InMYB21B Promotes Petal Cell Expansion and Flower Opening in Japanese Morning Glory (Ipomoea nil)

Flowers do not simply appear; they are built, piece by piece, through a carefully timed sequence of growth. For a plant to reproduce, its petals must expand from tiny buds into full, open displays, a process that requires cells to multiply and then swell to their final size. This swelling is driven by water rushing into the cells, which happens only when the cells accumulate enough sugar to draw the water in, much like a sponge soaking up liquid. At the same time, the cell walls must loosen to allow this expansion. While scientists have long known that sugar and water are the fuel for this opening, the specific genetic switches that coordinate the release of sugar and the loosening of walls just before a flower blooms have remained a mystery. Understanding this mechanism is key to knowing how plants time their most visible moment of life.

In a study of the Japanese morning glory, a flower famous for opening its blooms precisely in the morning, researchers have identified a critical genetic regulator that controls this final burst of growth. The team focused on a specific type of protein known as a transcription factor, which acts like a master switch, turning other genes on or off. They discovered that a gene named InMYB21B is essential for the flower to open. When the researchers disabled this gene using precise genome editing, the flowers failed to open entirely. Instead of swelling and unfurling, the petals remained small and tight, eventually withering and falling off the plant. The study reveals that without InMYB21B, the petals cannot break down their stored starch into the sugars needed to draw in water, nor can they loosen their cell walls to expand.

The researchers tracked the growth of the morning glory petals over several days, measuring their weight, length, and sugar content. They found that in normal flowers, the petals grow slowly for a while, then undergo a rapid expansion starting about twenty-one hours before the flower opens. During this critical window, the plant breaks down starch, a stored energy source, into glucose, a simple sugar. This spike in sugar increases the pressure inside the cells, pulling in water and causing the petals to swell. The team also observed that the genes responsible for breaking down cell walls and transporting water into the cells become highly active during this same period. It is a coordinated effort where sugar production, water intake, and wall loosening happen in unison to push the flower open.

To find the master switch behind this coordination, the scientists analyzed the activity of thousands of genes during the days leading up to the bloom. They used a computer method to group genes that rise and fall in activity together, looking for the ones that act as central hubs in the network. Two genes, InMYB21A and InMYB21B, stood out as central players. However, when the researchers created plants that lacked InMYB21A, the flowers opened normally, showing only a very slight reduction in size. In contrast, plants lacking InMYB21B were completely unable to open their flowers. The petals of these mutant plants stopped growing roughly a day before they should have, and the cells inside them remained tiny and unexpanded.

Further investigation showed that the cells in the mutant flowers were not only smaller but also far more numerous than in normal flowers. This suggests that InMYB21B does more than just help cells grow; it also signals them to stop dividing. In normal flowers, cell division slows down and stops as the petals prepare to expand, but in the mutant flowers, the cells kept dividing, resulting in a crowd of tiny cells that could not stretch. The study indicates that InMYB21B acts as a gatekeeper, ensuring that the transition from cell division to cell expansion happens at the right time. Without this signal, the developmental program stalls, and the flower never reaches its full size.

The absence of InMYB21B also disrupted the plant's ability to manage its energy and water. In the mutant petals, the breakdown of starch was severely impaired, and the levels of glucose did not rise as they should. Consequently, the genes responsible for loosening the cell walls and for moving water into the cells were not turned on. The researchers found that the genetic activity in these mutant flowers looked like that of a much younger flower, as if the developmental clock had been turned back. This delay in the genetic program prevented the physical changes necessary for the flower to open.

Interestingly, the loss of InMYB21B affected more than just the petals. The mutant plants also failed to develop functional reproductive parts. The male parts, which produce pollen, and the female parts, which receive it, did not mature correctly, rendering the plants sterile. This suggests that InMYB21B plays a broad role in the development of the entire flower, not just the colorful petals. While the related gene InMYB21A appears to have a minor role, InMYB21B is clearly the dominant force driving the final stages of flower opening in the Japanese morning glory.

This work provides a clear picture of how a single gene can orchestrate a complex biological event. By linking the production of sugar, the movement of water, and the physical expansion of cells, InMYB21B ensures that the flower opens at the precise moment required for pollination. The findings highlight that the dramatic opening of a flower is not just a passive event but an active, genetically controlled process that depends on the timely activation of specific metabolic and structural changes. Understanding this mechanism in the Japanese morning glory offers a window into how other plants might control their own blooming, revealing the intricate genetic choreography that allows life to unfold.