Reading the leaves: Decoding the secret cellular language of plants

Key Takeaways

• HHMI Investigator Keiko Torii and her team are investigating how plant cells communicate with each other to specify what kind of cell they will become, allowing them to form functional multicellular tissues and organs.
• In recent research, Torii and her team uncovered the signaling that happens between and inside cells to enable the correct development of stomata — the pores on leaves that allow plants to exchange gases and water.
• The researchers are also probing the shared signaling pathways plants use to balance stomatal development and their response to invading pathogens.
• Studying how plants grow, adapt, and defend themselves could help scientists develop drought-tolerant, water-efficient crops and provide insights into how cells communicate.

There’s more to growing a plant than just adding enough water and providing adequate sunlight.

Every new leaf, root, and flower depends on communication between the plant’s countless cells — an intricate cellular conversation that has enabled trees, grasses, and crops to survive on land for nearly half a billion years.

For the past 30 years, HHMI Investigator Keiko Torii has been listening to and decoding these cellular transmissions. Her research has revealed the molecular machinery that powers cellular communication and enables plants to adapt to changes in their environment — from conserving water in a drought to fighting off invading pathogens.

Research by Torii and her team at the University of Texas at Austin not only helps scientists develop crops that can withstand changing conditions but also provides insights into how our own cells stay resilient. Although plants and animals evolved independently, they both rely on sophisticated systems of cellular communication to stay alive.

“It doesn’t really matter whether a person is studying a particular type of cancer or bacterial communities or plant biology, it is all connected,” Torii says. “I think plants can give us a lot of hints that we can all learn from.”

Probing the Patterning of Plant Pores

Early in her career, Torii was part of a team that discovered ERECTA, a receptor that spans the plant’s cell membrane, allowing it to receive incoming signals from neighboring cells.

These communication signals allow cells to regulate the plant’s growth and development, including the patterning of stomata — pores on the leaf surface that allow gases and water vapor to be exchanged. ERECTA can adjust the number of stomata based on the signals it receives about the plant’s development or environmental conditions.

Researchers have shown that when ERECTA becomes activated, it triggers a signaling cascade inside the cell that eventually turns genes on or off. These genes include the “master regulators” of stomatal differentiation — with fun names like SCREAM, SPEECHLESS, and MUTE — that Torii’s team also identified. But exactly how this process works — and what happens to the receptor inside the cell when it’s activated — has remained elusive.

To get a full picture, Torii and her team recently examined proteins that interact with the receptor and are nearby when the receptor is activated. They identified proteins that shuttle ERECTA to the cell’s internal vesicles and, eventually, to the plant’s vacuoles — storage compartments that degrade the receptor. Companion research shows how activated ERECTA receptors are marked for ingestion and degradation.

“This whole routine has to be running for proper signaling,” Torii says. “If this flow is disrupted, then they cannot properly signal to organize the proper stomatal patterns.”

Balancing Signals for Growth and Defense

Torii and her team are also examining the interaction between the plant’s development and immune signals.

Plants have immune receptors that can detect invading pathogens and trigger an immune response, closing the plant’s stomata that bacteria use to invade the plant. Interestingly, these receptors and ERECTA look nearly identical and trigger similar pathways, even though they have different functions. The immune response pathway can also hijack the developmental pathway in certain conditions, reducing the number of stomata if pathogens are trying to invade.

“It is a big conundrum — how do cells recognize that this is a pathogen signal or a developmental signal and adjust the response accordingly?” Torii says.

Torii and her team recently uncovered the conditions that allow the immune signaling cascade to override the developmental pathway and regulate stomatal development. Using a new chemical they identified with their collaborators, the team perturbed the downstream pathway and identified the machinery shared by both signals.

Torii Lab's work hints that pathogens may increase the number of entry points, favoring infection. In return, plant immune signaling flows into the ERECTA pathway to fight back, reducing the number of stomata. The team is now working to figure out exactly how the plant tells the two signals apart.  

Building Better Crops

Although Torii works with Arabidopsis, a common model plant used by biologists, stomata evolved only once in land plants, so the genes, regulators, and pathways Torii and her team are identifying are important for all species — including crops that form the global food supply. The team is also examining how plants that live underwater and don’t need stomata shut down the genes required for their development.

Understanding how these pathways evolved and how different environments like heat, drought, pathogen infection, and future climate conditions impact these pathways could help scientists develop plants that can withstand changing conditions, like crops that use less water without compromising yield.

Torii says seeing the larger impact of her work is gratifying and far beyond what she could have imagined when she first started her research in Arabidopsis 30 years ago.

“The key regulators and pathways we identified seem to be super important for all land plants and it’s interesting to see how land plants evolved this pathway and how they utilize it,” Torii says.