Functional divergence of MFT paralogues in flowering promotion and organ development in Platanus acerifolia

In the quiet architecture of a plant's life, there exists a delicate switch that decides when to stop growing leaves and stems and start making flowers. This decision is not random; it is governed by a family of proteins that act as molecular messengers, traveling through the plant to tell specific parts when to change their identity. Among these messengers is a group known as MFT, short for Mother of FT and TFL1. These proteins are ancient guardians of plant development, found in everything from mosses to towering trees. In many plants, a single version of this protein helps regulate when seeds sprout or when a flower appears. However, as plants evolved, some species duplicated these genes, creating pairs of similar but distinct instructions. The question that has long intrigued scientists is what happens when a plant carries two copies of the same instruction: do they work together as a team, or do they split the job, each taking on a unique role?

This question is particularly urgent for the London plane tree, a majestic hybrid tree found in cities around the world. While celebrated for its shade and resilience, the tree has a notorious downside: it produces massive amounts of pollen in spring and sheds fuzzy, irritating seed hairs in summer. These biological byproducts cause significant discomfort for city dwellers and create a mess that is difficult to manage. Breeders have long sought a way to create sterile versions of this tree that do not produce these reproductive organs, but the molecular keys to stopping this process have remained elusive. To find them, researchers turned to the tree's own genetic code, specifically looking for the MFT genes that might control its flowering and fruiting cycles.

A team of scientists from the Wuhan Institute of Landscape Architecture and the Guangzhou Institute of Forestry and Landscape Architecture set out to isolate and study these genes in the London plane tree. They identified two specific genes, which they named PlacMFT1 and PlacMFT2. Despite being cousins in the same genetic family, these two genes had already begun to drift apart in their behavior. Both genes share a similar internal structure, composed of four sections of coding DNA separated by three non-coding gaps, a pattern common to this family of proteins. However, when the researchers looked at where and when these genes were active, a clear division of labor emerged.

The first gene, PlacMFT1, acts as a generalist. It is active in almost every part of the tree, from the stems and leaves to the developing flowers and fruits. It is most abundant in the stems, suggesting it plays a broad role in the tree's overall growth and maintenance. The second gene, PlacMFT2, is a specialist. It remains quiet in most parts of the tree but wakes up with intense energy specifically in the fruit. In the late summer, as the fruits mature, the levels of PlacMFT2 rise so sharply that they surpass even the most active genes in the cell. In young saplings, the two genes also show a complementary pattern: where one is silent, the other is active, ensuring that every part of the young tree is covered by at least one of these genetic signals.

To understand what these genes actually do, the researchers performed a classic experiment in plant science: they moved the London plane tree genes into a completely different plant, the small weed known as Arabidopsis. They forced the Arabidopsis to produce the London plane tree proteins in all its tissues, all the time. The results were immediate and revealing. Both genes, when overproduced, caused the Arabidopsis to flower much earlier than usual. This confirmed that both PlacMFT1 and PlacMFT2 retain the ancient, shared ability to tell a plant to switch from growing leaves to making flowers.

However, the two genes were not identical in their effects. The plants carrying the PlacMFT1 gene did not just flower early; they grew strangely. Their flowers fused together into large, disordered masses, their stems became elongated while their seed pods shrank, and their leaves curled up in an unnatural way. In contrast, the plants carrying the PlacMFT2 gene flowered early but looked otherwise normal, with no deformities in their flowers, leaves, or stems. This difference is crucial. It suggests that while both genes can trigger flowering, PlacMFT1 has acquired a second, more complex job: it helps shape the physical structure of the flower and the fruit. PlacMFT2, on the other hand, seems to focus solely on the timing of the transition, without interfering with the detailed construction of the organs.

The study indicates that the London plane tree has undergone a process called subfunctionalization. This means that after an ancient duplication event gave the tree two copies of the MFT gene, the copies did not simply copy each other's work. Instead, they split the original job into two specialized tasks. One copy took on the role of a broad developmental regulator, influencing how the tree grows and how its flowers form, while the other copy specialized in the specific timing of fruit development. This evolutionary split allows the tree to fine-tune its reproductive cycle with greater precision than if it relied on a single, all-purpose gene.

These findings offer a new perspective on how complex trees manage their growth and reproduction. For the London plane tree, understanding this genetic split provides a roadmap for future breeding efforts. If scientists can learn to manipulate these specific genes, they might be able to suppress the fruit-specific gene to stop the production of irritating seed hairs without harming the tree's overall health or its ability to flower. The research does not yet provide a finished solution, but it identifies the precise molecular levers that control the tree's reproductive cycle. By revealing how these two genetic cousins have learned to do different jobs, the study lays the groundwork for developing cleaner, more manageable street trees that can continue to provide shade without the seasonal nuisance of pollen and fuzz.