Some flowers smell like death, and for the same reason. Malodorous members of the Eurya (left, E. japonica pictured), Asarum (A. simile, middle) and Symplocarpus (S. renifolius, right) genera independently evolved changes in an enzyme that help them attract pollinating flies with the unlovely smell of rotting meat.
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Some plants stink of rotting meat or dung, which helps them attract flies for pollination. How plants make the carrion stench, which is usually produced by bacteria feasting on decaying corpses, has been a mystery until now.
Several types of plants have independently evolved to make the fetid odor thanks to a few tweaks in one gene, researchers report May 8 in Science.
Scientists in Japan used biochemistry and molecular and evolutionary genetics to determine that three unrelated plant lineages hit on the same evolutionary trick to produce the smell. First, a gene called SBP1 was duplicated. (Gene duplication is a pretty common occurrence in the evolution of most organisms, including humans.) Then the extra copy of the gene mutated, swapping a few amino acids in the enzyme it produces.
In a type of wild ginger (Asarum simile) and the East Asian eurya shrub (Eurya japonica), three changes were needed to bring the stink that these plants and some of their relatives share. But the Asian skunk cabbage (Symplocarpus renifolius) needed only two amino acid swaps to become malodorous.
SBP1 makes an enzyme that helps break down a chemical called methanethiol. Methanethiol is pretty smelly itself; it’s the compound that builds up in the mouths of some people with poor dental hygiene and gives them clinically bad breath, or halitosis. The original enzyme made by SBP1 — and related enzymes in humans, animals and plants — breaks methanethiol into hydrogen peroxide, hydrogen sulfide and formaldehyde.
The tweaked enzymes from the stinky plants instead links two methanethiol molecules into dimethyl disulfide, responsible for the much more putrid scent of rotten meat. (It’s also one of the chemicals hinting at extraterrestrial life that the James Webb Space Telescope detected in the atmosphere of exoplanet K2 18b.)
Among Asarum species, the ability to make dimethyl disulfide was gained and lost more than 18 times, the researchers estimate. There is evidence that plants are under evolutionary pressure to make the foul-smelling molecule, the team found. Those that do may attract more flies to pollinate them.
Imperfect extra copies of genes are often the source of new traits across evolution in many species. Duplicate genes can mutate without harming the function of the original gene, allowing room for innovation. For instance, poppy plants evolved the ability to make morphine by that route.
