Growing up, we’re taught to look and listen for dangers in the world around us, from oncoming traffic to fire alarms. As humans, we generally gloss over things like smell, because these other senses are just so central to our experience. Our experience, however, isn’t universal, and plenty of other organisms do use their sense of smell for these tasks, sniffing for signs of trouble before it’s upon them. It’s actually such popular way to survey one’s local environment that life forms that lack a proper nose even rely on smell as a way to stay safe, or at least prepare for the worst.
Preemptive protection for plants
The tall goldenrod (Solidago altissima) is a plant that, as the name would suggest, grows up to four feet high and sprouts small, yellow flowers at its top. It’s popular with populating insects, but also with bugs that can cause it harm, like goldenrod gall flies (Eurosta solidaginis). The flies like to lay their eggs in the stem of the goldenrod so the larvae can hatch surrounded by their first meal. Their munching might not kill the plant outright, but they induce the plant to grow a gall, or protective casing around the larvae, that can strain growth and reduce the flower’s seed production.
To protect against the flies larvae, tall goldenrod have evolved a very specific defense system. A plant under attack will produce jasmonic acid, a compound that can reduce the amount of nutrition the larvae can gain from eating the plant, constraining their growth and theoretically limiting their impact. Researchers have confirmed that this defense mechanism can be deployed to greater effect if the plant can prepare in advance, particularly after smelling danger in the air. The exact scent they pick up is the mating pheromones of male E. solidaginis flies. While the flies use this scent to communicate their intent to reproduce, tall goldenrod can use it as a way to minimize the damage for the eventual offspring.
Roundworms bolster themselves against bacteria
The tiny roundworm Caenorhabditis elegans is a bit more mobile than your average tall goldenrod, but they use smell to prepare for danger in a similar way. Instead of worrying about fly larvae, the microscopic nematodes keep a nose, or smell receptor, out for the scent of a strain of Pseudomonas aeruginosa bacteria called PA14. Even though the worms are technically mobile, they don’t bother fleeing a sniff of bacterial byproducts, and instead prime their cells’ immune response. When they do come in to contact with PA14, the prepared roundworms are much more likely to survive the encounter than peers that are caught off guard.
While roundworm safety is great, research into this dynamic has been more focused on understanding exactly how it works on cellular and chemical level. Scientists first had to confirm that the worms were able to truly prepare in reaction to the smell of bacteria, and not just reacting to any stimuli. They exposed the roundworms to smells from various bacteria, and found that their “noses” were discriminating enough to only react to odors from more dangerous sources.
The mechanism that drives the preparation is called heat shock response, and is common to both plant and animal cells. When a stress, like changes in heat, salinity or other stimuli, is encountered, the cell creates specific proteins to do extra repair work in any damage that cell already has. That way it will be more likely to withstand new damage caused by the new threat when it starts causing trouble. While humans aren’t necessarily concerned about the same strains of P. aeruginosa the nematodes are, we might like to imitate their cell’s sense of smell to fight off other diseases or even symptoms of aging that do affect us.
Source: Worms learn to smell danger, EurekAlert!