Most likely, the world has heard about the awesome rafflesia bloom, its rarity and inherent malodorous turnoff. But Filipino scientists are keen on further unlocking the mystery and meaning of the plant’s parasitic existence.
Nearly half, or 13, of the more than 30 rafflesia species currently identified and all endemic to Southeast Asia are found in the Philippines. One species, the Rafflesia arnoldii, which is native to the rainforests of Malaysia and Indonesia, is known for its biggest single bloom called “monster flower,” measuring three feet wide and weighing 15 pounds.
The smallest, the R. consueloae, was identified and described in 2016 by a research team from the University of the Philippines (UP) Diliman and Los Baños units while monitoring biodiversity in Pantabangan, Nueva Ecija. It is classified as critically endangered by the International Union for Conservation of Nature; other species endemic to Philippine forests are vulnerable or endangered.
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Appalling but appealing
Each rafflesia flower is infamous for its rotting-flesh smell—appalling to humans but appealing to scientists and its pollinators, the flies, for its pungent yet unknown floral delight.
Two centuries since the discovery of the first species, most research papers have described the plant’s physiological and genetic aspects. But comprehensive studies are very limited because of its rare presence in clusters at different stages of growth.
The Nueva Ecija rafflesia, originally discovered by local field assistants of the UP research team and thoroughly studied and submitted for species description by the late biologist Perry S. Ong, stands out among other species for its existence in numerous clumps on one host vine, making it ideal for the conduct of new research.
Named after Consuelo Rufino Lopez, the wife of Filipino industrialist Oscar M. Lopez, the R. consueloae measures only 9-10 centimeters in diameter in full bloom, or the size of a coffee cup.
According to Dr. Erika Bascos, a developmental biologist from UP Diliman, the R. consueloae clumps, numbering about 280 buds per host vine of the genus Tetrastigma (Vitaceae), allow researchers to conduct replication and perform sampling and experiments without fear of hurting the entire population.
Bascos, in her published article in the Planta journal, focused her research on the early stages of flower development that may hold the key to opening the secrets of the Rafflesiaceae family.
Like the previously discovered species, the R. consueloae are holoparasites, or not capable of photosynthesis, and thus complete their life cycle without exploiting a suitable host, the Tetrastigma vines.
“The research … gave us the opportunity to examine the early stages of infection within the host, and for the first time, we were able to see where the infection starts and determine the parts of the host that the infection may reach,” Bascos said.
She described the vegetative growth of R. consueloae, particularly in the early stages of “infection,” and its life cycle as an endophyte (preemergence of floral shoot from the host).
After an examination of a total of 1,278 root and 684 stem sections excised from Tetrastigma vines, it was found that R. consueloae infections were limited only to the roots of the host but did not pose a deadly threat to the host. However, it may have slightly affected the growth of host flowers, which is crucial in reproduction.
“The tissues obtained from different host individuals also had varying degrees of infection, alluding to a possible role of host resistance mechanisms and/or varying levels of parasite infectiousness,” Bascos said.
Showing microscopically how the single, isolated parasitic cells progressed to early floral buds, the study provided the first evidence that the R. consueloae may remain in vegetative form for more than half a year before reproduction.
Moreover, the floral shoot may develop even without the widespread growth of its endophyte within the host root, indicating that extensive vegetative growth is not a prerequisite to reproductive development, Bascos said.
The hosts, she added, could still be infected with endophytes even without external growth.
The R. consueloae tend to die without reaching full maturity inside their host in a natural process called “bud abortion,” Bascos said. Even without the presence of predators or pathogens or other factors, the buds abort on their own and stop developing.
“In a study conducted by Tolod et al., the mortality rate of the buds is 77.34% during the abortion,” Bascos said. “And among the flower samples they took, only 19.73% bloomed and reached maturity. Without developing, there will be no chance for fruit formation and pollination; therefore, natural reproduction won’t occur.”
Bascos’ findings could encourage further research on how to successfully propagate rafflesia seeds outside the living host.
“In the last few years, a number of rafflesia in-vitro seed germination and tissue culture experiments were reported, but none were successful,” she said, adding:
“There may be a unique chemical signalling happening between the host and the parasite, or there is something in the living tissues of the host that allows rafflesia to germinate and grow within the host.”
Due to their conservation status, turning their habitat into a tourist attraction will put the flowers in danger. Tourists could accidentally step on and harm the Rafflesia blooms, especially those under the forest soil.
Moreover, poachers collect the flowers for profit or for export.
The rafflesia remains a complex and delicate specimen to understand and study because of its unique nature to its host, Tetrastigma spp.. It has proven to be difficult—even impossible—to germinate and propagate outside its natural habitat.
Because of its parasitic nature, the Rafflesia family’s survival depends on its host. Conservation efforts by local authorities and biologists are thus crucial in maintaining both plant populations.
Carlo Frances Luayon is a former broadcast storyteller and currently works as a senior science communicator at the University of the Philippines’ College of Sciences in Diliman, Quezon City. —Ed.