The Sulfated PSY Peptide Negatively Regulates Receptor Kinase Activity to Promote Growth

Plants are masters of stillness. They cannot run from a storm, hide from a predator, or seek out the sun; they must grow exactly where they are planted. To survive, they rely on a complex internal language, a system of chemical signals that tells cells when to expand, when to divide, and when to stop. Among the most important messengers in this system are tiny protein fragments called peptide hormones. These molecules float between cells, delivering instructions that shape the plant's form. In many plants, these peptides carry a specific chemical tag—a sulfate group attached to a tyrosine amino acid. This tag acts like a key, allowing the peptide to fit into a lock on the cell's surface. That lock is a receptor, a protein that, once the key turns, triggers a chain reaction inside the cell. For decades, scientists believed that when a peptide hormone found its receptor, it switched the receptor "on," activating a machine inside the cell that drove growth. This was the standard rule for how plants read their chemical signals.

A team of researchers at Dartmouth College and the University of California, Davis, has now found that this rule does not apply to a specific family of growth hormones in moss. Using the spreading earth moss, Physcomitrium patens, as a model, they discovered that the receptors for these sulfate-tagged peptides work in the exact opposite way. Instead of turning on when they catch a peptide, these receptors are actually active and aggressive by default, constantly telling the plant to slow down its growth. The peptide hormone does not activate them; it silences them. When the hormone binds to the receptor, it turns the growth-inhibiting signal off, allowing the plant to expand. This discovery upends a long-held assumption about how plants control their size and development, revealing a mechanism where the absence of a signal is more dangerous than its presence.

The researchers began by looking at a mutant moss plant that could not produce the sulfate tag on its peptides. In a normal plant, the sulfate tag is added by an enzyme called TPST. Without this enzyme, the peptide hormones are incomplete and cannot fit into their receptors. The scientists expected that without the signal, the plant would simply fail to grow. Instead, they found that the receptors on the surface of the cells in this mutant were piling up in huge numbers. It was as if the cell, sensing that no key had arrived, had decided to install more and more locks on its door. These extra locks were not just sitting there; they were active. The researchers observed that this accumulation of unbound receptors caused the moss to become stunted, dense, and unable to form the leafy structures it needs to thrive. The plant was essentially stuck in a state of emergency, constantly being told to stop growing.

To prove that these receptors were the cause of the stunted growth, the scientists removed the receptors entirely from the mutant moss. They used a precise gene-editing tool to delete the genes that make the receptors. The result was immediate and dramatic. The mutant moss, which had been small and cramped, suddenly began to grow vigorously. It spread out across the petri dish, forming large, healthy leafy structures. This showed that the problem was not the lack of the peptide hormone itself, but the presence of the active receptors. The receptors were the brakes on the car; removing the hormone just meant the brakes were stuck on, but removing the receptors allowed the car to drive freely.

The team then investigated how these receptors actually worked. They created a version of the receptor that was broken in a specific way: it could still sit on the cell surface, but it could not send a signal inside the cell. When they introduced this broken, inactive receptor into the mutant moss, the plant grew normally. This confirmed that the receptor's ability to send a signal was the problem. The receptor was acting like a machine that was always running, constantly grinding down the plant's growth potential. The researchers found that when the receptor was active, it accumulated on the cell surface in high numbers. When it was inactive, it stayed at normal levels. This suggested that the act of signaling itself caused the receptor to build up, creating a feedback loop where more signaling led to more receptors, which led to even more signaling.

The final piece of the puzzle came from looking at what happens when the plant has plenty of the peptide hormone. In a normal plant, the hormone floats around and finds the receptors. When it binds, it does not turn the receptor on; it turns it off. This inactivation stops the receptor from sending its "stop growing" signal. It also causes the receptor to leave the cell surface, reducing the number of locks on the door. With the brakes released, the plant grows. The researchers also found that this system controls the production of the hormone itself. When the receptors are active and the plant is small, it makes less hormone. When the receptors are inactive and the plant is growing, it makes more hormone. This creates a delicate balance, a negative feedback loop that keeps the plant's size in check.

This study reveals a unique way that plants manage their development. In the moss, the receptors for these growth hormones are not passive receivers waiting for a command. They are active inhibitors, constantly working to restrain growth until the specific hormone arrives to silence them. This is different from the way many other plant receptors work, where the hormone is the switch that turns the machine on. The researchers showed that this mechanism is conserved, meaning it is a fundamental part of how plants control their size, even though the specific genes involved might differ between moss and flowering plants. By understanding that these receptors are naturally active and must be turned off to allow growth, scientists gain a new perspective on how plants regulate their size and respond to their environment. It suggests that for these specific signals, the plant's default state is to hold back, and growth is only possible when the signal arrives to release the tension.