Molecular Magic behind the “Touch me not” plant

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Image source: Bhuvaneshwari sampath (Own image)

 

When it comes to playing with plants, we all have experienced a greater pleasure by touching the leaves of touch-me-not plant and watching them go to sleep. But how it happens?

Plant taxonomy

Common names: Sensitive plant, touch-me-not plant, humble plant, shameful plant.

Scientific name: Mimosa pudica

Family: Fabaceae (Leguminosae)

Sub-family: Mimosoideae

Origin: This species originated in tropical Central and South America.

Habitat: A weed of wetter coastal areas. It is mostly found in plantation crops, disturbed sites, pastures, waste areas, parks, lawns, gardens and along roadsides.

 

Why Mimosa plant close when touched?

Mimosa show sensitivity to touch. This phenomenon is scientifically known as seismonastic movement. It is a result of quick drop in turgor pressure in the cells at the base of the leaflets. "Flaccid" cells cannot support the weight of the leaflet and they start to close.

What is the molecular mechanism behind the Mimosa pudica leaves to close?

When a plant detects a stimulus, it sends a signal to the top cell of the pulvinus (plant motor organ) via an action potential. Active transport keeps potassium and chloride, inside the cell at higher concentrations than outside the cell. However, when the action potential reaches a top cell, potassium and chloride specific ion channels are opened in response to the change in electric charge. This allows ions to move out of the cells through ion channels and triggering water to leave the cells through osmosis. Much of this water comes out from the cell vacuole- a large internal reservoir of water found in most plant cells. This phenomenon makes vacuole smaller and causing cell to shrink.

How so much water diffuses across the cell membrane quite so quickly? Mystery solved by Aquaporins channels.

The reason behind this fast process is the special water transporting channels called Aquaporins. Aquaporins are selective transporters, allowing only water to pass through while blocking the passage of other molecules or ions. A large amount of water can leave the cell very fast, in 1-2 seconds, which makes Mimosa leaves fold and droop rapidly.

How this process transmitted to neighbouring leaves?

The ions releases by the top cells are pumped into the bottom cells by active transport and consequently bottom cells will have high concentrations. The water moves in, again due to osmosis, and is stored in the vacuole, causing the bottom cells to swell. If a stimulus is strong, pulvini at the bases of the leaves and the petiole also react. However, in these pulvini the behaviour of the top and bottom cells is reversed, which means that activation drives the petiole downwards rather than upwards. So, the combined effect is that leaflets fold up, but the whole leaf is pushed closer to the ground, giving the appearance of lifeless.

Does it have any purpose for this peculiar behaviour?

There is no confirmed proof why touch-me-not plants behave in this way. But some researchers believe that they shrink out as a defence mechanism from other insects and herbivores. Of course, if you are a little carpenterworm looking for a supper from the plant, would you take the risk and go near a plant whose leaves twist, turn, fold and droop when you touch it, or would you prefer for a peaceful meal?

 

References:

Ahmad H, Sehgal S, Mishra A, Gupta R. Mimosa pudica L. (Laajvanti): An overview. Pharmacognosy Reviews. 2012;6(12):115-124.

Abramson CI, Chicas-Mosier AM. Learning in Plants: Lessons from Mimosa pudica. Frontiers in Psychology. 2016; 7:417.

Allen RD. Mechanism of the Seismonastic Reaction in Mimosa pudica. Plant Physiol. 1969 Aug;44(8):1101-7.

Eisner T. Leaf folding in a sensitive plant: A defensive thorn-exposure mechanism? Proceedings of the National Academy of Sciences of the United States of America. 1981;78(1):402-404.

Volkov AG, Foster JC, Ashby TA, Walker RK, Johnson JA, Markin VS. Mimosa pudica: Electrical and mechanical stimulation of plant movements. Plant Cell Environ. 2010 Feb;33(2):163-73.

 

Article by:

Bhuvaneshwari Sampath

Research Scholar

AU-KBC research centre, Anna University, Chennai

Email: mail2bhuvanaa@gmail.com

Posted By : ScienceIndia Administrator