Vincent Pan: Why are some seeds sticky?

In our latest post from Vincent Pan of Oklahoma State University explored the sticky world of seed mucilage and how it impacts granivory.

About the paper

Our whole journey down the seed mucilage rabbit hole was started when one summer Eric LoPresti, upon reading Fuller & Hay’s (1983) natural history note on sand coated Salvia columbariae seeds, got his mind blown. It happened that there is a long line of harvester ant nests along a road a few minutes bike ride from the lab, so we spent a few mornings trying out various pilot experiments. When wetted, the mucilage layer of a seed expands into a viscid gel that upon drying can bind the seed to whatever it is in contact with. We have often seen seeds stuck to rocks and leaves in the field, so one of the things we were interested in was whether such a phenomenon may help the seeds evade predation. Thankfully, the ants were very cooperative and more than eager to pick up the seeds we put out despite their clumsiness. As such, we were able to gather good data quickly.

Vincent Pan sticking more seeds to more things. Photo by Eric LoPresti.
Vincent Pan sticking more seeds to more things. Photo by Eric LoPresti.

In this study, we report that across a wide diversity of angiosperms, seed mucilage may help seeds escape granivory by gluing seeds strongly to the ground. Report of this granivory defence function of seed mucilage is not new; Gutterman & Shem-Tov (1997), Engelbrecht & García-Fayos (2012), and Yang (2013) already laid much of the groundwork. However, only a few species have been tested and it was unclear whether this function is broadly applicable, especially considering that the trait has evolved multiple times independently in many families and appears to be highly multifunctional. Indeed, while we found that seed mucilage was broadly protective against granivory, as we did in a sister paper about mucilage-mediated sand covering (LoPresti et al. 2019), species differed wildly in the degree of benefit they acquired.

A harvester ant (Pogonomyrmex subdentatus) attempting to dislodge a mucilage anchored seed (Gilia leptantha). Photo by Eric LoPresti.
A harvester ant (Pogonomyrmex subdentatus) attempting to dislodge a mucilage anchored seed (Gilia leptantha). Photo by Eric LoPresti.

We found that in certain groups, such as the rockroses, the attachment strength was often not strong enough to make a difference to granivorous ants. In others, such as flax, the same amount of mucilage appears to be more effective at discouraging granivory.

Thanks to the impressively meticulous work of Grubert (1974) examining dislodgement potential across hundreds of mucilaginous species, we were able to take these qualitative speculative hypotheses one step further by exploring broad trends across families and ecological contexts. Using Grubert’s data we found that “mucilage stickiness” was higher in annuals from hotter places with more solar radiation, although no single predictor explained the macroscale patterns particularly well. More closely related species have more similar mucilage stickiness, despite the high within-family and within-genus variation. This got us excited about looking at the evolution of structural and compositional aspects of mucilage that underlay these differences, as seed mucilage is a highly variable trait.   

About the research

            The diversity and multifunctionality of seed mucilage make it a particularly difficult trait to study in terms of its evolution and ecology. Beyond the challenge of measuring functional traits that is comparable and feasible across species, we are often puzzled by multiple conflicting alternative hypotheses that come from studies done on disparate taxonomic groups in very different environments. For instance, seed mucilage can both promote and reduce germination, as well as both facilitate zoochory (the dispersal of seeds by animals) and antitelechory (no dispersal). We have also done all sorts of things to mucilage producing seeds and often found completely opposite patterns among genera.

(A) Ocimum tenuiflorum (Lamiaceae) stained with Lugol's iodine with visible starch granules alongside cellulosic fibrils; (B, C, F) Linum grandiflorum (Linaceae), Lepidium sativum (Brassicaceae), and Salvia coccinea (Lamiaceae) stained with Ruthenium red that shows pectic sugars; (D) Matricaria chamomilla (Asteraceae) stained with methylene blue that shows cellulosic sugars; (E) Dried L. sativum mucilage under phase contrast with branching fibrils; (G) Unstained Salvia Rosmarinus (Lamiaceae).
(A) Ocimum tenuiflorum (Lamiaceae) stained with Lugol’s iodine with visible starch granules alongside cellulosic fibrils; (B, C, F) Linum grandiflorum (Linaceae), Lepidium sativum (Brassicaceae), and Salvia coccinea (Lamiaceae) stained with Ruthenium red that shows pectic sugars; (D) Matricaria chamomilla (Asteraceae) stained with methylene blue that shows cellulosic sugars; (E) Dried L. sativum mucilage under phase contrast with branching fibrils; (G) Unstained Salvia Rosmarinus (Lamiaceae).

            Reconciling these differences and to understand why mucilage evolved and did so repeatedly would require identification of good functional trait correlates and more extensive sampling of seed and mucilage traits. We are currently screening as many species as possible to identify the context for which certain mucilage functions are more important and the syndromes that comes with it.

About The Author

(Left) A contraption for measuring the terminal velocity of seeds. (Right) An early prototype for measuring wet attachment force of mucilage.
(Left) A contraption for measuring the terminal velocity of seeds. (Right) An early prototype for measuring wet attachment force of mucilage.

This mucilage seed predation project was actually the first ecological research I was involved in when I first joined the Rick Karban’s lab at UC-Davis as an undergrad assistant not knowing anything about anything. Hanging out with Eric LoPresti in the field tends to make natural history seem super cool, so I stuck around. It was interesting revising the same project a few years later when I became Eric’s lab technician at Oklahoma State University with a deeper appreciation of ecology that I realized mucilage was even cooler than I thought! I’m currently starting graduate school studying herbivory variability at Michigan State University. Because mucilage mediated plant defence is very easy to experimentally manipulate, I’m excited about the prospect of using mucilaginous seeds to test out plant defence theory.

Read the article in full here

References

  • Engelbrecht, M., & García-Fayos, P. (2012). Mucilage secretion by seeds doubles the chance to escape removal by ants. Plant Ecology, 213(7), 1167–1175. https://doi.org/10.1007/s11258-012-0074-9
  • Fuller, P. J., & Hay, M. E. (1983). Is Glue Production by Seeds of Salvia Columbariae a Deterrent to Desert Granivores? Ecology, 64(4), 960–963. https://doi.org/10.2307/1937217
  • Grubert, M. (1974). Studies on the distribution of myxospermy among seeds and fruits of Angiospermae and its ecological importance. Acta Biologica Venezuelica, 8, 315–551.
  • Gutterman, Y., & Shem-Tov, S. (1997). The Efficiency of the Strategy of Mucilaginous Seeds of Some Common Annuals of the Negev Adhering to the Soil Crust to Delay Collection by Ants. Israel Journal of Plant Sciences, 45(4), 317–327. https://doi.org/10.1080/07929978.1997.10676695
  • LoPresti, E. F., Pan, V., Goidell, J., Weber, M. G., & Karban, R. (2019). Mucilage-bound sand reduces seed predation by ants but not by reducing apparency: A field test of 53 plant species. Ecology, 100(10), e02809. https://doi.org/10.1002/ecy.2809
  • Yang, X., Baskin, C. C., Baskin, J. M., Gao, R., Yang, F., Wei, L., Li, L., He, H., & Huang, Z. (2013). Hydrated mucilage reduces post-dispersal seed removal of a sand desert shrub by ants in a semiarid ecosystem. Oecologia, 173(4), 1451–1458. https://doi.org/10.1007/s00442-013-2735-3

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