Ooh That Smell: Plants Use Emissions for Self-Defense

Musah's team used light and electron microscope technology to reveal the presence of sac-like, microscopic protuberances along the root shafts that appear, on root stimulation, to be associated with the odor emission.

ALBANY, N.Y. (January 27, 2016) -- Pity the poor plant. When faced with danger, animal species have their fight-or-flight response, but vegetation has no such option. Literally rooted in place, plants have taken defense to creative levels, employing thistles, thorns, barbs, nettles, spines, and prickles to cause attackers to sneeze, itch, cough, yelp, spit out, and just back away.

Now researchers at the University at Albany have found a new tool, previously unknown, that some plants use for their protection.

And it's not pretty.

When the roots of the incongruously named "shy plant" (Mimosa pudica) are disturbed, they emit a woozy, noxious soup of strong sulfuric compounds that unfortunately smells all too familiar -- the wrong side of socially acceptable.

In a study published in February's "Plant Physiology," UAlbany chemist and lead researcher Rabi Musah details the chemicals found in the plant's emissions, which comprise organic and inorganic sulfur compounds including, among others, sulfur dioxide, methylsulfinic acid, pyruvic acid, lactic acid, and thioformaldehyde, a rare and highly unstable compound never before reported as a secretion from a plant.

"We were able to identify and confirm the emission of these molecules from the roots by germinating seeds in petri dishes under sterile conditions, and monitoring the air around the roots by ambient ionization mass spectrometry," said Musah.

While emissions from plants are not unknown, what distinguishes M. pudica from others are previously unreported characteristics associated with its secretions. While most plant emissions originate in the above-ground, or aerial, parts of the plant, it's M. pudica's roots that are causing the big stink. Musah's team used light and electron microscope technology to reveal the presence of sac-like, microscopic protuberances along the root shafts that appear, on root stimulation, to be associated with the odor emission.

More curious is M. pudica's ability to discriminate between stimuli -- while the plant's emissions are strongest when its roots are touched by a finger or pulled from the soil, no such reaction, or emission, occurs when they're tapped with wood, glass, metal, or even soil (although dragging the roots across soil will produce the odor).

"It’s pretty amazing that the roots of a plant would be able to distinguish between different forms of matter in such a way as to respond to a human finger on one hand, but not respond to objects made of glass or metal on the other hand," said Musah, who is now investigating the mechanism of that .

Mimosa pudica, known colloquially and not surprisingly as sensitive plant, bashful plant, and touch-me-not, is noted for its seismonastic properties: in response to touch, water, wind, or a flame, its leaves will close in. It was chosen as a model to investigate how it and related plants are able to emit highly reactive sulfur compounds without themselves incurring tissue injury.

While no conclusions have been drawn regarding the purpose of the odor-emitting mechanism of the plant, Musah suggests that the sulfur compounds might function to deter below-ground herbivores, or perhaps influence the growth of nearby plants in order to give M. pudica a competitive advantage. This National Science Foundation- funded research will also pursue the environmental fate of the emitted organosulfur compounds.

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