Genome-wide Identification and Characterization of the 3- Dehydroquinate Dehydratase/Shikimate Dehydrogenase Gene Family in Rubus chingii Hu

Plants are not just passive greenery; they are chemical factories that constantly manufacture a vast array of compounds to survive, grow, and defend themselves. Among the most vital of these factories is a metabolic pathway known as the shikimate pathway, a fundamental route that plants use to build the building blocks for amino acids and a host of protective chemicals. At a critical junction in this pathway sits a unique enzyme that performs a double duty: it helps create shikimic acid, a precursor for many plant essentials, and it also steers the process toward making gallic acid. Gallic acid is a powerful phenolic compound found in many fruits and herbs, known for its antioxidant properties and its role as a direct precursor to ellagic acid, another beneficial compound. Understanding how plants control this specific enzyme is key to understanding how they regulate the production of these valuable chemicals, which often determine the nutritional and medicinal value of the plant.

Researchers turned their attention to Rubus chingii, a climbing shrub native to China that is prized both as a fruit and as a traditional medicine. The unripe fruits of this plant are used to treat various ailments, while the ripe fruits are eaten as a delicacy. The plant is particularly rich in gallic acid and ellagic acid, but scientists did not fully understand how the plant's genes controlled the production of these compounds. To solve this puzzle, a team of scientists from Zhejiang Sci-Tech University conducted a comprehensive study to map out the specific genes responsible for the enzyme that bridges the gap between shikimic acid and gallic acid. They wanted to know how many of these genes the plant possesses, where they are located, how they are structured, and when they are turned on or off during the plant's life cycle.

The team began by scanning the entire genome of Rubus chingii to find the genes that code for this specific enzyme. They identified exactly four genes, which they named RcDHD/SDH1 through RcDHD/SDH4. These genes are not scattered randomly; they are distributed evenly across just two of the plant's chromosomes. The researchers then examined the physical properties of the proteins these genes create. They found that the proteins vary in size, with some being quite large, containing over 1,300 amino acids, while others are smaller. Despite these size differences, all four proteins share a common structural core that allows them to function as enzymes. The study predicted that these proteins operate primarily within the nucleus and the cytoplasm of the plant cells, the two main compartments where cellular activities take place.

To understand how these genes evolved, the scientists compared them with similar genes found in other plants, ranging from rice and corn to soybeans and grapes. The analysis revealed that the four genes in Rubus chingii belong to a specific family group that is shared with other broad-leafed plants, or dicots, such as soybeans. They did not cluster with the genes found in grasses like rice or corn, suggesting that this group of genes has followed a distinct evolutionary path in broad-leafed plants. The researchers also looked at the structure of the genes themselves, noting that they contain between nine and fourteen segments, known as exons, which are the parts of the gene that carry the instructions for building the protein. This structural variation suggests that while the genes are related, they have diverged enough to potentially perform slightly different roles.

A crucial part of the investigation involved looking at the "switches" located near these genes. These switches, known as cis-acting elements, are sequences of DNA that tell the gene when to turn on. The researchers found that the switches for these four genes are sensitive to light, temperature, and various plant hormones. This means the production of the enzyme is likely influenced by the environment, such as changes in sunlight or the presence of stress. Specifically, the genes appear to have switches that respond to methyl jasmonate, a hormone involved in defense, as well as auxin and abscisic acid, which regulate growth and stress responses.

The team then moved from the computer to the greenhouse to see how these genes behave in a living plant. They measured the activity of the genes in different parts of the plant, including roots, stems, leaves, flowers, and fruits at various stages of ripening. They found that the genes are active everywhere, but their intensity varies. One gene, RcDHD/SDH4, was particularly active in the stems and leaves, while the others showed more consistent activity in the roots. When they tracked the genes during fruit development, a clear pattern emerged. The genes were most active when the fruit was small and green. As the fruit grew larger and began to change color, the activity of these genes dropped significantly.

This drop in gene activity was not random; it matched perfectly with the levels of the chemicals the genes help produce. The researchers measured the amount of gallic acid and ellagic acid in the fruit at each stage. They found that these compounds accumulated rapidly when the fruit was small and green, reaching their peak levels before the fruit turned red. As the fruit matured and the gene activity declined, the levels of these chemicals also stabilized or decreased. This tight correlation suggests that the genes are most critical during the early stages of fruit growth, driving the production of these protective compounds when the fruit is most vulnerable.

To confirm that these genes are indeed responsive to the plant's internal signals, the researchers treated the leaves with different hormones. When they applied methyl jasmonate, the activity of the genes surged, with some increasing their activity by more than five times. This confirmed that the plant uses these genes as part of its defense system, ramping up production when it senses a threat. Similarly, the genes responded to auxin and abscisic acid, though in different ways, with one gene showing a massive thirty-two-fold increase in activity when exposed to auxin. These results indicate that the plant has a sophisticated system for regulating these genes, turning them up or down depending on whether it needs to grow, defend itself, or manage stress.

The study concludes that the four genes identified in Rubus chingii are the primary controllers of the enzyme that links shikimic acid to gallic acid. Their activity is highest in the early stages of fruit development and in the leaves, which aligns with the periods when the plant needs to produce high levels of gallic acid and ellagic acid. The research provides a detailed map of how these genes are structured, where they are located, and how they respond to the environment. By understanding this genetic machinery, scientists can better appreciate how this plant produces its medicinal compounds and may one day use this knowledge to improve the quality of the fruit or enhance the production of these valuable chemicals for human use. The work lays a foundation for future studies that could explore how to manipulate these genes to boost the plant's natural defenses or increase the yield of its beneficial compounds.