Expression of the anti-predator gene in wild radish

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Lions, bears and other clawed creatures tend to top lists of nature’s most protective parents, but a new study has found that plants can also go to impressive lengths to protect their young.

Wild radish plants use spiky leaf hairs (trichomes) and toxic mustard oil to ward off cabbage butterfly caterpillars and other herbivores that eat their leaves. (Image credit: Casey Girard)

In a new study, published August 17 in the Proceedings of the National Academy of Sciences, Stanford researchers show that wild radish plants activate different anti-predator genes during key phases of their lives in response to caterpillar predation. Additionally, plants can also pass on these defensive strategies “on demand” to their offspring to prepare them for the predation they are likely to experience as seedlings and adults.

Scientists know that parent plants can pass on useful defenses to their offspring, but how plants allocate these defenses over time and from generation to generation is poorly understood. Understanding how plants protect themselves is important to humans for many reasons, explained lead author Rodolfo Dirzo, Bing Professor of Environmental Science in the School of Humanities and Science.

For example, aspirin, morphine and digitalis, a heart medication, are some of the many medicines derived from the chemical defenses created by plants. The natural defenses of plants can also be exploited by humans to protect crops and control pests.

“Human history has been heavily influenced by the evolution of plant-insect interactions,” said Dirzo, who is also a senior fellow at the Stanford Woods Institute for the Environment.

The radish war

Plants arm themselves with toxins, thorns and other chemical and physical defenses to ward off phytophagous animals, called herbivores. But tusks can be expensive for plants to produce, so they often spend their resources on herbivore defenses only when it benefits them the most.

In plants, the genes responsible for generating anti-predator defenses are often “turned on” via a chemical “switch” called DNA methylation. DNA methylation is an example of an epigenetic mechanism (“epi” meaning “on top of”) that alters the behavior of genes without altering the underlying DNA sequence of the genes themselves.

When Mar Sobral joined Dirzo’s lab as a postdoctoral researcher in 2011, she began studying how predation can affect heritable epigenetic changes.

In a study published in March 2021 in Frontiers in plant science, the team showed that wild radishes were much more likely to produce pink or purple flowers if their parents were attacked by caterpillars.

“Apparently, epigenetic changes related to increased physical and chemical defenses can also induce changes in pigment production because they have related pathways,” explained Sobral, who is now a researcher at the Universidade de Santiago de Compostela. in Spain and is the first author of both studies.

A chemical call to arms

To dig deeper into how plants evolve and pass on defenses to their offspring in response to predation, Sobral, Dirzo and Isabelle Neylan, a former undergraduate student in Dirzo’s lab, designed a multigenerational greenhouse experiment to explore interactions between wild radish plants and their main predator. , the caterpillar of the cabbage butterfly.

In wild radishes, anti-predator defenses manifest themselves in spiky leaf hairs and toxic mustard oil. The team wanted to know how wild radish plants distribute their physical and chemical defenses as seedlings and adults, and how defenses heightened during a particular life stage can be passed on from one generation to the next.

The greenhouse experiment allowed the researchers to explore their questions in a controlled environment – only cabbage butterfly caterpillars were allowed to graze on the wild radish plants, and the plants were given sufficient light, water, nutrients and a constant temperature.

The researchers hypothesized that wild radish plants would activate defenses in response to caterpillar attacks.

They also predicted that the offspring of plants attacked by caterpillars would have more tusks and be more likely to produce tusks when attacked by caterpillars.

Finally, since seedlings are fragile and more susceptible to fatal damage at this stage of their life, the team predicted that the ability to induce defenses would be strongest in seedlings.

The researchers tested their predictions using 160 wild radish plants and dozens of voracious caterpillars. The caterpillars were allowed to attack the plants for two weeks during two key stages of their life – when the seedlings had produced their first two leaves and when the adult plants were flowering. For comparison, a sample of control plants was kept safe from caterpillar attacks.

The researchers estimated the effectiveness of each plant’s physical defenses by counting the density of hairs on leaf samples. Their chemical defences, in the form of mustard oil exuded from the leaves, have also been collected and analyzed. Finally, leaf tissues from attacked and unattacked plants were analyzed for signs of DNA methylation.

The study confirmed that DNA methylation is a chemical call to arms in wild radish plants and that attacks by hungry caterpillars trigger DNA methylation in both parent plants and their offspring.

Experiments on the offspring of attacked wild radish plants revealed that the physical and chemical defenses of wild radishes were easily activated by predation at the seedling stage. However, only chemical defenses – not physical defenses – were deployed by adult plants in response to attack.

“There were several surprises,” Sobral said. “We did not expect that adult plants could be induced to produce tusks and can display the tusks they inherited from their ancestors. We assumed that these processes occur mainly at the seedling stage. »

Finding that wild radish plants can activate anti-predator defenses as seedlings and adults, but adults can only use chemical defenses, was also a surprise; it was generally thought that only seedlings could activate anti-predator defenses.

This study helps inform our understanding of the many complex plant-insect interactions on Earth, the researchers say.

About 50% of the species that scientists have discovered and named on the planet are made up of ‘higher’ plants (like ferns, conifers and flowering plants) and the insects that feed on them,” Dirzo said.

And those are just the species that we know of, he pointed out. “Thus, their interactions represent a central feature of Earth’s biodiversity,” he added. “We cannot understand the diversity of life on Earth without understanding the interactions between plants and the organisms that feed on them.”

Isabelle Neylan is now a graduate student at the University of California-Davis. Additional co-authors of the study, titled “Phenotypic plasticity in plant defense across life stages: inducibility, transgenerational induction and transgenerational priming in wild radishinclude Luis Sampedro of Consejo Superior de Investigaciones Científicas, Spain and David Siemens of Black Hills State University, South Dakota.

Funding for this research was provided by Stanford University.

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