The biology of the enigmatic corpse flower provides clues to its conservation

As dawn breaks, researchers and members of the Banao Indigenous people of the Philippines enter a secluded forest dense with ferns, moss, and fog. In the Luzon province, Kalinga hosts one of the world’s most enigmatic and extraordinary flowers—Rafflesia banaoana—and the crew hopes to catch it in bloom.

Plant biologists are prying insights from the Rafflesia species in hopes of protecting it in the wild, banking its seeds, and seeing blooms in botanical gardens outside of the plant’s native range. Image credit: Jeanmaire Molina (Pace University, New York, NY).

Over the centuries, Rafflesia has enchanted botanists, accruing epithets such as “the prodigy of the vegetable world” and comparisons to charismatic megafauna (the “panda of the plant world”). With flowers that stretch a meter across, Rafflesia arnoldii boasts the world’s largest flowers. Other plants in the genus bear smaller, but still impressive, blooms the size of dinner plates. Blood-red petal-like lobes speckled with white provide a feast for the senses, though not a particularly appetizing one. Known also as corpse flowers, Rafflesia blooms reek of rotting carrion, odors emitted to lure pollinators. (Rafflesia is not related to another species also called corpse flower, Amorphophallus titanum, which is found at botanic gardens around the world—a plant with an enormous putrid inflorescence of numerous flowers.)

Botanist Chris Thorogood trekked through dense forests in Southeast Asia looking for corpse flowers. Image credit: Chris Thorogood (University of Oxford Botanic Garden and Arboretum, Oxford, UK).

Adding to the intrigue, the Rafflesia plant is parasitic. Having shed its stem, roots, and leaves, the plant grows out of its host, namely, tropical grapevines in the genus Tetrastigma, absorbing the water and nutrients needed to survive. Parts of its life cycle remain a mystery, providing a puzzle for biologists. “There are more questions than answers when it comes to Rafflesia,” says Chris Thorogood, a botanist at the University of Oxford Botanic Garden and Arboretum in the United Kingdom.

Plant biologist Jeanmaire Molina is hoping to gather Rafflesia-colonized Tetrastigma vines from the Philippines for grafting, which has yet to succeed outside of the plant’s native range. She believes being able to grow the plants in a US botanic garden would greatly enhance conservation awareness. Image credit: Jeanmaire Molina (Pace University).1

However, the plant’s fate remains uncertain—researchers estimate that 60% of the 40 or so Rafflesia species are at risk of extinction (1). Plant biologists are prying insights from the plant itself that may aid efforts to protect it in the wild, bank its seeds, and see blooms in botanical gardens outside of its native range. Meanwhile, Rafflesia’s unusual life history has made it a living laboratory for studying interactions between plants that have shaped plant evolution.

Grafted for Success

In Kalinga, Thorogood, botanists Pastor Malabrigo Jr. and Adriane Tobias of the University of the Philippines Los Baños, and field guide Analeah Ricohermoso tread uphill following a river. Their mission: Find the flower and collect aromas and samples for DNA analysis. Although many specimens are easily accessible, some Rafflesia species grow in remote areas; treks into the wilderness help scientists acquire samples and understand the flower’s diversity.

After 7 hours of hiking, the group emerges onto a gravelly flat near the river. On a slope ahead, they spot around 30 Rafflesia banaoana plants, including about a half-dozen in full bloom. The sight, coupled with relief and sheer exhaustion, brings Thorogood to tears. “To spend time with a rare Rafflesia flower,” he says, “is the closest thing to magic” (2).

Aspects of Rafflesia’s long and largely hidden life cycle also contribute to the plant’s allure and offer insights into why it’s so tricky to study and propagate.

No one has ever observed Rafflesia seed germination, says Jeanmaire Molina, a plant biologist at Pace University in New York. Researchers assume that rodents feed on the fruit and excrete seeds near host vines. Ants may also carry and disperse seeds (3). But only certain species of Tetrastigma vines support Rafflesia blooms, and what sets hosts apart isn’t known. The seeds presumably infiltrate vines using enzymes to break down the Tetrastigma tissue. Once Rafflesia germinates, it grows as a thread called an endophyte that reaches into the host tissue. On the outside, there’s still no sign of a Rafflesia infection until a bud erupts on the woody grapevine, but researchers don’t know what triggers bud formation. The bud can take months to mature, sometimes growing as large as a cabbage.

Over the course of days, the bud opens to reveal the flower. The plant has separate male and female flowers; months after fertilization, the female flowers produce a rancid-smelling fruit that looks like cow dung and contains sawdust-sized seeds.

Despite the many mysteries of Rafflesia’s life cycle, researchers in Indonesia have had some success cultivating the plant (4). In 2004, botanist Sofi Mursidawati and colleagues at the Bogor Botanical Garden in Indonesia started experimenting with grafting. Decades had passed since anyone had tried this approach. “Everyone said ‘it’s too difficult, no guarantee it will be successful,’” she says. To Tetrastigma vines in their collection, they joined cuttings of infected Tetrastigma vines from Pangandaran Nature Reserve in West Java. “I never thought that the grafting technique would work,” says Mursidawati, who is now a botanist at Cibinong Botanical Garden in Indonesia. It didn’t—until 2010. “It was a big surprise when one morning in March, while raking the garden, we saw a bud that size almost as big as a tennis ball,” she says. The 17 blooms since have allowed researchers to make close-up observations—for instance, how Rafflesia develops within its host’s tissue (5).

As for growing Rafflesia from seed, success has been elusive. In 2012, the researchers tried to grow the plants by rubbing seeds onto cultivated Tetrastigma vines. The efforts seemed fruitless until 2022, when Bogor had a surprise bloom of Rafflesia arnoldii. Still, routine propagation from seed seems a long way off.

Mursidawati and her colleagues hope that by sharing their successes and insights with other researchers, they can help the plant thrive in collections around the world. The United States Botanic Garden in Washington, DC, has been funding Molina’s fieldwork to gather Rafflesia-colonized Tetrastigma vines from the Philippines for grafting using the same approach, which has yet to succeed outside of Rafflesia’s native range. “If we can replicate that in the US Botanic Garden, I can just imagine the conservation awareness that it will bring,” Molina says.

A Corpse Flower’s Chemical Clues

By investigating Rafflesia’s parasitic behavior, researchers hope to gain insights into its growth as well. Seeking to learn more about the behavior, Molina’s team investigated gene expression in Rafflesia’s seeds. They found genes known to respond to the compound karrikin, a seed germination cue for other plants that is present in wildfire smoke (6). Karrikin may signal to plants that wildfire has made space in the forest, allowing plants on the forest floor to emerge and fill the new gaps. In attempts to grow Rafflesia from seed, applying the compound might help to awaken seeds.

But Rafflesia growth doesn’t just depend on the parasite. The host plant—and its defenses—also plays a role in determining whether the corpse flower can take root. Pieter Pelser and colleagues have mapped associations between Rafflesia and Tetrastigma in the Philippines—not an easy feat; finding and identifying the hosts comes with its challenges. “That group of plants is just a pain in the butt,” says Pelser, a plant systemist at the University of Canterbury in Christchurch, New Zealand.

Pairing DNA analysis with a study of plant physical characteristics, Pelser’s team found that most types of Tetrastigma that the group studied could host multiple Rafflesia species. Four of the 11 Rafflesia species studied grew on multiple species of Tetrastigma. But, curiously, in some places, Rafflesia seemed choosy, only showing up on one type of Tetrastigma when others were available (7). Knowing these associations could help conservation managers select the best host plants. Meanwhile, Molina’s studies on host plant metabolites have hinted at compounds that might set poor hosts apart from more promising ones (8).

Evolutionary Marvel

Rafflesia, though, has proven to be more than a peculiar plant rife for conservation among corpse flower enthusiasts. It has also helped shape researchers’ understanding of how plants, in general, can evolve.

Indeed, Molina and colleagues turned up a remarkable event in Rafflesia’s evolution. In 2014, her team sequenced the mitochondrial DNA of Rafflesia legascae. “It was very controversial because it was the first study to have shown that the chloroplast genome can be lost in plants, even in parasitic plants,” she says (9). The chloroplast genome, which codes for plants’ photosynthetic machinery, is considered fundamental to plants. Other parasitic plants usually contain vestiges of the chloroplast genome, even if they’ve abandoned photosynthesis.

Rafflesia’s mitochondrial genome has offered further insight into plant evolution, suggesting a feat never before seen in plants. While studying the evolutionary relationships between members of a large group that includes Rafflesia and other tropical plants, such as passionflower and violets, evolutionary ecologist Charles Davis of Harvard University in Massachusetts found results that were shocking at the time: A region of Rafflesia’s mitochondrial genome placed it far from its relatives and close to its Tetrastigma hosts. “It was like a eureka moment,” Davis says. He realized that Rafflesia may have acquired genes from other plants without sex, a process called horizontal gene transfer. “At that point, we didn’t really think that horizontal gene transfer was common or even that present in any plant.”

While around 2% of Rafflesia’s nuclear genes seems to have been stolen from its host (10), the plant’s mitochondrial DNA contains up to 40% pilfered genes (11). Many questions remain about parasitic plants and horizontal gene transfer, such as how they get genes—perhaps they’re shuttled between plants by bacteria, viruses, and fungi—whether the genes function in their new hosts, and whether they replace native versions of genes.

The extremes of Rafflesia hint at what’s possible in plant evolution, Davis says. For example, Rafflesia and some of its close relatives have discarded huge chunks of their genome, dispensing with entire functional pathways.

Careful Conservation

To ensure this evolutionary marvel staves off extinction, researchers and conservationists will have to contend with the factors driving habitat loss and deforestation. Some farmers don’t realize that rare species are growing on forest land they seek to convert to farmland. To boost awareness and prompt local people to protect the plants, Indonesian community groups distribute news of blooming Rafflesia on social media. Publicizing the plants creates the risk that Rafflesia patches become a tourist attraction where buds get trampled, and the surrounding landscape is degraded. Hence, there’s a trade-off between awareness that garners local support and preserving fragile populations.

Collaborative efforts among scientists and others aim to aid conservation efforts. An online group started by Pelser’s team includes around 24,000 members, who assist each other with plant identification. The researchers train scientists and work with nongovernmental organizations. Educating people to identify plants, such as Rafflesia, motivates them to protect natural areas, Pelser says.

Thorogood and others are working to create a global network that would include biologists, government officials, conservation leaders, and people from local communities to share knowledge about practices and strategies for promoting Rafflesia’s survival. Conservationists and scientists are conducting surveys to map out the distribution and abundance of Rafflesia populations in Southeast Asia to get individual species listed with the International Union for Conservation of Nature and to improve the visibility of conservation needs.

“These are just inherently fascinating organisms, and that alone merits their existence in the world,” Davis says. The research thus far suggests that people should take notice of these plants for more than their foul smell and peculiar parasitism. These systems showcase the intriguing interdependencies of the host, parasite, and environment. “There’s really no plant like it,” Molina says, “and so to lose it is a catastrophe.”