Why Do Some Plants and Fungi (Like Stinkhorn) Smell Bad?

There are a few plants and fungi that rely on extremely offensive odors to attract insects and spread its pollen or spores. These odors, to people, stink like decomposing, rotten meat, but to flies they are highly attractive.

The stinkhorn is a phallus-shaped mushroom which produces a sticky spore goop which (you guessed it) stinks. Flies land on the sticky stinky fluid and fly away taking the spores with them, allowing the schroom to pop up quickly, even overnight, in weird places. I've come across a few types of these mushrooms growing up in Florida, and I can attest to the fact that they smell awful, more like feces than death. Some are kinda pretty though.

Stinkhorn

The carrion flower (Amorphophallus Araceae) is an enormous plant from Indonesia which emits a smell like rotting flesh to attract insects and beetles to transfer pollen. It has a tall stalk which contains smaller flowers, set into a few huge petals at its base. The plant can actually heat up the stalk to enhance the stinkiness of its odor emission.

Carrion flower

The rafflesia is a beautiful plant, techinically the largest single flower, found in the rainforests of Malaysian Borneo. The bloom can grow up to 3 feet across and weigh 15 pounds. It also emits an odor which smells like foul meat. I've been to Borneo three times, and each time I've tried to view one of these amazing flowers. However, their blooming is rare and unpredictable, and my visits have never timed well with a known bloom.

Rafflesia

These are just a few of the more famous examples of stinky plants, but what I was most curious about was what was the compound that made them smell offensive? How did a plant or fungus develop the scent of a rotting mammal as a good evolutionary tactic? While the specific chemical compound involved in these plants' scent is not completely known, there are simple amines present in rotting flesh (putrescine and cadaverine) which are also present in these plants' "nectar." Dimethyl sulfides have also been found in the carrion flower.

Putrescine and cadaverine are appropriately-named organic compounds produced by the breakdown of amino acids in tissues. They are thought to be growth factors necessary for cell division and are synthesized in small amounts in normal tissue, so perhaps these stinky plants just ramp up that process.

A scientist at UC-Berkeley, Dr. Rehan Khan, has recently shown that whether a scent will be reported by humans as offensive or pleasant can be predicted by the molecular weight and electron density of the compound. Compact and light-weight molecules, like limonene, smell pleasant, while the electron-dense and heavy molecules like butanol are reported as offensive. This was determined during a study of 150 molecules, comparing different properties of the molecules with the equivalent of professional sniff-tests.

"When we presented the preliminary results at a conference, a fragrance company said they flat-out didn't believe us," he said. The wary corporation sent his team 20 new molecules for the team to predict the pleasantness of, and when they returned their results, he said the company was "quite surprised."

There's an NPR podcast on stinky plants here.