URBANA, Ill. - For decades, herbicide safeners have been used as a defense mechanism against stress caused by herbicides in cereal crops such as maize, rice, grain sorghum, and wheat. Developing these chemical compounds (safeners) that work with new herbicides has largely been done through trial and error because of limited knowledge of what happens in the plant when safeners have been applied, either directly as a seed treatment or broadcast with the herbicide.
Researchers at the University of Illinois have recently found new insight into the mechanisms of how safeners turn on a defense "switch" in the plant to protect crops from herbicide injury by studying different lines of grain sorghum. This new understanding could help producers in broadening weed control options.
Dean Riechers and Pat Brown, both researchers in the Department of Crop Sciences, received funding this past week from the USDA Agriculture and Food Research Initiative (USDA-AFRI) under the Plant Growth and Development, Composition and Stress Tolerance Program to continue looking at safeners in grain sorghum.
"I like to use the word picture of safeners acting as an immunization or a vaccine," Riechers said. "We know what's happening, now we just need to know the how, the why, and the where.
Safeners are unique because this pathway is there for a reason. The herbicide just happens to come along and it gets detoxified. We want to know why that pathway is there in the first place. With safeners, we can turn it on and off whenever we want."
Safeners are a group of chemically diverse compounds with the unique ability to protect cereal crops from herbicide injury without reducing herbicide activity in target weed species. When a safener is applied, protection from herbicide injury is accomplished by increasing the expression of genes that cue the herbicide-metabolizing enzymes.
"Our recent research has contributed to identifying the proteins and enzymes involved in herbicide detoxification in cereal crops, as well as in gaining a deeper understanding of the basic mechanism of safeners for improving herbicide selectivity," Riechers said.
Brown explained that the researchers worked with several inbred sorghum lines that had been already genotyped. Some were treated with safeners (applied to the seed) and some were not, just as some were treated with herbicides and others were not. The plants were harvested at 12 days after planting, weighed and photographed.
The researchers found that safeners cause the massive accumulation of glutathione S-transferase (GST) proteins, predominantly in the coleoptile, or the protective sheath covering the emerging shoot of the plant. This indicated that herbicide detoxification occurs in this tissue before the herbicide ever reaches the sensitive new leaves inside the coleoptile.
However, some sorghum varieties didn't need safener to accumulate GST proteins. "It was pretty dramatic," Brown said. "I was also pleasantly surprised at how much genetic variability we saw, which is a bonus for this project. When a farmer buys sorghum to plant, it always has a safener on the seed already. A farmer would never plant unsafened sorghum seed because the herbicide would prevent it from emerging through the soil properly. Through this project, we're finding some varieties that don't even need the safener because they are naturally tolerant.
"We're trying to find this switch or switches that the safeners are turning on," he added.
The researchers hope that by identifying this genetic switch, the defense mechanism can eventually be used against other abiotic or biotic stresses that threaten plants like drought, insects, or disease. Brown said although they have only been working with dwarf lines of grain sorghum, the results could also apply to sweet or biomass sorghum. A next step could include looking at safeners in diverse maize lines.
The researchers added that a "pie in the sky" hope would be to someday understand why safeners only work in monocot plants, and just in cereal crops but not in grass weeds. "To figure why dicot plants, like soybean and cotton, don't respond to safeners would be the Holy Grail," Riechers said.
Brown explained an important part of the research is the connection with abiotic stress. "We tried to make the case that this grant isn't just about safeners, but also how plants can adapt to biotic and abiotic stresses. Right now we can treat a seed with a chemical and it protects the seedling a week later from herbicide. But what if one day we could treat seeds with other chemicals that would turn on defense responses in the plant until the plant was older or would protect against other abiotic or biotic stress?
"I think this is an idea with unlimited potential," he added.
"Another great selling point for farmers is that with more and more resistance in waterhemp and Palmer amaranth, we are running out of herbicides," Riechers said. "We could find a new herbicide for use with a GMO, but that could take 15 years to develop. But if you could find a new herbicide and a new safener to use with it, that would definitely broaden our weed control options."