Recently, a breakthrough in the field of brassinosteroid transportation has been achieved by Professor Sun Linfeng's team from the University of Science and Technology of China (USTC) in collaboration with Professor Eugenia Russinova's team from Ghent University in Belgium. They have identified the first transporter protein for brassinosteroids, namely the Arabidopsis ABCB19 protein, which transports brassinosteroids out of the cell.
This finding fills a crucial gap in the field of brassinosteroid transportation and holds significant implications for understanding plant growth, development, and agricultural production. The research results were published in Science on March 22nd.
Sun Linfeng (center) and team members analyzing the electron microscope structure of the ABCB19 protein. Photo by Zhou Xinyu.
Unraveling a mystery unsolved for over 80 years
Brassinosteroid, also known as "brassins," has long been a challenge to identify due to its extremely low levels in plants.
As early as 1941, American scholars discovered that corn pollen extracts could promote soybean growth, but the exact reason was unknown at the time. In 1970, they extracted an active substance from rapeseed pollen, which similarly stimulated soybean growth, naming it "brassinolide."
It wasn't until 1979 that researchers, through optimized purification processes, extracted 4 milligrams of the active substance from 227 kilograms of rapeseed pollen. They used techniques such as X-ray and mass spectrometry to analyze the chemical structure of the active substance, finally unveiling its mysterious identity as "brassinolide."
With further research, scientists gradually realized the importance of brassinosteroids in plant growth regulation. In 1996, it was classified by the academic community as the sixth major class of plant hormones, following auxins, abscisic acid, cytokinins, ethylene, and gibberellins.
Despite its low abundance in plants, brassinolide exhibits significant efficacy. "It can regulate various aspects of plant growth, flowering, breeding, and enhance plant adaptability to stresses such as drought, salinity, pests, and diseases, which are crucial for plant development and survival," explained Sun Linfeng, co-corresponding author of the paper.
The application of brassinolide is also very cost-effective. Just a trace amount of brassinolide can significantly increase the yield of economic crops. It is now widely used in agricultural production.
As living standards improve, people demand higher safety and quality standards for agricultural products. Sun Linfeng believes that brassinolide, as an efficient, broad-spectrum, non-toxic, and harmless new type of plant growth regulator, will have vast application prospects.
Brassinolide is synthesized inside cells and then transported outside the cells to exert its effects. However, for over 80 years, who is responsible for the transportation process and what it entails has been a mystery.
In this study, Sun Linfeng's team unexpectedly discovered the first "transporter" of brassinolide, the Arabidopsis ABCB19 protein, while investigating the transport process of the first major class of plant hormones, auxins.
Unexpected discovery of the "hidden function" of ABCB19 protein
In previous studies, the Arabidopsis ABCB19 protein had been regarded as the "transporter" of auxins, a result widely accepted in the academic community.
However, in this study, Sun Linfeng's team made a new discovery.
"At first, we were trying to explain how the ABCB19 protein binds to and transports auxins, but experiments showed that the protein's binding and transport of auxins were not significant," said Sun Linfeng. Due to the lack of satisfactory experimental results, this work was temporarily shelved.
Later, during a discussion with colleagues in plant biology, Sun Linfeng was inspired.
"Suppose the ABCB19 protein transports auxins. In that case, when it is disrupted, mutant protein plants would exhibit characteristics of auxin signal defects. However, in experiments, we noticed that ABCB19 protein mutant plants did not show consistent phenotypic changes with other auxin transport protein mutant plants," Sun Linfeng said.
So, the team boldly questioned whether the ABCB19 protein could transport other plant hormones besides auxins?
Challenging established authority is a significant challenge, but the team decided to give it a try.
Generally, proteins require energy to transport plant hormones. Therefore, by monitoring energy consumption, one can preliminarily determine whether the protein reacts with plant hormones.
First, the team added auxins to the ABCB19 protein and found that it did not promote energy consumption by the protein, confirming that the protein did not transport auxins well.
Similarly, the team monitored the energy consumption of the ABCB19 protein in various plant hormones such as gibberellins and brassinolide. "Surprisingly, we observed that the energy consumption of the ABCB19 protein was significantly abnormal in brassinolide," said Sun Linfeng.
They established a system using radiolabeled tracking substances to track the movement of substances, effectively implanting a "locator" on brassinolide, proving that the ABCB19 protein can transport brassinolide in an artificially constructed simulated cell environment.
To further understand the appearance of the ABCB19 protein, the team used cryo-electron microscopy technology to analyze its high-resolution three-dimensional structure, observing intuitively how the ABCB19 protein binds to and transports brassinolide.
Sun Linfeng's team, in collaboration with Eugenia Russinova's team, further confirmed in plant cells that the ABCB19 protein can transport brassinolide and promote its signaling regulatory effects.
"These results unexpectedly revealed the 'hidden function' of the ABCB19 protein and clarified many previously observed differences in phenotype, prompting us to re-examine the physiological role of this protein," said Sun Linfeng.
Filling the research gap in brassinolide signaling pathways
Food security is a major strategic issue for the country. Sun Linfeng said, "Basic research on plant hormones contributes to increased agricultural productivity and efficiency."
Since joining the University of Science and Technology of China and establishing his team in 2017, Sun Linfeng has focused on "transport and signal regulation of plant hormones." In recent years, they have studied the transport processes of important plant hormones, unveiling the appearance of several key "transporters" through cryo-electron microscopy technology and using various biochemical research methods to answer how they transport plant hormones.
The team has revealed the structure of the auxin "carrier" protein, PIN1, shedding light on fascinating biological phenomena like plant "phototropic growth." Their findings, published on August 2, 2022, in Nature, offer a clearer understanding.
Building on this, the team explored targeted inhibitors of PIN proteins, discovering that the anti-inflammatory and analgesic molecule, naproxen, can inhibit PIN protein function. This paves the way for safer and more effective pesticide development, as detailed in their July 2023 publication in Plant Communications.
In 2023, the team also elucidated the three-dimensional structure of the plant hormone abscisic acid "carrier," ABCG25 protein, with results published in Nature Plants.
It's fair to say that the Sun Linfeng team has been relentlessly pursuing plant hormone research.
As of the latest research findings, the top five transport proteins for plant hormones have been identified, leaving only the brassinosteroid transport protein as an unknown. "When we confirmed that the ABCB19 protein is the brassinosteroid transport protein, we were truly excited. Our work fills a research gap in the brassinosteroid signaling pathway," says Sun Linfeng.
"The preference of the ABCB19 protein for transporting brassinosteroids rather than auxins is an intriguing discovery and a significant contribution to the field," remarked one reviewer.
In the future, Sun Linfeng will lead the team in further analyzing and modifying the ABCB19 protein, providing more insights into utilizing brassinosteroids to enhance agricultural production. Simultaneously, this lays the groundwork for designing and developing small molecule modulators targeting this protein, offering more avenues for their use as pesticides and plant growth regulators in agriculture to increase crop yields.
For more information on the related papers, visit: https://doi.org/10.1126/science.adj4591.
