The extremely rare flower of the titan arum, also known as the corpse flower, is famous for its intense smell of carrion. Plant biologists have now determined the formation process of the substances and the genetic basis that are responsible for this unusually foul-smelling floral scent. They also surprisingly identified a previously unknown fragrance component. They also discovered how and why the plant’s huge flower stalk warms up before flowering – an unusual phenomenon in plants.
The titan arum (Amorphophallus titanum), native to Indonesia, is known for two things: its rare deep red flowers and its unusual smell. The huge flower only blooms every five to seven years, usually quite suddenly overnight and for just a few days. It gives off a pungent sulfurous smell that attracts pollinators such as flies and carrion beetles. Because the scent of the flowers is reminiscent of rotting meat and fish, the plant is also commonly referred to as the corpse flower. This distinctive scent is believed to consist of a cocktail of smelly sulfur compounds.
How exactly the plant produces this mixture of smells was previously unclear. It has also been a mystery to scientists until now how and why the central flower stalk of the corpse flower – the up to 3.50 meter high cone-shaped spadix – warms up to eleven degrees Celsius above the ambient temperature shortly before it blooms. Such heat production is common in animals, but unusual for plants.
Heat production by titan arum is similar to animal thermogenesis
A team led by Alveena Zulfiqar from Dartmouth College in the USA has now investigated this phenomenon in more detail. The biochemists looked for the genetic basis and the biological mechanisms behind the production of the heat and the odorous chemicals of the titan arum. The researchers took tissue samples from a 21-year-old corpse flower named Morphy, which is in the greenhouse at Dartmouth College. When this flower was blooming in 2016, they collected a total of nine samples from different parts of the spadix over three nights. “The blooms are rare and short-lived, so we only had a small window of time to study these phenomena,” explains senior author Eric Schaller, also from Dartmouth College.
On the one hand, Zulfiqar and her colleagues examined which chemical molecules were present in these titan arum samples. They also extracted the RNA from the tissue and sequenced it to find out which genes were active in the plant cells at flowering time. “This allows us to see which genes are expressed and which are specifically active when the spadix heats up and emits smells,” says Schaller.
The chemical analyzes showed that the scent mixture actually consists of sulfur compounds: “The most important odorous substances in titanium arum are dimethyl disulfide and dimethyl trisulfide,” write the biochemists. These chemicals also give off rotting corpses. The RNA analyzes showed, on the one hand, that genes that ensure the production of certain proteins, so-called alternative oxidases, were increased in Morphy’s flower stalk during flowering. These are similar to a group of proteins found in animal cells. There they interrupt the storage process of chemical energy in the mitochondria, whereupon the animals increasingly release this energy as heat. The researchers conclude that there appear to be similar uncoupling proteins in the corpse flower that enable the thermogenesis of the flower stalk. These genes were particularly active at the beginning of flowering and at the spadix tip.
Sulfur compounds cause the smell of carrion
Genes that are involved in the transport and metabolism of sugar, amino acids and sulfur were also active when the titan arum blossomed, as the RNA analyzes showed. This could explain where thermogenesis gets its energy from, and at the same time fits with the observation that the odor released by the corpse flower is due to volatile sulfur compounds. To find out exactly how these scents are produced, the team isolated additional tissue samples from the plant during a later flowering period of Morphy. They used mass spectrometry to determine the content of various amino acids in the samples. This allows conclusions to be drawn about the molecules made from these substances.
The result: At the beginning of flowering, the titan arum flower contained high concentrations of the sulfur-containing amino acid methionine, especially at the tip of the spadix. However, samples taken a few hours later had significantly less methionine. The crucial thing is that this amino acid can be used to produce sulfur-based compounds that evaporate easily when heated and produce pungent odors – such as the previously identified substances dimethyl disulfide and dimethyl trisulfide.
Fishy smelling substance identified
In addition, the researchers discovered surprisingly high concentrations of another amino acid in the samples from the edge of the spadix: arginine. This can be used to produce the organic compound putrescine – a volatile odorant that occurs in dead animals when they begin to rot. Follow-up analysis confirmed that Morphy produces this fishy substance during the flowering period. Zulfiqar and her colleagues not only confirmed the suspected mixture of sulfur compounds, but also identified another, previously unknown component of the corpse flower’s scent mixture: putrescine.
Together, these molecules cause the pungent stench of the titan arum flower. The thermogenesis of the flower stalk could help the plant produce its scents quickly and distribute them effectively, the team suspects. In follow-up studies, it now wants to find out whether flowers housed together synchronize their blooming to collectively increase smell levels and attract even more pollinators.
Source: Alveena Zulfiqar (Dartmouth College) et al.; PNAS Nexus, doi: 10.1093/pnasnexus/pgae492