This week’s organism is a creeping annual or perennial plant native to South America and Central America. It is a member of the legume family and like many of its relatives it can form a symbiosis with nitrogen-fixing bacteria (see my recent post on Medicago truncatula). However, it is more well-known for its ability to fold up its leaves rapidly when touched and is also known as “the sensitive plant”. The video below shows Mimosa pudica in action:
As you can see the leaflets fold up in response to touch. The speed of this response is dependent on the strength of the touch stimulus. In the video above the pencil hit the leaf hard and quickly so the leaflets folded very quickly but a gentle touch results in slower closure that tracks up the leaf. See the second video:
It is thought that the movement is a defence mechanism hoping to deter herbivores and perhaps dislodge insects that try to eat it. A few minutes after the touch stimulus the leaves will usually reopen. The leaves also close up at night (when they are not needed for photosynthesis) and reopen each morning.
But plants don’t have a nervous system or muscles! How is the rapid movement generated? At the base of the leaflets are groups of cells that act a bit like a joint. Normally during the day these cells are full of water (turgid) and this gives the cells a fairly rigid structure that holds the leaflets out. When the leaflet is touched an electrical signal from the site of contact is transitted through the cells of the leaflet. When the electrical signal reaches the cells at the base of the leaflet it triggers the release of potassium ions (K+) and chloride ions (Cl– ) and subsequently the release of water from the cells. This causes the cells to become more flaccid (bit like a deflating air mattress losing its firm shape) and the “joint” bends closing the leaflets. The stimulus can be transmitted between cells along the leaf so that the whole leaf can close up in response to a single touch.
The electrical signals stimulated by touching Mimosa pudica are not unlike those generated in mammalian nervous systems. However, electrical signals in Mimosa pudica travel over 1000 times slower at 20–30 mm/s compared to in mammalian nerves 100 m/s (2). In the mammalian nervous system electrical signals travel along specialist cells called neurones and at the boundary between two neurone cells (synapse) chemical neurotransmitters are released to activate an electrical signal in the following neurone. This is not the case in Mimosa pudica. The leaf cells are electrically connected by cytoplasmic bridges (plasmodesmata) so a single electrical signal can travel through many leaf cells towards the cells at the base of the leaflet.
Rapid folding of its leaves is not the only herbivore defence Mimosa pudica has. The plant contains a toxic amino acid called mimosine (leucenol). Despite its toxic effects in animals there is evidence that it can neutralise the venom of poisonous snakes (2,3) so it may have the potential to be developed into a drug in the future.
Special thanks go to the creators of the videos and the university professor that introduced me to this fascinating plant!
(2)Fromm and Lautner (2007) Electrical signals and their physiological significance in plants. Plant, Cell and Environment. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3040.2006.01614.x/pdf
(3) Mahanta and Mukherjee (2011) Neutralisation of lethality, myotoxicity and toxic enzymes of Naja kaouthia venom by Mimosa pudica root extracts http://www.sciencedirect.com/science/article/pii/S0378874100003731
(4) Mahadeswaraswamy YH (2011) Daboia russelli venom hyaluronidase: purification, characterization and inhibition by β-3-(3-hydroxy-4-oxopyridyl) α-amino-propionic Acid. http://www.ncbi.nlm.nih.gov/pubmed/21682681