Researchers
have discovered a mechanism for the selective transportation of sugar
and hormones in plants. The results also clarify that sugar
transportation is necessary for male fertility in plants, which means
pollen production. The findings were published in Proceedings of the National Academy of Sciences.
Transporters
are proteins that exist on cell membranes. Each transporter binds to a
substrate and carries it across the cell membrane. Some transporters
carry nutrition, while others carry molecules that, for example, help
cells to communicate with each other. One class of transporters found in
both plants and animals is Sugars Will Eventually Be Exported
Transporter (SWEET). SWEET distributes sugar, such as sucrose, in
plants. Once the SWEET binds to sucrose, it moves across the plant,
distributing it so that the plant can get nutrition and grow. Here,
sucrose, which is carried by the transporter, is called a substrate.
Importantly,
SWEET has more than one substrate. For example, some types of SWEET in
Arabidopsis plants also transport a hormone called gibberellin (GA),
which controls plant growth and reproduction. These types of SWEET have
the ability to transport sucrose and GA, but the physiological relevance
of their respective transport activities is not understood. Both
sucrose and GA are essential for plant growth and development, and each
has a unique structure. “How SWEET can bind to both of these different
structures, and how SWEET selects to transport sucrose or GA, were not
clear”, said the senior author.
Furthermore,
when some types of SWEET are mutated in plants, the plant becomes male
sterile, meaning that it cannot produce pollen. However, the reasons for
this sterility were unclear. It may have been due to a lack of sucrose
transport, a lack of GA transport, or both. To examine these questions, a
research team combined molecular dynamic simulation with classic
genetic methodologies. In their experiments, they focused on a type of
SWEET called SWEET13, which is known to transport both sucrose and GA,
and is also necessary for male fertility.
To
understand how sucrose and GA bind to SWEET and are transported by it,
they first performed a technique called molecular dynamic simulation.
This technique simulated a docking between SWEET13 and sucrose, and a
docking between SWEET13 and GA, based on their structures. Using these
computational simulations, the researchers predicted amino acids in
SWEET13 that recognize sucrose and GA. Subsequently, using cell
cultures, they confirmed whether these amino acids were necessary for
the sucrose and GA transport activities of SWEET13. A particular site of
SWEET13 with an amino acid called asparagine and serine was important
for sucrose and GA transport activities, respectively.
Next,
to clarify how the male sterility occurred, the researchers replaced
one of the aforementioned amino acids, generating SWEET that transports
only sucrose and SWEET that transports only GA. With SWEET that
transports only sucrose but not GA, the plant produced pollens. However,
with SWEET that transports only GA but not sucrose, the plant failed to
produce pollens. These results mean that it is SWEET’s transport of
sucrose, not GA, that is necessary for pollen production.
“By
combining molecular docking, molecular dynamics simulation and genetic
methodologies, we successfully tested the selectivity of substrates
[i.e., sucrose and GA] separately”, said the author. In addition, there
are other transporters besides SWEET that carry multiple different
substrates. “Our methodologies can examine other transporters and
substrates, and that would advance research on transporters”, the author
said. Understanding more about sugar and hormone transport in plants
could lead to improved agricultural methods that are more efficient and
sustainable than current practices.
https://www.pnas.org/doi/epdf/10.1073/pnas.2207558119
http://sciencemission.com/site/index.php?page=news&type=view&id=publications%2Fsweet13-transport-of&filter=22