After a decades-long search, researchers say they have finally found an elusive group of proteins that help plants survive stresses such as drought, cold and heat that may increase with climate change.
The results came about thanks to a technique similar to methods used to screen for new drugs, and a unique collaboration of scientists from Canada, U.S. and Spain, said Peter McCourt, one of the co-authors of the paper published Thursday in Science Express.
Scientists have known for decades that a hormone called abscisic acid (ABA) helps plants survive drought and other stresses. But there were key things they hadn't been able to understand, said McCourt, a cell and systems biologist at the University of Toronto.
"For the last 20 years if not longer, people have been trying to identify the gene that perceives ABA and turns that into a protective response," he added. "It was the holy grail — once we had that, maybe we could finally understand how the hormone works."
'It was the holy grail — once we had that, maybe we could finally understand how the hormone works.' — Peter McCourt, University of Toronto biologist
Sean Cutler, a former University of Toronto researcher who is now at the University of California Riverside, led the research, collaborating with McCourt, researchers from the University of Ontario Institute of Technology, a Spanish university and several U.S. institutions.
Together, they found four genes for proteins called receptors that bind to ABA and trigger a response from a small flowering plant called Arabidopsis.
McCourt said the receptors haven't been tested in other plants. But additional studies have already shown that adjusting the way plants use ABA improves drought tolerance in crops like corn and canola.
Because of that, McCourt said he would be surprised if the findings of the recent study don't work in other plants.
Could be tailored for agriculture
Plants in nature do use ABA to respond to drought and other stresses, but the response level may not be that high due to tradeoffs such as reduced growth, McCourt said.
"But those tradeoffs may not be important in a farmer's field and so we can really manipulate the pathways to really optimize or tailor it for agriculture," he said.
The new discovery could help researchers and farmers do that.
Researchers had long tried to find the receptor among many proteins that bound to ABA, but that had been very difficult, McCourt said, partly because hormones often stick to molecules that aren't receptors.
His research group had tried unsuccessfully for more than a decade to make a plant mutant that couldn't respond to ABA. If they had done so, they could look in its genetic code for the altered gene. That would likely have been a defective copy of the receptor.
As it turned out, the problem was that there are 13 related receptors for ABA.
"If you knock out any one, the other 12 will compensate," McCourt said. "You won't see a difference in the plant."
Researcher screened thousands of compounds
Cutler's research group got around that by screening thousands of chemical compounds for their response in plants, starting the work while he was still at the University of Toronto. That is a technique called chemical genomics and is similar to how drug researchers screen chemical compounds for their response in animal cells.
"It was a very novel approach," McCourt said.
Cutler finally found a molecule called pyrabactin that made plants behave as though they had been exposed to ABA. However, unlike ABA, pyrabactin only bound to one of the 13 receptors, making it easier to find that receptor and the gene that coded for it. He was then able to find the rest based on their similarities.
Cutler then contacted other researchers in the same field, such as McCourt, who were independently finding some of the same receptors using other techniques. Cutler proposed that instead of competing, they combine their results, and the other researchers agreed. In the end, they were able to conclusively verify four out of the 13 receptors
They found that knocking out the gene for one receptor had no effect, but knocking out two or three reduced the plant's response to ABA.
"If you knock out four of the major ones, then the plant no longer responds to ABA," McCourt said. In fact, ABA has many different functions in the plant, and pyrabactin mainly affects germination, not drought resistance. However, McCourt said researchers will now screen other chemicals for ABA-like effects on drought tolerance.
That may lead to a cheap, stable compound similar to ABA that can be sprayed on plants during droughts, he added.
ABA itself cannot be used in agriculture because it is expensive and light sensitive, said a University of California Riverside news release.