In most plant species, sucrose is the main form of assimilated carbon produced during photosynthesis. Sucrose is essential for plant growth, as it provides a source of carbon to produce new molecules, but also energy for the plant cells. Sucrose has also an associated role as a signalling molecule, by regulating the growth of new organs, accumulation of storage proteins, and flowering in plants. Long-distance sucrose distribution from the green source tissues, generally leaves, to energy-demanding sink tissues (flowers, fruits, new organ in formation) is mediated by a specific and highly modified vascular tissue called phloem. The transport of sucrose in the phloem is an active transport, as sucrose is loaded in the conductive tissue by specific proteins from the SUC/SUT family. The SUC1 transporter from A. thaliana is located on the membrane of cells and use the proto-motive force to drive the loading of sucrose. Despite their key role in plants, the working mechanism of these SUCs transporters is not yet well understood.
A team of researchers from the Aarhus University recently published a new study in Nature Plants to understand the precise mechanism of action of the SUC1 transporter. They used X-ray diffraction data collected at I04 and I24 beamlines at Diamond to determine the 3D structure of this transmembrane protein. They wanted to understand how SUCs protein recognise sucrose, and how transport is proton coupled. As sugar transport is a key feature in plants, understanding how proteins can fine-tune the sugar concentration in conductive tissue is fundamental. Lead author of this study, Dr Bjørn Panyella Pedersen explained:
Active sucrose transport and loading into the phloem determines the turgor pressure. This pressure creates the vascular flow of nutrients (sucrose and all other components of the sap), and determines which parts of the plant will grow in response to environmental signals. Ultimately, we hope our research will help to augment control of growth and morphology in plants.
For their study, the team used a well-known plant model, Arabidopsis thaliana. This plant is widely used as model because it has a sequenced and annotated genome, and huge collections of mutant lines exists, allowing characterisation of plants where a specific gene is not expressed. Furthermore, this plant has a fast life cycle and produces numerous seeds.
In this study, the researchers present the structure of SUC1, and key elements to explain both the recognition of sucrose by the transporter, and the active transport by proton coupling. They produced SUC1 transporters and performed in-vivo assays to determine if the protein was functioning, and then proceed to solving the structure at the microfocus beamline I24. Dr Bjørn Panyella Pedersen says:
We have used Diamond’s beamlines I24 and I04 for our research since 2014, both in person and by remote data collection. We have always been very happy with the support and quality of the beamlines at Diamond. Brexit and the Corona years have made our access to the facility more challenging at times but with the help from the support staff we have been able to maintain our work at Diamond.
As already mentioned, the SUC1 activity is using the proto-motive force to function. As such, it requires an acidic amino acid in the transmembrane domains. The authors showed that the protein possessed only one amino acid that is consistent with this description (the aspartate located at position 152). After modifying this amino acid, the protein completely lost its function, showing that this specific aspartate is critical for the function of SUC1. Dr Panyella Pedersen noted:
For our structural biology efforts we find that access to state-of-the-art beamlines and cryoEM microscopes are paramount. Diamond provides both and has stayed a frontier research infrastructure since the inception of the research group. We will continue our work to understand the molecular mechanisms behind transporters and we hope Diamond will play a big role in this.
The research done by this team is important, as it highlights a major process in plants. Sucrose has many functions in plants, understanding how the transport is done is a first step to decipher more complex interactions between sucrose and regulatory pathways.
To find out more about the I24 beamline or discuss potential applications, please contact Principal Beamline Scientist Robin Owen: [email protected].
To find out more about the I04 beamline or discuss potential applications, please contact Principal Beamline Scientist Ralf Flaig: [email protected].
Bavnhøj, L., Driller, J.H., Zuzic, L. et al. Structure and sucrose binding mechanism of the plant SUC1 sucrose transporter. Nature Plants (2023). https://doi.org/10.1038/s41477-023-01421-0
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