The amazing Agrobacterium
Mary-Dell Chilton’s plant science discovery continues to feed the world
The last four decades have seen the key crops of the world go through a quiet revolution. While farmer’s fields may look unchanged to the casual observer, the crops growing there have been transformed —higher yielding, fighting off diseases better and using water more efficiently than ever before.
In part this is down to traditional selective breeding, crossing different varieties to create hybrids with desired traits, such as coping with a specific pest. However, this process can be slow going and the results are difficult to predict or control.
A more efficient way of producing these more productive and resilient crops is through genetic modification, a direct approach where scientists can precisely place beneficial traits into the plant’s genetic structure. This is faster and more reliable than conventional breeding, but getting the right material to the right place in the genetic structure is a complex scientific challenge.
Nature’s own genetic engineer
The story of how scientists unlocked the secrets to genetic transformation of plants is one of groundbreaking innovation, but it starts in a surprisingly humble place: the soil.
Pick up a handful of soil and you will be holding millions upon millions of invisible microbes, including a remarkable bacterium called Agrobacterium tumefaciens. These bacteria carry out some essential functions such as helping decompose organic matter and binding together soil, but they also interact with plants in a unique way.
When Agrobacterium encounters a wounded plant, it can infect the injury site and cause rapid cell division, creating tumour-like growths known as crown galls.
The queen of the Agrobacterium
Though scientists knew since the early 1900s that Agrobacterium causes crown galls, a crucial question remained unanswered – how was the bacteria altering plant DNA? A key figure in solving this mystery was plant science pioneer Mary-Dell Chilton.
Born in Indianapolis in 1939, Chilton went to the University of Illinois to study chemistry but was quickly drawn to the emerging field of molecular biology. In an interview given to the National Inventors Hall of Fame, she said: “When I was a graduate student, we were just beginning to work out the genetic code, and I was tickled to death by the experiments that people were doing to determine what the code was. I was fascinated. I fell in love.”
After earning her Ph.D. in chemistry with a focus on bacterial transformation, Chilton joined the University of Washington. While there, she encountered research publications claiming that fragments of Agrobacterium DNA had transferred into the plant cells of crown galls in tobacco plants — a potentially groundbreaking discovery, since this kind of transfer was thought impossible without strong genetic similarity between bacterial and host DNA.
Chilton and her team set out to test the hypothesis, but they found no evidence for it. Something was not adding up.
The queen of the Agrobacterium
Though scientists knew since the early 1900s that Agrobacterium causes crown galls, a crucial question remained unanswered – how was the bacteria altering plant DNA? A key figure in solving this mystery was plant science pioneer Mary-Dell Chilton.
Born in Indianapolis in 1939, Chilton went to the University of Illinois to study chemistry but was quickly drawn to the emerging field of molecular biology. In an interview given to the National Inventors Hall of Fame, she said: “When I was a graduate student, we were just beginning to work out the genetic code, and I was tickled to death by the experiments that people were doing to determine what the code was. I was fascinated. I fell in love.”
After earning her Ph.D. in chemistry with a focus on bacterial transformation, Chilton joined the University of Washington. While there, she encountered research publications claiming that fragments of Agrobacterium DNA had transferred into the plant cells of crown galls in tobacco plants — a potentially groundbreaking discovery, since this kind of transfer was thought impossible without strong genetic similarity between bacterial and host DNA.
Chilton and her team set out to test the hypothesis, but they found no evidence for it. Something was not adding up.
The missing piece
When most people think of DNA they think of the famous double helix structure, but with bacteria things work differently. Genetic information is stored not only in chromosomal DNA but also in circular molecules called plasmids.
The reason Chilton and her team hadn’t been able to find evidence of DNA transfer was because they were looking in the wrong place. Just because the chromosomal DNA didn’t transfer doesn’t mean there wasn’t any transfer happening at all.
Researchers at the University of Ghent in Belgium discovered that virulent strains of Agrobacterium contained unusually large plasmids — tumor-inducing or 'Ti' plasmids. This turned out to be the missing piece of the puzzle.
When Chilton and her team looked for fragments of Ti plasmid in the DNA of transformed plant cells, they found them, revealing how DNA transfer actually takes place. By proving their initial hypothesis wrong, they made a major scientific breakthrough.
But she didn’t stop there. Chilton and her team discovered how to disarm the Ti plasmid so that it could transfer DNA without causing crown galls, which led them to create the world’s first transgenic plant.
Dr. Mary-Dell Chilton retired from Syngenta in 2018 after 35 years of service, but her contributions to agriculture and humanity will endure for generations to come.
In 1983, Chilton joined CIBA-Geigy, a Syngenta legacy company, to lead its biotechnology research, helping to develop plants with beneficial genes and seeds to ensure improved genetics could reach growers worldwide. Her research led to the development and refining of a key technique for the improvement of crops – Agrobacterium mediated transformation (AMT).
AMT: The agricultural gamechanger
AMT revolutionized agriculture. AMT enables scientists to develop crops that carry specific proteins targeting pests, and hybrids with multiple protective traits against a range of threats. Even the latest innovations in soybean science still make use of AMT.
The economic and environmental benefits of Chilton’s research are almost impossible to quantify. Crops developed through AMT are genetically protected against pests and diseases, reducing the need for chemical intervention while increasing yields.
Growers can now produce more food using fewer resources, a crucial advantage as we face the challenges of climate change and a growing population.
For her contributions, Chilton has received dozens of high-profile honours, including the prestigious World Food Prize in 2013 and the National Medal for Technology and Innovation from then-President Joe Biden in 2023.
With the world population projected to increase by two billion people in the next 30 years, innovations that help farmers produce higher yields without depleting environmental resources are increasingly essential. Chilton’s breakthrough work remains a vital tool in helping to feed the world, now and into the future.
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