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In addition to being a symbol of love and romance, roses are commonly known for their sharp spikes – thorns that protrude from the stems to ward off animals looking to chew on the buds.
They are not the only plant that has this defense mechanism: other flowers, such as spider flowers or mulberry treesthe flowering shrub responsible for raspberries and blackberries, has sharp spikes, as do certain cultivated plants such as tomatoes, eggplant, barley and rice.
But how did all these species, many of which evolved separately over millions of years, come to have the same spiny feature? A team of international researchers has discovered that the answer lies in your DNA, tracing its origins to an ancient family of genes responsible for the spines in all of these variations, according to a new study. published Thursday in Science magazine.
The findings not only open the door for scientists looking to create spine-free variants, but also provide insights into the evolutionary history of an extremely diverse genus of plants, experts say.
The evolution of thorns
Contrary to pop culture references, roses not have thorns, which are the sharp woody tips of certain shrubs and trees, including locust and citrus trees. Instead, the flowers have spines that form on the bark of the plant, equal to how hair grows.
Thorns have existed for at least 400 million years, dating back to when ferns and their relatives first appeared with thorns on their stems. Since then, the trait has appeared — and disappeared — at different points in evolutionary time, said study co-author Zachary Lippman, a plant biologist and professor of genetics at Cold Spring Harbor Laboratory on Long Island, New York.
One of the most diverse genera of plants, known as Solanum – which includes crops such as potatoes, tomatoes and eggplant – first grew thorns 6 million years ago. Today, the genus has more than 1,000 species that appear throughout the world, with about 400 of them called “prickly solanum” for their spines, according to the University of Utah.
When a common feature such as spines appears independently in different lineages and species, it is known as convergent evolution, and occurs when species adapt in a similar way to certain environmental needs. Wings are another example of a feature that has evolved this way among different species of birds, as well as in other animals such as bats and even some types of squirrels that have a wing-like structure, said Lippman, who is also an expert on birds. Howard Hughes Medical Institute investigator.
Thorns and thorns are an evolved defense against herbivores — animals that eat plants — and can also help with growth, plant competition and water retention, according to the study. Until then, it was unknown what exactly caused ferns and other unrelated plants to grow thorns. Now, study authors have discovered that an ancient gene family known as Lonely Guy, or LOG, served as the trait’s guardian, turning it on and off in different species over millions of years.
Lonely guy genes
By removing thorns from several species, including roses and eggplants, the authors found that a LOG gene was responsible for the thorns in about 20 types of plants studied. LOG-related genes are found in all plants, even in mosses, which are considered the first dry land plant, Lippman said. The genes are responsible for activating a hormone known as cytokinin, which is important for a plant’s basic functions at the cellular level, including cell division and expansion, which in turn affects the plant’s growth.
“It’s not that there was a common ancestor that had spines, and then it radiated for 400 million years to everyone else, and then they were lost sporadically. In fact, what it appears is that they can be obtained easily in different strains,” Lippman said. “Now the question is: How often does convergent evolution not just be the trait we see, but the genes behind the trait?”
He added: “I think our study is probably the first to really demonstrate the power of these tools (genetic and genome sequencing) to span such a wide evolutionary distance to ask this classic question about convergent evolution in plant or animal organismal evolution. .”
The discovery adds a valuable tool for researchers analyzing the extent of protection that spines offer against herbivores. This level of defense has previously been difficult to assess, since manually removing spines from already grown plants — to test whether they are more vulnerable without them — damages the tissue and can compromise the plants’ health, said Tyler Coverdale, assistant professor of biological sciences at the University of Notre Dame who was not part of the new study.
“By eliminating spines with targeted genetic mutations, we can better understand the ecological role of plants’ physical defenses,” Coverdale said in an email. “Spines are a key evolutionary development that allows plants to resist herbivory, which is why many Solanum thorns are found in areas with historically high herbivore diversity. Without this fundamental innovation, it is possible that Solanum would be much more restricted in its scope and diversity.”
Removing spines genetically
Before this discovery, another method of removing thorns from plants was to try to breed the plant with another variation that had naturally lost its thorns, Lippman said, which is why there are some thornless rose species today.
But now that the gene responsible for the spines has been identified, scientists can remove them using genome editing techniques such as CRISPR, a method that scientists use to modify the DNA of living organisms. Targeted gene editing can easily create more variations and has fewer repercussions on plant growth and fruit production, Coverdale said.
“This study not only tells us more about the specific evolution of spines, but also provides us with insights into the mechanics of how to engineer plant developmental pathways for agricultural improvement,” said Vivian Irish, a plant biologist and professor in the department at Yale University. . molecular, cellular and developmental biology. Irish was not part of the new study, but was the senior author of a 2020 Study who found thorns growing on plants through the activity of stem cells.
“(LOG genes) have been repeatedly co-opted (a biological change in the function of a trait) in different plant species for the formation of spines, and also lost repeatedly in lineages where spines are lost. …(C)o-option on many different levels may be nature’s rule of thumb, and this innovation, in many cases, may well reflect the reuse of old genes in new ways,” she added in an email.
For agricultural purposes, removing the thorns could facilitate harvesting and pave the way for the delivery of lesser-known products to supermarkets.
One example the authors use is desert raisins, which are berries grown on thorny bushes native to Australia. With the thorns removed, the fruit could be grown much more easily and would be more similar to common supermarket fruits like blueberries and strawberries, Lippman said.
“It’s really about having more knowledge… and understanding the importance of mutations in providing us with the foods we eat at the scale that we eat them, and knowing that there’s more potential out there,” Lippman said. “The more we understand behind the scenes, the more we have a chance to tweak the system, or the engine, if you will, to make it even better.”
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