Who would win the 100 metre sprint, Usain Bolt or a plant?
At face value this seems like a ridiculous match up; the fastest man on the planet against a fundamentally stationary plant. However, over the course of this article I hope to convince you that this question is actually quite reasonable and that our immobile neighbours on this planet can truly be lively company.
Animals (with a few exceptions) move with the use of structures such as nerves and muscles, which can yield forceful and rapid movements. Although plants do not contain similar structures and have definitely skipped leg day at the gym, they are still able to move - just in a different way.
Their apparent static nature can often be misleading, as plants are capable of movements at speeds and scales spanning several orders of magnitude. Rooted to the ground and established in a general location for their entire life time, plants are exposed to many hazards such as high winds, extreme temperatures, drought, floods and grazing herbivores. This means they require movement strategies to enable them to survive in a dangerous and unforgiving environment. Plants do not have eyes or ears, but, the key to their ability to move is the fact they can sense aspects of their environment such as light, touch and gravity. This sensory capability underpins how plants are able to move.
So plants are mobile. But which one would have a chance against Bolt? To find out, we need to explore what types of movement plants are capable of.
The most fundamental movements in plants are tropisms. Tropisms involve directional growth of the plant (in the stem or roots). For example, the movement known as phototropism is a plant’s growth response to light. It is regulated by proteins that act as photoreceptors, sensing the light, and the plant hormone auxin, which causes cell elongation. Shoots are positively phototrophic, meaning they grow towards the light. This is achieved through asymmetrical distribution of auxin in the shoot apex, concentrating the auxin on the side furthest from the light exposed surface. Growth consequently occurs at a faster rate on this side of the stem, causing curvature towards the light source. Plants need this light for photosynthesis, a process which involves the conversion of carbon dioxide and water into sugars for the plant. Therefore, if a plant is able to adapt and grow towards an area with higher light intensity it will have a survival advantage.
Another example of tropic movement is gravitropism. The first force that seedlings experience, gravity, triggers growth responses in plants. Shoots have negative gravitropism and so grow away from the ground in order to out-compete neighbouring plants for light. Roots grow with gravity, leading them to propagate further into the soil to seek out nutrients. This not only enables them to thrive but also provides structural support and stability to the plant.
Few people realise that plants also grow in response to touch – thigmotropism. Examples of thigmotropism include the bending and coiling of climbing plants against a wall or trellis, or a plant growing roots away from obstacles in the soil along the path of least resistance. Although essential and acrobatic, trophic movements are slow. So much so, that spotting them may need technologies such as time-lapse photography to capture. To uncover the faster realms of plant dynamics, the marathon tropic events must be left behind to focus on the sprinters of the plant world – nastic movements.
Nastic movements do not involve growth, but instead harness the power of electrical signalling and elastic potential. This allows their motions to be more rapid, often requiring high-speed cameras to be visible to the human eye. Much like tropisms, there are different types.
A common risk for plants is that they will be damaged by grazing herbivores. With such a high risk, plants have developed a rapid defense response to leaf injury, termed traumatonasty. Wounding of the leaf induces a signal which rapidly spreads throughout the plant at speeds of around 15 mm/s. Signal propagation is mediated by glutamate-receptor-like (GRL) proteins, which are structurally similar to vertebrate ionotropic glutamate receptors involved in neurotransmission in humans (among other species). Trautomonasty might be seen tenuously as the plant alternative to human nerve transmission. However, on the starting blocks, a human’s nerves will be firing at over 100m/s, still at speeds considerably ahead of this type of plant movement.
The plant Mimosa pudica illustrates another interesting, slightly faster example of nastic movement – thigmosnaty. Similar to thigmotropism, movement occurs when the plant is stimulated through touch. Stimulation in this way causes the leaflets of the plant to close in succession, a wave of movement that travels at about 20mm/s. The swift closing of these leaves is controlled by an action potential triggered by chloride ions. Movement of chloride and potassium ions laterally through plasmodesmata (cytoplasmic channels joining two adjacent plant cells through the cell wall), causes the pulvinar action potential to spread. The closing of the leaflets themselves results from a change in internal water pressure leading to the folding movement in this plant.
One of the most well-known examples of nastic movement is seen in the Venus Fly trap, Dionea muscipula. This plant switches to a carnivorous strategy when soil conditions are nutrient limiting and relies on catching small animals, usually insects, to survive. The fly trap consists of two convex lobes with sensitive hairs on its surface (usually only three or four). Animals are drawn to the nectar in the centre of the plant and move across the lobes, triggering the sensory cells of the hairs. This mechanical stimulus is then converted into an electrical signal initiating an action potential, which spreads rapidly (20cm/s). If a second action potential is fired within 20 seconds the trap will snap shut within half a second– capturing the prey. The Venus Fly trap is undeniably fast, but still not a suitable competitor for our human athlete.
Some of the fastest nastic movements are a result of seed and spore ejection. In the plant Polypodium glycyrrhiza, ejection takes place in a structure termed the leptosporangium. Spores are contained in the sporangium, which has an outer cell layer called the annulus. When the spore is reaching maturation the cells in the annulus lose water and shrink. This reduction in cell volume is converted into bending energy, causing slow backwards bending of the annulus. Elastic strain is built within the annulus walls, until a critical pressure of -20MPa, causing cavitation and a large increase in annulus cell volume. This change in volume causes the sporangium to spring back into its original position and launch the spores’ huge distances into the air, reaching ejection speeds of over 10 m/s.
Although plants do not run around like Usain Bolt, they clearly have a range of opportunities for movement. The slower growth movements are more analogous to a marathon race for the plant. However, the nastic plant movements start to compete with the dynamic speeds seen in the animal kingdom. While most are not mechanically suitable competitors for a 100m dash, some nastic events such as spore and seed ejection occur at speeds (10m/s) that only just fall short of the Olympic sprinter’s gold medal acceleration of 12m/s. So, although, disappointingly, Usain Bolt will win the 100m race over a plant, I hope this trivial comparison highlights that the secret world of plant movement is more exciting and fast paced than you may have imagined.