Disruption of the single-copy GOLDEN2-like gene underlies the classical yellow and yellow-mutable mutations of Japanese morning glory

In the quiet green machinery of a leaf, tiny factories called chloroplasts work tirelessly to capture sunlight and turn it into food. These factories rely on a specific set of instructions to build their internal structures, particularly the stacked membranes where the light-harvesting happens. Without these instructions, the factory cannot assemble correctly, and the plant loses its vibrant green color, turning a pale yellow instead. Scientists have long known that a family of proteins, which act like master switches for these instructions, is essential for this process. In many plants, there are several copies of these switches, so if one fails, another can often take its place. This redundancy makes it difficult to study exactly what happens when a single switch is lost, because the backup systems hide the true effect.

A team of researchers turned to the Japanese morning glory to solve this puzzle. This flower is famous in the scientific community for its classical yellow mutation, a trait that has been observed for over a century. The plant with this mutation has leaves that are a sickly yellow-green, and in some cases, the mutation is unstable, causing green patches to appear randomly on the yellow leaves. By studying these specific plants, the researchers found a unique opportunity: unlike most plants, the Japanese morning glory possesses only a single copy of the master switch gene responsible for chloroplast development. This means there is no backup system to hide the consequences of a broken gene. The scientists set out to identify the exact genetic cause of the yellow color and to see how the absence of this single gene reshapes the leaf's internal world.

The researchers discovered that the gene responsible for the yellow color is called InGLK. In the stable yellow mutant plants, this gene was broken by a tiny error: a four-letter insertion in the genetic code that shifted the entire reading frame, rendering the protein useless. In the unstable lines, which show green patches on a yellow background, the gene was interrupted by a jumping piece of genetic material known as a transposon. This piece of DNA had inserted itself into the middle of the gene. When the transposon jumped out again, it sometimes left the gene functional, allowing the plant to grow green tissue. This mechanism explained why the unstable plants could revert to green in certain spots, while the stable mutants remained yellow everywhere.

To understand what was happening inside the leaves, the team examined the pigment levels and the microscopic structure of the cells. They found that the yellowish-green background tissue contained significantly lower amounts of the pigments that give leaves their color and drive photosynthesis. However, the ratio between the two main types of green pigment remained the same as in healthy plants. When they looked at the chloroplasts under a powerful microscope, the difference was stark. The chloroplasts in the yellow tissue still had some internal structures, such as membrane layers and starch-like grains, but they lacked the organized, stacked arrangement seen in healthy green leaves. The internal membranes were sparse and disordered. In the green patches that appeared on the unstable plants, and in the plants where the jumping DNA had completely left the gene, the chloroplasts looked normal again, with fully formed stacks and rich pigment.

These findings confirm that the loss of this single gene is enough to disrupt the entire development of the chloroplast, reducing pigment production and scrambling the internal architecture. Because the Japanese morning glory has only one copy of this gene, the plant cannot compensate for the loss, making the effects of the mutation clear and total. The study shows that this single gene is the primary driver for building the photosynthetic machinery in this species. By providing a system where a single genetic change leads to a complete and visible transformation of the leaf's structure, these yellow mutants offer a clear window into how plants build their solar power plants, free from the confusion of backup genes.