Plants use many strategies to disperse their seeds, but among the most fascinating are exploding seed pods. Scientists had assumed that the energy to power these explosions was generated through the seed pods deforming as they dried out, but in the case of ‘popping cress’ (Cardamine hirsuta) this turns out not to be so. These seed pods don’t wait to dry before they explode. A recent paper in the scientific journal Cell offers new insights into the biology and mechanics behind this process.
Several teams of scientists from different disciplines and countries including Oxford Mathematicians Alain Goriely and Derek Moulton and colleagues from Oxford's departments of Plant Sciences, Zoology and Engineering and led by Angela Hay, a plant geneticist in the Department of Comparative Development and Genetics at the Max Planck Institute for Plant Breeding Research (MPIPZ), worked together to discover how the seed pods of popping cress explode. A rapid movement like this is rare among plants; since plants do not have muscles, most movements in the plant kingdom are extremely slow.
But the explosive shatter of popping cress pods is so fast that advanced high-speed cameras are needed to even see the explosion. Richard Bomphrey, of the Royal Veterinary College at the University of London, explains: “Because the seeds are so small, aerodynamic drag slows them down immediately.” To compensate, the seeds are accelerated away from the fruit and get up-to-speed extremely quickly. In fact, they accelerate from 0 to 10 metres per second in about half a millisecond, “which is super fast!” says Bomphrey.
Hay’s teams of scientists discovered that the secret to explosive acceleration in popping cress is the evolutionary innovation of a fruit wall that can store elastic energy through growth and expansion, and can rapidly release this energy at the right stage of development.
Previously, scientists had claimed that tension was generated by differential contraction of the inner and outer layers of the seed pod as it dried. So what puzzled the authors of the Cell paper was how popping cress pods exploded while green and hydrated, rather than brown and dry. Their surprising discovery was that hydrated cells in the outer layer of the seed pod actually used their internal pressure in order to contract and generate tension. The authors used a computational model of three-dimensional plant cells, to show that when these cells were pressurised, they expanded in depth while contracting in length, “like the way an air mattress expands in depth, when inflated, but contracts in width,” explains Richard Smith, a computer scientist at MPIPZ.
Another unexpected finding was how this energy was released. The authors found that the fruit wall wanted to coil along its length to release tension, but it had a curved cross-section preventing this. “This geometric constraint is also found in a toy called a slap bracelet,” explains Oxford Mathematics's Derek Moulton. In both the toy and the seed pod, the cross-section first has to flatten before the tension is suddenly released by coiling. Unexpectedly, this mechanism relies on a unique cell wall geometry in the seed pod. As Moulton explains, “This wall is shaped like a hinge, which can open,” causing the fruit wall to flatten in cross-section and explosively coil.
According to Hay, their most exciting discovery was the evolutionary novelty of this hinged cell wall. They had evidence from genetics and mathematical modelling that this hinge was needed for explosive pod shatter, “but finding the hinge only in plants with explosive seed dispersal was the smoking gun,” says Hay.
These findings reinforce the description of evolution as a "tinkerer, not an engineer", made by the scientist Francois Jacob. It appears that the sophisticated mechanism of explosive seed dispersal in popping cress evolved via tweaking the shape of already-existing cellular components.
When asked what implications their results will have for other researchers, Smith answered: “It is likely that other processes in plants that were previously attributed to passive shrinkage by drying are in fact active processes, especially in green, hydrated tissues.”
This study is a good example of how the recent trend towards interdisciplinary, collaborative science can lead to a global understanding of the biological and physical mechanisms at play in a complex process. The authors of this Cell paper built up a comprehensive picture of explosive seed dispersal by relating observations at the plant scale all the way down to the cellular and genetic scales, and systematically linking each scale. As Oxford Mathematics's Alain Goriely says, “this approach was only made possible by combining state-of-the-art modelling techniques with biophysical measurements and biological experiments.”
The image above is of a mathematical model explaining the explosive dispersal of seeds from a common garden weed.