Other than being a symbol for love and romance, roses are commonly known for their sharp spikes — prickles that protrude from the stems to ward off animals looking to munch on the buds.
They’re not the only plant to have this defense mechanism: Other flowers such as spider flowers or brambles, the flowering shrub responsible for raspberries and blackberries, bear the sharp points, as do certain crop plants such as tomatoes, eggplants, barley and rice.
But how could all these species, many of which evolved separately over the course of millions of years, come to have the same spiny feature? A team of international researchers found that the answer lies in their DNA, tracing the origin to one ancient gene family that’s responsible for the prickles in all these variations, according to a new study published Thursday in the journal Science.
The findings not only open the door for scientists looking to create prickle-free variants but also provide insight into the evolutionary history of an extremely diverse genus of plants, experts say.
The evolution of prickles
Contrary to pop culture references, roses do not have thorns, which are the sharp woody points of certain shrubs and trees, including honey locusts and citrus trees. The flowers instead have prickles that form from the skin of the plant, similar to how hair grows.
Prickles have been around for at least 400 million years, dating back to when ferns and their relatives emerged with some bearing prickles on their stems. The trait has since then popped up — and disappeared — at different points in evolutionary time, said study coauthor Zachary Lippman, plant biologist and professor of genetics at Cold Spring Harbor Laboratory on Long Island, New York.
One of the most diverse plant genera, known as Solanum — which includes crops such as potatoes, tomatoes and eggplants — first gained prickles 6 million years ago. Today, the genus has more than 1,000 species that appear throughout the world, with around 400 of those referred to as “spiny solanum” for their prickles, according to the University of Utah.
When a common trait, such as prickles, appears independently across different lineages and species, that is known as convergent evolution, and occurs when species adapt similarly to certain environmental needs. Wings are another example of a feature that evolved in this way among different species of birds, as well as other animals such as bats and even some types of squirrels that have the winglike structure, said Lippman, who is also a Howard Hughes Medical Institute investigator.
Prickles and thorns are an evolved defense against herbivores — animals that eat plants — and can also aid in growth, plant competition and water retention, according to the study. It was previously unknown as to what exactly caused ferns and other unrelated plants to grow the prickles. Now, the study authors have found that an ancient gene family known as Lonely Guy, or LOG, has served as a gatekeeper for the trait, switching it on and off in different species across millions of years.
Lonely Guy genes
By removing prickles from various species, including roses and eggplants, the authors found that a LOG gene was responsible for the prickles in about 20 types of plants studied. LOG-related genes are found in all plants, even dating back to mosses, which are regarded as the first dry-land plant, Lippman said. The genes are responsible for activating a hormone known as cytokinin that is important for a plant’s basic functions on a cellular level, including cell division and expansion, which in turn affects the plant’s growth.
“It’s not that there was one common ancestor that had prickles, and then it radiated out over 400 million years to all these others, and then they were lost sporadically. In fact, what it looks like is that they seem to be quite readily gained in different lineages,” Lippman said. “Now, the question is, how often is convergent evolution not just the trait that we see, but the genes behind the trait?”
He added, “Our study is, I think probably the first to really demonstrate the power of those tools (genetic and genome sequencing) to span such a wide evolutionary distance to ask this very classic question about convergent evolution in organismal evolution of plants or animals.”
The discovery adds a valuable tool for researchers looking into the extent of protection the prickles offer against herbivores. This level of defense has previously been challenging to assess since manually removing prickles from already grown plants — to test whether they are more vulnerable without them — damages the tissue and can compromise plants’ health, said Tyler Coverdale, an assistant professor of biological sciences at the University of Notre Dame who was not part of the new study.
“By knocking out prickles with targeted genetic mutations, we can more fully understand the ecological role of physical plant defenses,” Coverdale said in an email. “Prickles are a key evolutionary development that allow plants to withstand herbivory, which is why many of the spiny Solanum are found in areas with historically high large herbivore diversity. Without this key innovation, it is possible that Solanum would be much more restricted in its range and diversity.”
Genetically removing prickles
Before this discovery, another method of removing prickles from plants was to attempt to breed the plant with another variation that had naturally lost its prickles, Lippman said, which is why there are some rose species without the spikes today.
But now that the gene responsible for the prickles has been identified, scientists can remove the prickles utilizing genome editing techniques such as CRISPR, a method scientists use for DNA modification of living organisms. Targeted gene editing can create more variations with ease and has fewer repercussions to the plant’s growth and fruit production, Coverdale said.
“Not only does this study tell us more about the evolution of prickles specifically, it also provides us with insights into the mechanics of how to engineer plant developmental pathways for agricultural improvement,” said Vivian Irish, plant biologist and a professor in Yale University’s department of molecular, cellular and developmental biology. Irish was not part of the new study but was the senior author of a 2020 study that found thorns grow on plants through the activity of stem cells.
“(LOG genes) have been repeatedly co-opted (a biological shift in a trait’s function) in different plant species for the formation of prickles, and also repeatedly lost in lineages where prickles are lost. … (C)o-option at many different levels might be nature’s rule of thumb, and that innovation in many cases might well reflect re-using old genes in new ways,” she added in an email.
For agricultural purposes, removing prickles could make harvesting easier and pave the way to get lesser-known produce into grocery stores.
An example the authors use are desert raisins, which are berries grown on prickly bushes native to Australia. With the prickles removed, the berry could be cultivated with much greater ease and would be more similar to common grocery store berries such as blueberries and strawberries, Lippman said.
“It’s really about having more knowledge … and understanding how important mutations were to give us the food that we eat at the scale that we eat it, and knowing that there’s more potential out there,” Lippman said. “The more that we understand under the hood, the more we’re going to have a chance to tweak the system, or the engine, if you will, to make it perform even better.”
