Study reveals ‘sticky situation’ for seeds in warming regions

A seed is a small package a plant uses to start new life. Inside it has everything it needs to flourish in a new location. This treasure store is also a tasty meal for some animals, that the plant needs to protect. Eric LoPresti and colleagues examined how some seeds defend their contents with stickiness. When wet, a sticky seed can attach to certain objects like rocks or the ground, and when its coating starts to dry, it anchors itself to prevent getting stolen away. But many places are getting hotter and drier. In an article published in the Journal of Ecology, LoPresti and colleagues ask what happens to those defenses when the world gets warmer?

It’s tough to be a seed. As small, living organisms without the ability to move on their own, seeds must be able to endure harsh weather conditions, withstand movement by wind or animals, and protect themselves from being eaten. The probability of a seed making it to adulthood is low, sometimes even one in a million. With constant threats looming, plants have evolved a myriad of tactics and defenses to get their seeds out into the world and give them the best chance for success.

Most plants try to avoid keeping their seeds close, so the natural enemies that affect the mother plant are as far away from the seed as possible. Since plants cannot physically move on their own, they utilize the nature around them to disperse and protect their seeds. Lightweight seeds can be carried by the wind. Hard outer shells can protect seeds from being eaten. Seeds embedded in fruits can be moved via animal consumption and deposited elsewhere. More recently, scientists like LoPresti and colleagues have started to examine an understudied and unusual mechanism of seed survival that can be found in numerous plants including chia, basil, salvia and rosemary: being sticky.

In certain landscapes, like deserts, seeds face warm, dry temperatures and attacks from seed-eating organisms called granivores. Without many places to hide, some seeds have developed a “sticky” outer coating to prevent being taken away and eaten. When the coating becomes wet and then dries, it causes the seed to stick tight to surfaces, just like glue. As the glue dries, it is almost impossible to re-wet. So once the seed is stuck, it’s stuck for good. This sticky glue, which researchers refer to as “mucilage,” not only helps the seeds maintain their ground, but also deters granivores like harvester ants from removing and eating them.

One of the main enemies of seeds in desert areas, harvester ants use their jaws to loosen the seeds on the surface and haul them to back to their nest to be eaten. Harvester ants chip away at the glue bit by bit to remove a seed. The stronger a seed’s glue, the less likely an ant will be to remove it because the effort begins to outweigh the reward. Many factors can affect how well a glue sticks to its surface, especially heat and humidity.

In a world that is getting warmer and drier, LoPresti and colleagues sought to better understand how increased temperatures and drying speeds affect a sticky seed’s survival against harvester ants. In the lab, the botanists wetted seeds of ten different plant species and dried them at various temperatures and speeds. They then tested the mucilage’s strength against being dislodged from a surface and the probability of being removed by harvester ants in the field.

The study found that as temperatures increased, the amount of force needed to dislodge the seeds decreased. Although responses varied between the ten seed species, on average, seeds dried at the highest temperatures only required about 30% of force to be dislodged, compared to those dried at room temperatures. The study found similar results with drying speeds. The seeds with mucilage that dried quicker were more easily dislodged than those that dried normally. Unsurprisingly, when researchers put these seeds to the test against harvester ants, the seeds that underwent accelerated drying were more likely to be taken by harvester ants.

With increasing temperature and drying speed, it was found that the seeds were not able to connect enough to the surface to deter the ants from prying them away, leading to an overall decrease in seed survival. LoPresti and colleagues also mention that the composition of mucilage itself may be an important factor in an ant’s decision to attempt removal, which suggests that mucilage plays an even greater role in seed defenses than is currently understood.

Although this study examines only one piece of the “sticky seed” puzzle, it highlights the importance of adaptability in a changing world. As many regions get hotter and drier, sticky seeds may need to develop stronger mucilage attachments to survive against granivore attacks.

This study is just the start. LoPresti and his team plan to investigate other factors, such as the chemical make-up of the seed mucilage and seed surface area and volume, which may affect mucilage strength and the probability of removal by granivore, hoping to better understand the future of these incredible small, sticky seeds.

READ THE ARTICLE

LoPresti, E.F., Stessman, M.E., Warren, S.E. and Toll, K. (2022) “Drying conditions alter the defensive function of seed mucilage against granivores,” The Journal of Ecology. Available at: https://doi.org/10.1111/1365-2745.14044.

Gina Errico is a master’s student at Oklahoma State University studying the effects of fungal spillover from coffee plantations onto tree seedlings in nearby forests. Interested in the intersection between human agency and ecosystem functioning, Gina spends her days as a researcher and nights as a science communicator, working to bridge the gap in ecological knowledge in society. She plans to pursue a career in science communication upon graduation, but for now you can find her writing her thesis, working on science communication initiatives or tending to her houseplants.