Study: Soybeans seem to inherit the bad memories of their parents

Fast facts

Experiments show soybean progeny react to stressors placed on parents

FAYETTEVILLE, Ark. — When soybean plants survive attacks from insects and periods of drought, they remember.

PLANT MEMORY — Researchers with the Arkansas Agricultural Experiment Station have shown how soybean plants can pass on adaptive traits to progeny after experiencing drought and herbivory  (U of A System Division of Agriculture photo by Manish Gautam)

While plants don’t remember in the way animals do, research out of the Arkansas Agricultural Experiment Station shows that soybean plants can pass on adaptive responses to stress — like those hungry insects — across generations without changing their DNA.

Scientists call this kind of adaptation across generations “transgenerational plasticity,” and the consensus has been the independent stressors of drought and herbivory, or animals feeding on plants, can induce gene expression — possibly through epigenetics. Unlike genetic changes, or mutations, epigenetic changes are reversible and don’t change the DNA sequence, but rather how an organism read its DNA sequence.

Implications for agriculture

In soybean — one of the world’s most important crops — researchers with the experiment station, the research arm of the University of Arkansas System Division of Agriculture, have found the first evidence that drought and insect herbivory can create lasting, transgenerational effects. These stressors not only affect the parent plants but alter the traits and defenses of their offspring.

The research demonstrates some of the positive and negative impacts the stressors have on a plant’s progeny and could be used to develop more resilient crops in the same season.

As a vaccine can build immunity, techniques such as “priming” and “hardening” in the early vegetative stages might enable the plants to withstand future setbacks with minimum reduction in yield, according to Rupesh Kariyat, associate professor of crop entomology in the entomology and plant pathology department for the Division of Agriculture and the Dale Bumpers College of Agricultural, Food and Life Sciences.

“This gives us the opportunity where we can manipulate the degree of stress of soybean to bolster defenses early in the season without compromising the final yield of the crop,” Kariyat said. “But there is a catch — we have yet to quantify the threshold under drought and herbivory stress that may cause more harm than good to the plants.”

For the past two years, Kariyat and doctoral students Manish Gautam and Insha Shafi have looked at how the caterpillars of two insects — soybean looper and fall armyworm — interact with soybean plants, and their effects on parent and progeny plants in a variety of situations including the coincidence of drought and sequential herbivory.

INTERACTIONS — Rupesh Kariyat, associate professor of crop entomology for the Arkansas Agricultural Experiment Station, investigates plant-insect interactions. (U of A System Division of Agriculture photo)

Kariyat, who holds the Clyde H. Sites Endowed Professorship in International Crop Physiology, notes that climate change is already making insect threats worse.

“Insects are getting bigger, and they’re going through multiple generations each year,” he said. “That leads to increased pesticide use, which isn’t sustainable.”

Improving soybean resilience through stress memory could reduce pesticide dependence, with significant ecological and economic benefits. While U.S. farmers typically purchase fresh seed annually, in Brazil and Kenya, many farmers rely on saved seeds to avoid the high cost of commercial varieties. In such systems, traits passed from parent plants to offspring become especially relevant.

Pros and cons of stress memory

Kariyat and his doctoral candidates conducted multi-layered experiments to test if soybean plants under drought stress are more vulnerable to insects, and how much of the parental plant’s memory — its response to stress — is passed on to its progeny to cope with specific kinds of herbivory.

Progeny of stressed plants had seeds with higher nitrogen and protein content, key markers of plant fitness. Offspring also produced more flowers and had a greater density of trichomes, the tiny hair-like structures that defend against pests. Kariyat noted that these positive effects were strongest when parent plants experience both drought and herbivory pressure.

Despite these advantages, the stress memory came at a cost — strong defenses but weaker growth. Stressed progeny showed reduced yield, including a higher rate of empty pods.

Defensive trichomes also declined with maturity, suggesting that the enhanced defenses may be short-lived or age-dependent. The research suggests that there is a costly trade-off between survival and productivity, Kariyat said.

Kariyat concluded that their research so far points to stress memory in soybeans being a double-edged sword. While it can improve defense and early vigor, it also leads physiological trade-offs that ultimately reduce fitness and yield.

Experiment spotlight: Caterpillars on a bridge

To see whether insects could detect past drought stress in soybeans, the researchers built tiny bridges between previously drought-stressed plants and plants that had consistently received enough water. They observed soybean looper caterpillars pausing, rotating their heads and often reversing course back to the healthier plants, the study noted.

The scale of damage by the caterpillars on consistently watered plants was significantly higher than the plants that had recovered from drought stress, supporting the “plant vigor hypothesis” that pests prefer robust hosts.

First contact matters

Most previous studies focus on a single stressor, but real farm conditions often involve multiple. Kariyat’s team tested what happens when soybean looper and fall armyworm attack in sequence. What they found was surprising.

When soybean looper came first, soybean plant progeny had 40 percent higher nitrogen content, more flowers and pods, and better protein levels than when the order was reversed.

While this might suggest herbivory can prime soybeans for better performance, Kariyat cautions the effect depends on the type, timing and severity of stressors.

“It’s not that herbivory always improves the plant performance, but the type, severity and combination of stressors determine whether the responses would be beneficial or detrimental,” Kariyat said. “From the research so far, while moderate or minimum biotic stress may induce resilience in soybeans — and this is also found in other systems — the combined abiotic and biotic stress may lead to exhaustive performance, triggering expensive defensive responses that often compromise yield and fitness.”

For example, the hair-like structures on plants called trichomes increased in the progeny of plants that had been exposed to drought and herbivory treatments more so than those just for drought. However, when sequential herbivory was tested, there was no difference in trichome density, which suggested the plants were investing in physiology and fitness traits over physical defenses when coping with just herbivory.

This indicates that drought may play a major role in tipping the balance from beneficial stress to harmful overload.

Read the research

Shafi is co-advised by Ioannis Tzanetakis, professor of plant virology for the experiment station and director of the Arkansas Clean Plant Center.

To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website. Follow us on X at @ArkAgResearch, subscribe to the Food, Farms and Forests podcast and sign up for our monthly newsletter, the Arkansas Agricultural Research Report. To learn more about the Division of Agriculture, visit uada.edu. Follow us on X at @AgInArk. To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit uaex.uada.edu.