Unlocking the Tiny Secrets to Grow the World’s Largest Flowers

At some point in their lives, all plant biologists and enthusiasts face the same awkward and uncomfortable question: “what is your favourite plant?” While I don’t have a good answer for that, if you ask Dr Jeanmaire Molina, she won’t hesitate and will tell you that her favourite plant in the world is Rafflesia. And to be fair, these plants are actually quite intriguing.

Rafflesia is a genus with 41 species native to the tropical forests of Southeast Asia, from Thailand to Central Malaysia. These plants –known as padma (Indonesian and Malayan) or patma (Javanese) in local languages– arouse everyone’s curiosity for several reasons. First, they hold the record for the largest flowers in the world, with some species producing flowers about one meter in diameter. If that wasn’t flashy enough, these flowers emit heat and have a distinctive smell of decaying meat that attracts the carrion flies that pollinate them. Second, these plants belong to a select group of plants to which only 1% of known flowering plant species belong: the parasitic plants. While there are many different types of parasitic plants, Rafflesia is known to be a holoparasite, which means they do not manufacture their own food through photosynthesis as most plants do. Instead, it uses an absorptive structure called haustorium to extract water and nutrients from the plants they infest –which, in the case of Rafflesia, are only the vines of the genus Tetrastigma. Third, and probably one of its most striking features, is that Rafflesia has lost the chloroplast genome completely, raising questions about whether we can even still consider it a plant.

Being as fascinating as they are, it’s no surprise that these plants have been featured frequently in pop culture, from Vileplume from Pokémon to Stranger Things’ Demogorgon. However, these plants are critically endangered, with 60% of the species of this genera facing a severe risk of extinction due to the degradation of their natural habitats. For this reason, different efforts have been undertaken to propagate them, and here is the challenge (and associated headaches) started.

Dr Jeanmaire Molina is originally from the Philippines, the country with more Rafflesia species (at least 13). However, her first encounter with Rafflesia was during a field trip to Malaysia in 2004 as part of her graduate studies. In an interview with Botany One, Molina commented that this love-at-first-sight encounter, as she described it, paved the way for her life’s mission: understand the biology of Rafflesia species and look for effective ways to propagate them. This field trip was the beginning of Molina’s adventure to study the “Panda of the Plant World” and, naturally, looking for funding for her research –including a crowdfunding campaign titled “If Lady Gaga could wear a flower, it would be Rafflesia”. In the end, it wasn’t Gaga but the United States Botanic Garden, a Federal Agency with more than two centuries of experience in plant conservation, that funded Molina’s research to try to propagate the “Panda of the Plant World”, as she called it.

To date, the best way to propagate the Rafflesia species seems to be grafting Rafflesia-infected Tetrastigma to an uninfected rootstock –a technique that has been used in Bogor Botanical Gardens (Bogor, Indonesia) to produce several blooms for their live collections. In contrast, efforts to propagate Rafflesia species via seeds have been unsuccessful overall. In a previous study by Molina and her colleagues from the United States Botanic Garden, they tried to induce the germination of R. speciosa seeds using several renowned growth regulators, including strigolactones, which are known to promote the germination of other parasitic plants. However, no matter the compound or its concentration, no seed germinates. This same frustrating scenario has been repeated in several other papers by different research groups, leading Molina to want to delve into the biology of Rafflesia seeds, an interest that resulted in his most recent paper, published in the journal Plants, People, Planet. And when I say “delve”, I really mean it since this research was the first one to build and study the transcriptome of Rafflesia seeds, that is, the genes expressed in them.

Molina and her team evaluated the transcriptome of Rafflesia speciosa seeds with those of four other plant species: Arabidopsis, Striga, Cuscuta, and Anoectochilus. Arabidopsis serves as a model plant species, and its genes have been widely studied, making it a mandatory reference for almost any molecular biology study. On the other hand, Striga and Cuscuta are also parasitic plants similar to Rafflesia, and having them in the study could inform if Rafflesia seeds work like those of other parasites. Finally, Anoectochilus is a mycoheterotrophic orchid that depends on mycorrhizal fungi during seed germination. The latter species was included in the study to assess the possibility that Rafflesia depends on microorganisms for germination, which should be reflected in its genes.

The results of this work showed that Rafflesia seeds express a set of genes also found in parasitic plants’ seeds. These genes are known as “core parasitic genes” and are related to processes common to all parasitic plants, processes common to all parasitic plants, such as the degradation of host tissues and haustoria formation.

Still, the authors were surprised by some notable absences in the Rafflesia transcriptome: D14 and genes related to carotenoid synthesis. D14 is a gene responsible for orchestrating the germination of Striga seeds in response to strigolactones, and inhibiting carotenoid synthesis has proven to be a key issue for the germination of parasitic plant seeds. The absence of these genes in the Raflesia transcriptome explains the failure to germinate its seeds using techniques from other parasitic plants. Remember that Molina and his team were unsuccessful in using strigolactones to stimulate Rafflesia germination? This study shows why: the seeds don’t have the genes to respond to this compound! Similarly, inhibitors of carotenoid synthesis did not stimulate the germination of this species, as it does not produce these substances in the first place. Taken together, these results indicate that the metabolism of the “Panda of the plant world” is more different than we expected, and its propagation cannot be carried out with the same logic as other parasitic plants.

Another notable absence in the Rafflesia seed transcriptome was RAM2, a gene associated with mycorrhizal symbiosis in Anoectochilius. This result suggests that Rafflesia may rely on something other than this type of interaction with fungi to obtain nutrients. Instead, the Rafflesia transcriptome exhibited a higher proportion of genes related to fatty acid metabolism compared to the other studied plant species. This finding highlights the significance of fatty acid breakdown and metabolism in Rafflesia seed germination, especially since these species are known to have oily endosperms. Rafflesia’s seed transcriptome also showed several genes associated with the formation of the haustorium, a structure stimulated by various compounds produced by its host Tetrastigma. As a result, Molina and her team suggest that “testing these compounds in germination experiments of Rafflesia seeds may be worthwhile in the future”.

Overall, the study of Molina and her team took us a step closer to understanding the seed biology of Rafflesia –a knowledge that holds the promise of unravelling the key for its successful propagation and conservation. Hopefully, future studies in Rafflesia germination can build on the study of Molina’s team and (fingers crossed!) be amused by successful results rather than new headaches. More than that, the team has big aspirations:

“We believe that Rafflesia has significant untapped engagement potential. We hope this seed transcriptome study brings us closer to a reality in which botanic gardens all over the world grow Rafflesia for conservation while increasing public awareness and appreciation of an evolutionary marvel.”

The partnership between Molina, the United States Botanic Garden and new colleagues from the Philippines, is already carrying out new field trips and several new propagation trials. Hopefully, their research will help us to continue elucidating the biology of these outstanding species, bringing us ever closer to a technique that will allow us to appreciate Rafflesia in the nearest botanical garden and its natural habitat and that will help more people to find in Rafflesia a new favourite plant.

READ THE ARTICLE:
Molina, J., Wicaksono, A., Michael, T. P., Kwak, S. H., Pedales, R. D., Joly‐Lopez, Z., … & Purugganan, M. D. (2023). The seed transcriptome of Rafflesia reveals horizontal gene transfer and convergent evolution: Implications for conserving the world’s largest flower. Plants, People, Planet. https://doi.org/10.1002/ppp3.10370

Carlos A. Ordóñez-Parra
Carlos (he/him) is a Colombian seed ecologist currently doing his PhD at Universidade Federal de Minas Gerais (Belo Horizonte, Brazil) and working as a Science Editor at Botany One. You can follow him on Twitter at @caordonezparra.

Spanish and Portuguese Translation by Carlos A. Ordóñez-Parra. (in progress)

Cover: Rafflesia arnoldii. Image: LukeTriton / Wikimedia Commons