Some experimental organisms, like maize or sorghum, are primarily agricultural plants — far more corn is grown globally for food and animal feed than for scientific research. Others, like Arabidopsis thaliana, have no practical use at all. Still others are related to crop plants, though not cultivated themselves, or have other properties that make them interesting outside of a strictly scientific or agricultural context. Datura stramonium, commonly known as jimsonweed, is one of these.
Jimsonweed is a member of the nightshade family, like potatoes, eggplant, tomato and peppers. It is native to the Americas, but has spread all over the world. Unlike its delicious cousins, jimsonweed is toxic and should not be consumed. It has hallucinogenic properties, but the specific chemicals that cause its psychotropic effects tend to cause delirium rather than an enjoyable altered state and the toxic effects can be extremely dangerous. Jimsonweed is for all intents and purposes what the last part of its common name implies — a weed.
Datura came into its own as an experimental organism in the 1920s and 30s through the work of Albert F. Blakeslee at the Carnegie Institution’s research facilities at Cold Spring Harbor Laboratory. Blakeslee had been working with Datura since the 1910s. He was using it to study variability and inheritance patterns, and also recommended it “for illustrative purposes in classes in genetics,” to demonstrate Mendelian ratios, as he and his collaborator Amos G. Avery wrote in a 1917 paper. What made Datura so useful? Although Datura plants could grow over 6 feet high, they could also be “grown to maturity in a 3-inch pot.” Selfing, or mating a plant with itself, was technically easy to accomplish, and the plants produced huge numbers of seeds. Blakeslee’s students had worked out that “if all seeds were sown and grew to plants of the same size and productiveness, it would take only five generations at this rate, including the first plant, to cover the entire surface of the earth…and there would be left over sufficient seed to sow several of the planets in addition.” In addition to this, Datura was a good model organism for those interested in tracing the form and activity of chromosomes. As plant scientist Rob Martienssen explains, Datura has “beautiful chromosomes,” and Blakeslee could thus determine “the chromosomal constitution of individual plants very easily.”
Blakeslee was working with several colleagues, including Amos G. Avery, who was in charge of the gardens and greenhouses belonging to the Carnegie Institution’s Department of Genetics at Cold Spring Harbor. The key discovery that Blakeslee and his group made (and which Blakeslee and Avery published in a co-authored paper in 1937) was that a chemical called colchicine could be used to multiply the number of sets of chromosomes that plants have. He could, in other words, create polyploids of specific plants on demand. This had implications for plant breeders as well as geneticists. In terms of the study of genes and their functions, it was important to know that genetic changes could be induced chemically. Previously, in the late 1920s, Hermann J. Muller had shown that x-ray exposure could induce genetic mutations in fruit flies. And indeed, scientists had known about polyploidy in plants for several decades, and Blakeslee and others had succeeded in inducing it in other ways, for example through extremes of temperature. But the colchicine method was more reliable. It was also significant that chromosomes could be multiplied through chemical means, although the reasons for this and its implications had yet to be fully worked out.
In general, Blakeslee’s colchicine discovery added another way to deliberately change an organism’s genetic setup. Why were geneticists so interested in being able to induce mutations? Mutations can lead to changes in the organism’s phenotype, which helps pinpoint which genes are related to which traits. In the days before genes could easily be ‘knocked out’ and genomes sequenced, how and in connection with what other traits mutations were inherited helped map an organism’s genome. Finding plants with interesting mutations or strange new chromosomal arrangements was key to genetic research, but nature tended not to produce these at a rate convenient for geneticists. Colchicine allowed scientists to give nature a shot of adrenaline: as Blakeslee and Avery wrote in their 1937 paper, “colchicine treatment only speeds up a process which occurs naturally at rare intervals, and offers the possibility of quickly getting results which might take a lifetime if one waited for the more leisurely process of nature.” This was exactly the same language that scientists at Brookhaven would use about radiation breeding in the 1950s — zapping plants with a reactor created a bonanza of potentially significant mutations, all at the touch of a button.
Phenotypical changes were useful for plant breeders as well. Soon after Blakeslee and his colleagues had made the discovery, he was in touch with the laboratory director of the New York Botanical Garden, Arlowe B. Stout, to let him know that “we have just run across the fact that we can introduce tetraploidy rather readily by chemical means…I am looking around for a variety of other things beside Datura and the few other things that I have tested. People at the Department of Agriculture and the Princeton Rockefeller Institute have asked me to double up chromosomes in some sterile species hybrids of tobacco which they want to get turned into double diploids, and the Department of Agriculture in Ottawa wants me to double up wheat-agropyrum sterile hybrid. I should like to try out a number of representative things that would be of theoretical interest and also some that would be of practical importance.” As Blakeslee put it soon after to the New York State Conservation Departments chief aquatic biologist, Dr. Emmeline Moore: “This seems of very general application in plants and bids fair to be of considerable value both to the theoretical geneticist and to the practical plant breeder.” Breeders were always interested in potentially useful and valuable phenotypic changes, everything from larger fruits or greater yield in the case of crop plants to new colors or larger flowers in ornamental plants.
It wasn’t just professional scientists and plant breeders who were interested in colchicine and its striking genetic effects. Colchicine could produce strikingly beautiful and unusual ornamental plants, and while hobby gardeners did not typically buy it and use it themselves, they were interested in the results obtained with it by horticulturalists. Over the next several decades, the technology moved from the lab to the lawn as seed companies began to produce polyploid plants for ornamental purposes. In the early 1960s, Newsday’s veteran gardening writer Bea Jones published a number of columns about flowers that had been developed or substantially changed through the use of colchicine. Zinnias, for example, had been transformed since the 1930s. Jones assumed her readers would be interested in the scientific details behind this, and in a different article described how seeds were treated with colchicine, what polyploidy was and how it affected important characteristics of plants, such as fertility and flower size. “The Gloriosa Daisy is one of the notable examples,” she explained in 1962, “a huge blossom obtained from the common black-eyed Susan.” A decade later, she placed this technique in a longer historical perspective for her readers, emphasizing the Long Island roots of this flower, including a description of the work of Blakeslee at Cold Spring Harbor.
Blakeslee’s work on Datura, in other words, had a mixture of practical and theoretical consequences. One of they key practical consequences of Blakeslee’s work on polyploidy in plants — other than flower breeding — is explained by Rob Martienssen in an interview with CSHL Library and Archives in May of 2022, recounting Blakeslee’s attempt to cross what turned out to be a tetraploid plant with a plant that had fewer sets of chromosomes:
“He tried to do genetic crosses with it and found that it could cross to itself perfectly fine, and gave lots of seeds, but when he crossed it as a male to the same population of jimsonweed that it had come from, so the progenitor, all the seeds died in what looked like a programmed seed abortion and actually quite a useful one because if you want to make a seedless plant, this is how it’s done.”
Blakeslee, in other words, had made the first of a series of discoveries that would ultimately lead to the production of the seedless fruit — bananas, grapes, watermelon, and so on — so common today.
Blakeslee, though, didn’t know that that particular plant was tetraploid when he made the cross. He determined that later on — it was a difference in the size of the plant’s fruit case that had originally drawn his attention to it. (Not all the polyploid plants that Blakeslee and his collaborators worked with were ones they had created themselves — a few were found by chance among ‘normal’ plants. Polyploidy can come about through natural means too.)
The effects of this cross of a tetraploid with a plant of the same species that had a different number of sets of chromosomes led to an important theoretical point:
“What he discovered was something called the triploid block, where if the dose of maternal to paternal chromosomes is not as it should be, then the seeds die. Specifically, if you have two doses coming from the paternal and one from the maternal, then the seeds will abort. He then looked at this in many, many different species of flowering plants and found that pretty much every flowering plant did it.”
Blakeslee’s work on Datura in the 1930s, in other words, had far-ranging theoretical and practical consequences. It’s also worth noting the similarities and differences between Datura and Arabidopsis as experimental organisms. Blakeslee was using Datura to study heredity in the 1920s and 1930s because its chromosomes were easy to work with and because the plant itself was simple to grow, self and propagate. At that time, the role of DNA was not known, and scientists debated what a ‘gene’ really was, physically and conceptually. Nearly a century later, scientists are now using Arabidopsis in a different way: as a genetic model system for plants in general, in which the sequence and location of each gene is known, and the effects of specific changes in the nucleotide sequence can be explored.
Photo Credit: Antoinette Sutto, Cold Spring Harbor Laboratory.