Plant researchers have been wrong for years about how plant growth and development are regulated, as Yao Xiao pointed out during his PhD project. They overlooked the role of a protein that turns out to be essential, just as it is in the development of animals and yeasts. In plants, it regulates the transport of the plant hormone auxin.
That a plant seed develops into a growing, well-structured organism is mainly attributable to the plant hormone auxin. It is produced in certain cells and transported to where its activity is needed, partly through plant vessels, partly by a cell-to-cell process. The auxin transport must be fine-tuned, and Yao Xiao wanted to know how this is achieved. ‘When I was young, I was eager to contribute to applied science,’ he says. ‘But I grew more and more curious to know how things work, so I came to prefer fundamental research.’
In a publication in Nature Plants, he describes the most exciting part of his research. He showed that the kinase PDK1 (in full: 3-phosphoinositide-dependent protein kinase 1) regulates auxin transport. A kinase is a protein that can activate other proteins by transferring a phosphor group. PDK1 activates another kinase, which activates yet another, and in the end this activates PIN, the protein that determines the cell-to-cell transport of auxin. By indirectly activating PIN, PDK1 plays a central role in plant development.
This was more than just a discovery; it was a complete surprise. Before this, there were serious doubts about the role of PDK1 in plant development.
‘Although PDK1 was known to be essential to the development of animals and yeasts, it was thought to be unimportant in plant development. But we discovered that this was a mistake.’
‘It was known that PDK1 is essential to the development of animals and yeasts, as the master regulator of kinases,’ Xiao explains. ‘These organisms are unviable if this protein doesn’t function well. But when the gene that codes for it was mutated in the model plant Arabidopsis thaliana, which should have meant that PDK1 lost its function, nothing happened. Seeds that carried the mutation developed normally. So, PDK1 didn’t seem to play the same role in plant development as it does in animals and yeasts.’
When studying how the transport of auxin is regulated in plants, Xiao decided to have a closer look at PDK1, despite its assumed unimportance. And then he discovered, quite unexpectedly, that the gene mutations applied in earlier research did not really destroy the protein’s function. ‘Researchers had been wrong about this for years,’ he says. Using the new CRISPR-Cas9 technique of gene editing, he then made new mutants in which PDK1 was knocked out. And this had a strong effect: mutant plants did not grow well. They were dwarfed, had abnormal vascular tissue and low fertility.
So, PDK1 is essential to plant development after all, just as it is for animals and yeasts, emphasising a common origin of organisms. Xiao’s finding, apart from shaking up fundamental plant research, may be applicable to plant breeding science as well.
Spending his childhood in a small village where his parents grew crops like rice and maize, Yao Xiao (China, 1989) developed an interest in nature. When finalising his Biology Master’s in Plant Sciences, he met Remko Offringa from IBL, applied for a PhD project with him and came to Leiden. His research focused on the cell-to-cell transport of auxin, the main plant hormone that controls growth and development. Xiao currently holds a postdoc position at the Technical University of Munich. In the future, he hopes to return to China, start his own research group, continue his research on kinase and apply his knowledge to plant breeding.