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With a smell of rotting flesh the flowers of certain species of wild ginger are unlikely to be used in a wedding bouquet – although they are irresistible to carrion-loving flies. Now researchers say they have worked out how the sulphurous scent is produced.
Scientists say the odour is down to small changes in an enzyme that prevents bad breath in humans.
“Some organisms have an extraordinary trait which is seemingly difficult to evolve, but such characteristics can evolve in a simpler manner than one would imagine,” said Dr Yudai Okuyama, the first author of the research from the National Museum of Nature and Science in Japan.
Writing in the journal Science, Okuyama and colleagues reported how they first investigated the origins of a key chemical known to be behind the sulfurous smell produced by some species of Asarum, or wild ginger.
This chemical, called dimethyl disulfide (DMDS), is thought to be produced from a substance called methanethiol. Methanethiol is known to cause bad breath in humans and is formed in plants and animals as the amino acid methionine is broken down.
The researchers confirmed this by feeding a form of methionine labelled with carbon-13 atoms to a species of wild ginger, A. fudsinoi, and found the DMDS released also contained carbon-13.
The team then looked at an array of different species of Asarum to identify genes whose activity varied with the amount of DMDS produced.
The work led to the identification of a gene that gives rise to a selenium-binding protein. Such proteins are found across the plant and animal kingdom, and typically convert methanethiol into less harmful substances. In humans this mechanism prevents halitosis.
The researchers found the “normal” version of this gene was present in all of the plants and animals they considered, including the various species of Asarum. However, they found the latter also had a mutant form of the gene that produces a protein that turns methanethiol into DMDS. This mutant gene is more active in foul-smelling species.
The team said the change in function appeared to be down to a small number of mutations in the gene, with only two or three changes in the amino acids of the protein needed for the shift to occur.
Further work showed Eurya and Symplocarpus plants also have selenium-binding proteins that can turn methanethiol into DMDS, explaining why some species smell bad, with the team noting they appeared to have evolved independently.
However, not all foul-smelling plants produce their scent in the same way: the team found species of Amorphophallus – a group that includes a plant known as the “corpse flower” – did not have a selenium-binding protein that converts methanethiol into DMDS.
“We think some similar enzyme that belong[s] to a different protein family might be responsible for the step,” said Okuyama.
