Young plants pay a big price for fighting off disease

From playground coughs to forest blights, the young suffer more from disease than adults. This pattern stretches across ecosystems and species. Children in schools catch colds more frequently. Seedlings wilt under fungal attacks while mature plants stand strong.

Scientists have long wondered why evolution hasn’t corrected this vulnerability. Shouldn’t early life be better protected, given how deadly infection can be at that stage?

A new study from University of Maryland (UMD) biologists brings some clarity to this age-old puzzle. Their work doesn’t focus on toddlers or baby animals, but on plants. Specifically, they looked at Silene latifolia, or white campion, a common wildflower in Europe and parts of North America.

This plant is often infected by a fungal disease called anther-smut. It doesn’t kill the host but stops it from reproducing by destroying its pollen-producing parts.

A costly defense in youth

The researchers found that young plants face a serious trade-off. Those that resist disease early on do so at a steep cost. Their growth slows down, and their reproductive success later in life drops significantly.

“It’s a mystery why young organisms don’t evolve stronger disease resistance because getting sick early in life can be deadly,” said study co-author Emily Bruns, an assistant professor of biology at UMD.

“Our findings suggest that a hidden trade-off is involved, stopping them from being able to completely fight off a disease.”

To uncover these patterns, the team grew 45 genetically different lines of the white campion under controlled lab conditions. They tracked how each one responded to the anther-smut fungus as seedlings and later in life.

Early immunity drains energy

The results showed that baby plants with stronger resistance produced far fewer flowers and seeds as adults – even when grown in fields without any disease.

This surprised the researchers. Why should defense in early life cost so much? The answer lies in how plants allocate energy.

Young plants, like young animals, operate with limited internal resources. What they spend on defense cannot go toward growing roots, leaves, or reproductive tissues later.

“We found that young plants paid a higher ‘cost’ for fighting the disease compared with adult plants,” Bruns said.

“Trying to fight off the fungus was more difficult and resource-consuming for these baby plants. They only have so much energy to spend. If baby plants spend it on disease defense, they can’t put it toward future growth.”

This concept is important. Adult plants with high resistance show no such penalty. They’ve already developed their roots and leaves and can afford to spend more on immune defenses.

Seedlings, on the other hand, gamble with every drop of stored energy. Using it on immunity might leave them alive, but too weak to reproduce.

Evolution is halted in young plants

To test whether this cost could explain why strong juvenile resistance doesn’t evolve, the researchers created a mathematical model. It revealed that the energy price of resistance is high enough to halt the evolution of stronger defenses in young plants.

If this cost didn’t exist, plants with better early resistance would outcompete others and wipe out the disease. But because resistance drains future reproductive potential, evolution holds back.

“Some young plants ‘pay the cost’ and survive into adulthood, but they make fewer flowers, meaning they’re less able to reproduce,” Bruns explained. “But most remain susceptible as babies, allowing the disease a toehold.”

Even more surprisingly, the negative effects didn’t appear immediately. Young plants that resisted the disease looked healthy during the first growing season. But when they reached their reproductive peak in the second year, the impact showed. These plants bloomed far less than expected.

Gender shapes the impact

The researchers also found a striking difference between male and female plants. Males suffered much higher penalties when they invested in resistance.

Bruns thinks this might be because male white campion plants produce more flowers than females. Their reproductive success depends on spreading as much pollen as possible, so losing energy to defense hits them harder.

“Male plants produce many more flowers than females to spread their pollen as widely as possible – making the cost of diverting resources to disease resistance especially steep for males,” she said.

This insight points to how evolution can apply pressure differently across sexes. In plants as in animals, reproductive strategy affects how disease shapes the body. The same infection might challenge one sex more than the other, depending on where they spend their biological resources.

Why young plants’ weakness matters

While this research focuses on a wildflower, its implications stretch far. Many species – from humans to fish – show a similar pattern: greater vulnerability in youth. Bruns believes this study offers a new lens for thinking about why that happens.

Because juveniles carry the disease burden for many populations, understanding why they remain vulnerable could help us design better approaches in agriculture, conservation, and health.

“Nature is full of infectious diseases. Understanding the different checks and balances between hosts and pathogens helps us understand how evolution has shaped these relationships over millions of years,” Bruns said.

Future experiments and applications

The team doesn’t plan to stop with this study. They now want to know if timing matters. What happens if the disease hits just a little later – after seedlings grow their first true leaves and rely less on stored energy? Could resistance become less costly then?

They also plan to explore another angle: the protective effect of adult plants. If resistant adult plants surround seedlings, would that lower the local disease pressure? Could the adults, in effect, guard the next generation by absorbing the fungal threat?

These questions could open new research paths – not just for ecologists, but also for people trying to reduce disease in crops or manage infections in wildlife.

Evolution has no perfect solutions. But studies like this reveal the complex calculations that every organism, even a wildflower seedling, must make.

The study is published in the journal Proceedings of the National Academy of Sciences.

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