrose-H+ symport mechanism in the plasma membrane of sugar beet leaves[3], and the regulation of sucrose transport by proteins and gene expression patterns in other plant species[4][5]. This new study extends our knowledge to the important process of sucrose uptake in sugar beet taproots and opens up possibilities for breeding more efficient sugar-producing plants. By improving the efficiency of sugar transporters like STP13, it may be possible to increase the amount of sucrose that sugar beets can store in their taproots, which would make the crop more productive. This, in turn, could lead to a more sustainable source of sugar and bioenergy, reducing our reliance on fossil fuels and helping to mitigate the effects of climate change. The work of the Julius-Maximilians-Universität Würzburg team is thus not only a triumph of plant biology but also a promising development for the future of sustainable agriculture and energy.

GeneticsBiochemPlant Science

References

Main Study

1) Sugar beet PMT5a and STP13 carriers suitable for proton-driven plasma membrane sucrose and glucose import in taproots.

Published 11th April, 2024

https://doi.org/10.1111/tpj.16740

Related Studies

2) Surcose transport in isolated plasma-membrane vesicles from sugar beet (Beta vulgaris L.) Evidence for an electrogenic sucrose-proton symport.

https://doi.org/10.1007/BF00391867

3) Proton-Coupled Sucrose Transport in Plasmalemma Vesicles Isolated from Sugar Beet (Beta vulgaris L. cv Great Western) Leaves.

Journal: Plant physiology, Issue: Vol 89, Issue 4, Apr 1989

4) Source-Sink Regulation Is Mediated by Interaction of an FT Homolog with a SWEET Protein in Potato.

https://doi.org/10.1016/j.cub.2019.02.018

5) Sucrose Transporter ZmSut1 Expression and Localization Uncover New Insights into Sucrose Phloem Loading.

Journal: Plant physiology, Issue: Vol 172, Issue 3, Nov 2016


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