Forever Young

In the animal kingdom, some species live like Peter Pan: They never grow up. For example, the axolotl, native to Mexico, keeps its juvenile features even after reaching reproductive maturity. This phenomenon is called neoteny, and if it happened in humans, we would look like toddlers for our entire lives.  But neoteny isn’t just limited to animals. In fact, it’s surprisingly common in plants, too.  “Plants probably have more examples of neoteny than animals,” says Scott Poethig, John H. and Margaret B. Fassitt Professor Emeritus of Biology. In career-culminating research published in the Proceedings of the National Academy of Sciences, Poethig uncovered the genetic underpinnings behind neoteny in plants. The work reveals how a single molecular switch can freeze a plant in its juvenile state—a finding Poethig says holds big implications for conservation, horticulture, and more. A Genetic Switch in Every Plant

Scott Poethig, John H. and Margaret B. Fassitt Professor Emeritus of Biology (Image: Jia He)

Poethig first became fascinated by plants in graduate school, when he investigated corn mutants. His later work in Arabidopsis, a small plant in the mustard family, led to the discovery of a molecule that controls when plants develop their adult features. The molecule, called miR156, is a type of microRNA, a class of RNA that helps regulate gene expression. It turns out, according to Poethig’s work, that miR156 is key for the juvenile-to-adult transition for Arabidopsis. This transition is separate from reproductive maturity, and generally involves physical traits that help the plant adapt to different stages of its life. For example, juvenile leaves in some plant species may be better suited for rapid growth in moist, competitive environments, whereas adult leaves fare well in dry, high-light conditions.  Poethig found that Arabidopsis contained high levels of miR156 as juveniles, which prevented the genes associated with adult traits from expressing, but those levels naturally decreased into adulthood.

By analyzing RNA in juvenile and adult samples of eucalyptus from San Francisco, acacia trees from Australia, and ivy and oak from Penn’s own campus, Poethig confirmed miR156 levels controlled the juvenile-to-adult transition in plants all around the world.  “Every plant does this, and nobody had figured out the mechanism before,” Poethig says.  Why Some Plants Never Grow Up Like axolotls, some plant species never make the juvenile-to-adult transition. Take Acacia, a genus of about 1,000 types of shrubs and trees. A handful of these retain their complex juvenile compound leaf structure rather than developing a simplified adult leaf structure. Poethig wanted to know whether miR156 was the reason.  As a graduate student in Poethig’s lab, Aaron Leichty, now a researcher at the USDA Plant Gene Expression Center and Professor in the Department of Plant and Microbial Biology at University of California, Berkeley, grew more than 100 Acacia species in Penn’s greenhouse. “Every plant we grew for the first time was a surprise for us, since neither one of us had really seen these plants in the wild,” Leichty says.  Leichty measured how long each tree stayed in its juvenile phase and used DNA sequencing to chart the evolutionary relationships between different species. The results revealed that neoteny likely evolved independently at least seven times in Acacia, and higher levels of miR156 likely played a role in some of these events.  Over the years I’ve made it my goal to convince people that plants are interesting. They matter—to you, to me, to the planet.

Poethig and Leichty are still investigating the master genes behind this process. “We think that this prolonged juvenility is due to a mutation in just one or two genes, which is a pretty big deal,” Poethig says. “Some people question the process of evolution, but we might be seeing how a single mutation can give you an entirely different plant.” The Importance of Understanding  Mapping acacia revealed an unexpected pattern: These “forever young” species clustered along Australia’s southwest and southeast coasts, regions with cool, humid climates, while their adult-leafed relatives dominated the continent’s hot, arid interior.

This suggests, according to the researchers, that keeping juvenile traits offers a competitive advantage in certain environments. “It’s likely that neoteny is an adaptation to temperate and subtropical ecosystems, where rapid growth is key to competition,” Leichty says.

The research also points to practical applications: Because miR156 acts as a master regulator of growth, tuning its expression could help scientists tailor plants to different needs. For example, manipulating the molecule could simplify plant propagation or help fit crops to local climates, creating varieties that grow faster, use water more efficiently, or resist pests better. And by prolonging the juvenile phase, when leaves are more efficient at photosynthesis and contain less thick, woody substances, changing miR156 could also make biofuels easier to produce.  Ultimately, Poethig says he hopes this work will broaden how scientists think about plant development. “You have to remember that plants change throughout their lifetime,” he says. “Plant biologists tend to be focused on flowering, but other developmental changes matter just as much.” And after a lifetime unraveling those hidden transitions, he wants to share them with everyone. “Over the years I’ve made it my goal to convince people that plants are interesting,” he says. “They matter—to you, to me, to the planet.”