Transcriptomics of C 3 +CAM Peperomia species reveals potential initial steps toward a CAM pathway

Plants are masters of survival, but they face a constant, difficult trade-off. To grow, they need to pull carbon dioxide from the air to build their tissues, yet to do this, they must open tiny pores on their leaves. Opening these pores lets in the gas they need, but it also lets precious water escape. In hot, dry places, this can be a fatal leak. Most plants, known as C3 plants, open their pores during the day when the sun is out. However, in arid environments, this strategy often leads to dehydration before the plant can finish its work. To solve this, some plants evolved a different strategy called Crassulacean Acid Metabolism, or CAM. These plants flip the schedule: they open their pores at night when the air is cooler and water loss is minimal, storing the captured carbon to use during the day while their pores remain shut. This adaptation allows them to thrive in deserts and on dry rocks where other plants would wither.

For decades, scientists have studied how plants make this switch, focusing on lineages that have fully committed to the CAM lifestyle. But nature often offers a middle ground. Some plants, known as C3+CAM, do not fully abandon their original daytime schedule. Instead, they keep their pores open during the day but can also tap into the nighttime strategy when conditions get tough. Understanding how a plant moves from a standard daytime routine to this flexible, dual-mode existence is like watching the first steps of a major evolutionary leap. It reveals how complex traits begin to form, often through subtle tweaks rather than a complete overhaul of the system.

A team of researchers turned their attention to a group of tropical plants called Peperomia, which are found in the forests of Hawaii. This genus is unique because it is the only known family of plants in its broad botanical group, the Magnoliids, to have developed CAM. While there are nearly 1,600 species of Peperomia, very few have been studied for their ability to use this water-saving trick. The researchers wanted to know if the Hawaiian species, many of which grow as epiphytes on trees where water is scarce, had developed this ability. More importantly, they wanted to see what was happening inside the plant's cells to allow this transition. By looking at the plants' genes, the team hoped to find the specific molecular switches that turn a standard plant into one that can survive drought by changing its daily rhythm.

The team began by gathering dried leaf samples from twenty-five different native Hawaiian Peperomia species held in museum collections. They analyzed the carbon isotopes in these leaves, a method that acts like a fingerprint to tell if a plant is using a standard daytime strategy or a nighttime one. The results were surprisingly quiet. Most of the species showed isotopic signatures typical of standard C3 plants, with no obvious signs of CAM activity. However, carbon isotopes can sometimes miss weak or flexible versions of the trait. To get a clearer picture, the researchers grew five specific species in a controlled environment and subjected them to a week of drought. They measured how much carbon the plants absorbed and how much acid accumulated in their leaves, a chemical sign that the plant was storing carbon at night.

The physiological tests revealed a hidden story. While most of the plants behaved like standard C3 species, two of them, Peperomia blanda and Peperomia sandwicensis, showed clear signs of CAM activity. Under drought stress, these two species began to accumulate acid in their leaves, a process that happens when plants fix carbon at night. They were not full-time CAM plants, but they were C3+CAM: they could switch on this water-saving mode when they needed to. The third species, Peperomia mauiensis, remained a standard C3 plant, showing no such shift even when dry. This confirmed that the ability to use CAM exists in these Hawaiian plants, but it is not universal across the group.

To understand how this switch works, the researchers looked at the genetic instructions inside the leaves of these three species. They examined the activity of genes that are known to be part of the CAM pathway, such as those that help capture carbon at night. They found something unexpected. The genes for the core CAM machinery were actually present and active in all three species, including the one that did not show CAM behavior. The genes were not missing or broken; they were simply running at different volumes. In the two species that could switch to CAM, these genes turned up their activity significantly at night, especially when the plants were thirsty. In the standard C3 species, the same genes were present but did not show this strong nighttime surge.

The key to the difference appeared to be a specific gene called PPCK. This gene produces a protein that acts as a regulator, preparing the plant's carbon-capturing enzymes to work efficiently at night. In the two CAM-capable species, PPCK showed a sharp increase in activity during the dark hours, and this activity spiked even higher when the plants were under drought stress. In the standard C3 species, this gene did not show the same rhythmic pattern. The researchers also looked at the network of genes that work alongside PPCK. They found that in the CAM-capable plants, PPCK was tightly linked to a gene that controls the plant's internal clock, known as RVE. This connection suggests that the plant's internal timekeeper is directly talking to the CAM machinery, telling it when to wake up and start working.

The study suggests that the evolution of CAM in these plants did not require the invention of entirely new genes. Instead, it seems to have happened by changing how existing genes are regulated. The genetic tools were already there, waiting in the wings. The transition to a water-saving lifestyle involved simply turning up the volume on specific genes at the right time of day and linking them to the plant's internal clock. The gene PPCK appears to be the bridge between the standard daytime plant and the flexible CAM plant. By increasing its activity at night and connecting it to the circadian rhythm, the plant gains the ability to store carbon when water is scarce.

This finding offers a new perspective on how complex traits evolve. It suggests that the path from a standard plant to a drought-resistant one might be shorter and more accessible than previously thought. The researchers did not find a massive genetic overhaul, but rather a subtle rewiring of existing systems. The ability to switch modes seems to rely on a few key regulatory changes that allow the plant to respond to stress. While the study focused on a specific group of Hawaiian plants, the mechanism they uncovered—using existing genes and tweaking their timing—could be a common theme in how plants adapt to changing climates. The researchers noted that many other plants might possess this hidden flexibility, waiting for the right conditions to reveal it.

The work also highlights the importance of looking beyond the obvious. If the team had relied only on the initial carbon isotope tests, they would have concluded that these Hawaiian plants were standard C3 species. It was the combination of careful physical measurements under stress and a deep dive into the genetic code that revealed the true nature of these plants. The study confirms that Peperomia blanda and Peperomia sandwicensis are indeed C3+CAM species, capable of shifting their metabolism to survive dry spells. It also points to PPCK as a critical regulator in this process, acting as a switch that connects the plant's internal clock to its survival strategy. As scientists continue to map the evolutionary history of these plants, understanding these initial steps will be crucial for seeing how life adapts to the world's most challenging environments.