Microscopic precision in sugar transport

Sugar is essential for both plants and humans. It serves as an energy source, a signaling molecule, and a building block - hence, plants have evolved intricate mechanisms to control where sugar is transported within their tissues. This study focuses on the transport of sugars from the plant’s main vascular system to other areas such as leaves, fruits, and pollen.

In the plant’s vascular system, sugar flows predominantly as sucrose - a combination of glucose and fructose. Before it can be used by the cells, sucrose is broken down into its two components, at which point transport proteins step in.

“You can think of it a bit like our blood vessels,” explains PhD student Camilla Gottlieb Andersen, the study’s lead author. “Sugar flows through the plant's 'veins' as sucrose, but it needs to reach other places - like a fruit or a growing leaf. These cells aren’t directly connected to the main system, so specialized transporters, which we studied, move the sugars where they’re needed.”

The researchers discovered that the transporter STP6 has a broader sugar profile than STP10. STP6's ability to transport fructose is linked to its sugar-binding pocket being just slightly less precisely shaped. This small "looseness" allows it to accommodate multiple types of sugars.

“If a transporter is extremely good at recognizing one specific sugar, it also becomes highly selective. But if it’s just a little less precise, it can actually accept a wider range of sugars,” says Camilla Gottlieb Andersen.

Frog eggs and crystals: Digging deep in the lab

“We investigated two transport proteins, STP6 and STP10, which both move sugars into plant cells. Interestingly, STP10 only transports glucose, while STP6 can also handle fructose,” Andersen explains. “And these differences come down to something as tiny as a single amino acid in the binding pocket - though there may be other factors we don’t yet know.”

To understand how STP6 and STP10 operate, the researchers solved their 3D structures using crystallography and tested their functions in living systems. Among their tools were frog eggs - perfect single-cell test systems.

“It might sound odd, but frog eggs are invaluable in transport research. We can express our proteins in them and measure which sugars they take up. It’s one of the most precise methods available,” Andersen explains.

Crystallography is an advanced technique where proteins form tiny crystals that are then analyzed down to the atomic level. Though demanding and rarely used today because of its complexity, it offers unparalleled detail.

“It’s a bit old-school, but it gives us a resolution that lets us see exactly how the sugar sits in the pocket. That’s where we see the tiny difference that makes STP6 and STP10 behave so differently,” Andersen says.

Why does this matter?

While this is fundamental research, seemingly far removed from real-world application, the results point toward broader possibilities. Understanding sugar transport could lead to developing plants that use their energy more efficiently - and perhaps even defend themselves better against diseases.

“If we can understand how plants allocate sugars, maybe we can encourage them to produce more fruit, more pollen, or grow more efficiently,” says Andersen. “Imagine a strawberry plant that invests less energy in leaves and more in berries.”

Sugar transport also plays a role in plant defense. Certain pathogens, like rust fungi, feed on plant sugars. If a plant can restrict sugar access, it might become more resistant.

“We’ve seen that a mutation in a sugar transporter can make a plant immune to certain diseases. This opens up the possibility of tweaking these transporters to enhance the plant’s natural defenses,” Andersen explains.

The future of plant breeding and green transitions

Camilla Gottlieb Andersen emphasizes that this is basic science - but with significant potential for addressing climate challenges, food production, and sustainability.

“We’re not necessarily dreaming of GMOs. But if we can identify natural variants of plants with the ‘good’ versions of these sugar transporters, we could breed for those traits - or use CRISPR to give them a gentle nudge,” she says.

And in a world where food needs to be grown on less land - such as in urban farming or vertical greenhouses - this knowledge could be game-changing.

“Imagine a tomato plant that produces twice as many tomatoes because it uses its energy more wisely. That would be a major breakthrough for the agriculture of the future.”

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Conflicts of interest

The authors declare no competing interests.

The scientific article

PNAS: Comparative analysis of STP6 and STP10 rationalizes molecular selectivity in Sugar Transport Proteins

Camilla Gottlieb Andersen, Laust Bavnhøj, Søren Brag, Jan Heiner Driller, Anastasiia Bohush, Bjørn Panyella Pedersen. 

More information

PhD Student Camilla Gottlieb Andersen
Department of Molecular Biology and Genetics
Aarhus University
camillaga@mbg.au.dk

Professor Bjørn Panyella Pedersen
Department of Molecular Biology and Genetics
Aarhus University
bpp@mbg.au.dk