Cold-dependent ELF6 accumulation drives H3K27me3 demethylation to regulate flowering time response to low ambient temperature

Plants do not have calendars, yet they know exactly when to bloom. This timing is a matter of life and death; flowering too early or too late can mean the difference between a bountiful harvest and a failed season. To get it right, plants constantly monitor their surroundings, particularly the temperature. They have evolved intricate systems to sense whether the air is warming or cooling and to adjust their internal clocks accordingly. In the model plant known as Arabidopsis, scientists have long understood that specific genes act as brakes on flowering, holding the plant back until conditions are favorable. Among these, a group of genes acts as the primary gatekeepers, preventing the plant from rushing into bloom during a cold snap. However, the molecular machinery that allows these genes to react so precisely to a drop in temperature has remained somewhat of a mystery, especially regarding how the plant's internal genetic switches are physically flipped on or off by the cold.

A new study published by researchers from Spain, Germany, and China has peeled back the layers of this process, revealing a specific mechanism where temperature directly influences the accumulation of a protein that acts as a molecular eraser. The team focused on two proteins, ELF6 and JMJ13, which function as histone demethylases. In simple terms, these proteins remove a chemical tag called H3K27me3 from the DNA packaging inside the cell. This tag usually acts like a heavy lock, keeping genes turned off. By removing the lock, the proteins allow genes to be read and expressed. The researchers discovered that these two proteins work together to ensure that the plant's flowering brakes are applied correctly when the weather turns cold. Without them, the plant fails to delay flowering as it should, essentially losing its ability to sense the chill.

The investigation began by looking at mutant plants that lacked these proteins. When the researchers grew plants missing both ELF6 and JMJ13, they observed a striking difference in how the plants responded to a drop in temperature. Under normal laboratory conditions, these mutant plants flowered earlier than usual. However, when the temperature was lowered to a cool 16 degrees Celsius, the mutant plants failed to show the expected delay in flowering that wild-type plants display. Instead, they continued to rush toward blooming as if the cold were not there. This indicated that the two proteins are essential for the plant to recognize and react to low ambient temperatures. The researchers confirmed this finding using different genetic versions of the mutants, ensuring the result was robust and not a fluke of a single experiment.

To understand why this happened, the team looked at the specific genes that control flowering. They focused on a set of genes known as the FLC-clade, which includes FLC and FLM. These genes produce proteins that act as repressors, stopping the plant from flowering. The researchers found that in the mutant plants lacking both ELF6 and JMJ13, the levels of these repressor genes dropped significantly, particularly the gene known as FLM. This drop in gene activity meant the "brakes" on flowering were not being applied. Further analysis showed that this was not just a matter of the genes being turned down; the chemical locks on the DNA, the H3K27me3 tags, had accumulated excessively in the mutant plants. This accumulation prevented the genes from being read properly. In healthy plants, the ELF6 and JMJ13 proteins remove these locks, allowing the FLM gene to be active and keep the plant from flowering too soon.

The study went deeper to see when and how these proteins interact with the DNA. The researchers tracked the location of ELF6 and JMJ13 within the plant cells as the seedlings grew. They found that the behavior of the two proteins was quite different. The JMJ13 protein was present at the gene locations constantly, regardless of the temperature or the age of the plant. It acted as a steady, background presence. In contrast, the ELF6 protein behaved dynamically. In young seedlings, ELF6 only gathered at the gene locations when the plants were grown in the cold. At warmer temperatures, it was largely absent from these spots during the early stages of growth. This suggests that the cold itself triggers the plant to produce or recruit more ELF6 protein specifically to the genes that control flowering.

To confirm this, the researchers measured the actual amount of ELF6 protein inside the cells. They found that in seedlings grown at low temperatures, the amount of ELF6 protein increased significantly compared to those grown at standard temperatures. This increase happened very early in the plant's life, right when the seedling is just emerging. This timing is crucial because it ensures that the plant's flowering brakes are engaged immediately upon exposure to cold, preventing a premature bloom. The study also ruled out the idea that other related proteins, such as REF6, were the primary drivers of this specific cold response, and it showed that the effect was not dependent on a different gene called MAF2. The primary driver of the temperature sensitivity was clearly the FLM gene, with FLC playing a secondary role.

The findings paint a clear picture of how a plant translates a physical sensation of cold into a genetic command. When the air cools, the plant increases its supply of the ELF6 protein. This protein then travels to the DNA of the FLM gene and removes the chemical locks that would otherwise silence it. With the locks removed, the FLM gene is free to produce the repressor proteins that tell the plant to wait before flowering. This mechanism ensures that the plant does not flower during a cold spell, which could damage its reproductive organs. The research highlights that the plant's ability to adapt to its environment relies on a delicate balance of chemical tags on its DNA, and that specific proteins act as the sensors and erasers that maintain this balance. By identifying this pathway, the study provides a deeper understanding of how epigenetic mechanisms—the chemical modifications that control gene activity without changing the DNA sequence itself—allow plants to survive and thrive in a changing climate.