Chloroplast Genome-Based Characterization of Cytochrome b₆f Complex Encoding Genes in the Indonesian Strawberry (Fragaria × ananassa 'Mencir')

Deep within the green cells of every plant lies a tiny, ancient power plant called the chloroplast. These organelles are the engines of photosynthesis, the process that turns sunlight into the energy plants need to grow. Inside them sits a separate, circular piece of DNA, distinct from the main genetic library found in the plant's nucleus. This chloroplast DNA acts as a reliable historical record, changing very slowly over time and rarely getting mixed up like the DNA in our own cells. Because of this stability, scientists often look to it to understand how plants are related to one another and how they have adapted to their environments. For farmers and breeders, knowing the precise genetic makeup of a crop is essential. It helps them select the best varieties to survive in specific climates, ensuring that the food on our tables remains abundant and resilient.

In the highlands of Indonesia, a specific variety of strawberry known as 'Mencir' has become a local favorite. It is prized for its large size, sweet flavor, and its ability to thrive in lower altitudes where other strawberries struggle. However, despite its agricultural importance, the complete genetic blueprint of its chloroplast had never been fully mapped. To fill this gap, researchers at Universitas Gadjah Mada and Mulawarman University set out to read the entire chloroplast genome of the 'Mencir' strawberry. They used a modern sequencing technique that reads long strands of DNA at once, allowing them to assemble the full picture without missing any pieces. Their goal was not just to list the genes, but to understand the structure and evolution of a specific set of genes that build a critical machine inside the chloroplast: the cytochrome b6f complex. This machine acts as a bridge, shuttling electrons between two major stages of photosynthesis, and its proper function is vital for the plant's survival under the intense sunlight of the tropics.

The researchers successfully assembled the complete chloroplast genome of the 'Mencir' strawberry, revealing it to be a circle of DNA measuring 155,568 base pairs in length. This structure follows a standard pattern found in flowering plants, with a large single copy section, a small single copy section, and two identical repeating regions that help stabilize the genome. The team found that the DNA is slightly richer in two specific chemical building blocks, adenine and thymine, than in the other two, a common trait in plant chloroplasts. When they compared this new map to those of other strawberries, both wild and cultivated, they discovered that the overall structure is remarkably similar across the board. The genes are arranged in the same order, and the repeating sections sit in the same places. This high degree of similarity confirms that the 'Mencir' strawberry is indeed a member of the Fragaria family, closely related to other known varieties like 'Royal Royce'.

While the overall map was stable, the researchers looked closely at the specific genes that build the cytochrome b6f complex, which are named petA, petB, petD, petG, petL, and petN. They found that these genes are organized in two distinct ways. Four of them, petA, petG, petL, and petN, are simple and continuous, containing no interruptions. The other two, petB and petD, contain internal segments called introns that must be removed before the gene can be used to make a protein. This structural difference is a trait shared with other land plants, reflecting an ancient evolutionary history. The team then used advanced computer modeling to predict the three-dimensional shapes of the proteins these genes create. The results showed that the core parts of the machine, which do the heavy lifting of moving electrons, are incredibly rigid and stable. Their shapes are so similar across different strawberry varieties that the computer models barely showed any difference. In contrast, the smaller parts of the machine, which act more like structural supports, showed slightly more flexibility in their shapes, though they remained highly consistent.

A deeper look at the genetic code revealed that these genes are under intense pressure to stay exactly the same. When the researchers compared the DNA sequences of these genes across many different strawberry species, they found almost no changes. Some of the genes, specifically petG and petN, showed absolutely no variation at all among the species they studied. This lack of change is not a sign of a lack of diversity in the plants, but rather a sign of strict necessity. The proteins these genes build are so critical to the plant's ability to capture energy that even a tiny mistake in their structure could be fatal. Consequently, nature has ruthlessly filtered out any mutations that might alter them. This intense conservation means that these specific genes are too stable to be useful for telling different strawberry varieties apart, as they do not change enough to create unique fingerprints for each type.

Instead of serving as a tool for distinguishing between varieties, the study highlights that these genes are better understood as a model for how nature preserves essential functions. The researchers identified other parts of the chloroplast genome, specifically the spaces between genes, that do change more frequently. These variable regions, such as the area between the trnH and psbA genes, are much better candidates for DNA barcoding, a method used to identify plant species. The study also confirmed that the 'Mencir' strawberry uses a specific preference for certain codons, the three-letter words of the genetic code that spell out amino acids. This preference leans heavily toward letters that end in A or T, a pattern driven by the natural mutation rates of the plant rather than by any specific breeding effort.

The findings provide a solid genetic foundation for the Indonesian 'Mencir' strawberry, confirming its identity and its place within the broader family of strawberries. By mapping its chloroplast genome, the researchers have created a reference point that can help future breeders understand how this local variety adapts to its environment. The study underscores that while the machinery of photosynthesis is built from highly conserved, unchanging parts, the spaces between them hold the key to understanding diversity. For the 'Mencir' strawberry, which has already proven its worth in the fields of Yogyakarta, this genetic clarity opens the door to further improvements, ensuring that this resilient and delicious fruit continues to thrive in the tropical landscape.