Why plants are touch-sensitive?

Have you ever wondered how much plants have evolved to adapt to the environment they live in as well as to adjust to changing environmental factors? One of the most interesting adaptations of the plant world is being sensitive to touch and external stimuli. Same as the animal kingdom, plants do respond to physical touch, they experience and demonstrate their involvement related to the stimuli. Let`s explore the wonders of touch sensitivity in the plant world through this article.

💡 Do you know?

The Greek word for touch is “thigma”

The growth or turning movements of plants are called “tropic” movements.
Non-directional responses to stimuli that occur more rapidly than tropic movements are known as “nastic movements”.

Touch-based reactions of plants can be categorized into a few types based on their nature of reactions due to touch.

Thigmotropism: The directional growth orientation or movement of a plant concerning external touch
Thigmonasty: Non-directional rapid movements of a plant due to touch or contact
Thigmomorphogenesis: Alteration of growth patterns of the plant due to external touch or contact.

Why plants have become touch-sensitive?

Maintaining support and stability

Plants need support and strength if their structure is not strong enough to withstand their own weight, branch and leaf structure. Due to this reason, the plants have adapted to use special structures like “tendrils” to climb and acquire support and strength when they are growing. These tendrils help them to facilitate their tropic movements towards the preferable environments for the plants. These tendrils are a modified stem, petiole or leaf structure that forms a coiling structure to climb through a supporter. Tendrils grow in a curve by employing a process known as “differential growth”. In positive thigmotropism, the side of the tendril opposite to the side of contact will grow at a faster rate than the contact side. In some cases, the cells on the contact side will compress, which enhances the curving response. Therefore, the non-contact side begins to elongate faster than the rest of the tendril, while the contact side compresses. This causes the tendril to curve toward the site of contact. The cells on the opposite side of the tendril sense the stimulus through specialized epidermal cells on the tendril. When the tendril touches an object, these epidermal cells control the differential growth of the tendril. This differential growth can result in the tendril completely circling the object within five to ten minutes.

Figure 01: Coiling behaviour of the Passion Fruit plant -Passiflora edulis (Source: Tendrils of passion fruit plant) — *Figure 01: Coiling behaviour of the Passion Fruit plant -Passiflora edulis (Source: Tendrils of passion plant)*

This coiling behaviour of the tendrils is mainly controlled by the auxins and the ethylene with the presence of ATP(Adenosine triphosphate) and light. These tendrils exhibit self-thinking abilities allowing them to sort out their structures and other objects. These capabilities of plants have established the structure of the plant stronger along the growth period.

*Figure 02: Cucumber (Cucumis sativus) tendrils winding in a supporter (Source: Cucumber tendrils )*

As a defence mechanism

Plants maintain different kinds of mechanisms to respond to herbivory. Several evolutionary processes in plants have created structural features such as spines, thrones and trichomes to physically chase away the herbivores. Some other plants emit Volatile Organic Compounds (VOCs) to impede herbivores as well as to attract carnivores who feed on the respective herbivores feed on the plant.

Some of the plants bear their defences inside the plants and are triggered when the plant is touched or harmed as the last line of defence. Idioblast is a mechanism triggered by the plant Dumb cane (Dieffenbachia seguine). When the plant cells are damaged, the calcium oxalate crystals (Raphides) are detonated into the bodies of the threat. These Raphids contained poisonous enzymes which can cause even paralysis in animals.

Figure 03: Dumb Cane plant - Dieffenbachia seguine (Source : Dumb cane plant) — *Figure 03: Dumb Cane plant – Dieffenbachia seguine (Source : Dumb cane plant)*

The morphological changes of plants play a huge role in defence against herbivores. The most widely known Touch-Me-Not plant shows contraction of its leaves and branches under the touch. This behaviour convinces the predator that the plant is wilted or not suitable to consume. This touch-induced nastic movement of Mimosa pudica is known as thigmonasty.

Facilitating pollination

Have you ever wondered how plants facilitate their pollination using their touch sensitivity?

*Figure 04 : Loasaceae Flower and its stamens (Source: Loasaceae Flower)*

It has been discovered that there is a complex mechanisms regulating the pollen release via thigmonastic stamen movement found exclusively in Loasaceae subfamily Loasoideae. Initially, the anthers are immature and closed, and maturation takes place one after another, beginning with the stamens that are directed towards the center of the flower. Pollen viability is limited and pollination success can depend on the presence of fresh, viable pollen. As a result, the anthers of Loasaceae open only just before the stamen is supposed to move or during stamen movement. Once stamen movement is started the stamens quickly (within 1–2 min) perform a movement of 90–120° from the inner side of the petals to the center of the flower. The stamen movement of an individual flower has a minimum rate in the absence of pollinator visits. This “autonomous stamen movement” is dependent on light and temperature. The stamen movement rates can be drastically accelerated if pollinators are rich and interact with the plant. When harvesting nectar, the flower pollinators have to stimulate specialized structures of the flower, which are called nectar scales, by bending them outwards to approach the nectar. The mechanical stimulus of this movement begins the (thigmonastic) stamen movement. The rate of thigmonastic movement is positively correlated with the frequency of the pollinator visits. And since only mature anthers move and present their pollen in the center of the flower, anther maturation is accelerated or slowed down accordingly. Within hours following the last stamen movement, the stigma becomes receptive and the style begins to lengthen, marking the end of the staminate phase, in which all of the stamens in a flower have migrated towards the center. Following effective pollination, petals and androecium quickly wither and shed.

Seed dispersion

*Figure 05: Balsam plant (Impatiens balsamina) seed capsules burst under external force (Source : Balsam seeds)*

Plants have adjusted to expand their next generation in an optimum way. So, several mechanisms have been adopted by plants to disperse their seeds to a maximum area. Explosive seed dispersal mechanisms play a major role in this regard. Balsam plant (Impatiens balsamina) is this kind of a special plant which uses a mechanism called “explosive dehiscence” to disperse their seeds to greater distances. When the seed capsules of this plant are mature enough, they are sensitive to touch and burst off under tension.

This mechanism is based on the asymmetrical lignin deposition within Endocarp B cells. Here the Lignin provides the strength to store and release kinetic energy when triggered by an external touch. The asymmetrical nature of lignin deposition provides uneven expansion and contraction of the fruit valve producing energy and seeds for expelling seeds at high velocity.

Optimizing resource allocation

When surviving in the environment, plants need to manage their resources effectively to establish their existence for a considerable period. Therefore mechanisms like nyctinasty have been evolved in plants to manage their resources effectively. The plant movements occur in response to the light changes happening with the day and night changes. Most of the nyctinastic movements are reversible, including the closing and opening of plant leaves and leaflets or flowers. These movements in plants help them to optimize resource allocation by minimizing energy consumption as well as reducing the energy they consume. As photosynthesis is not possible at night, the closure of leaves and leaflets would minimize the surface area of leaves touching the air which would also reduce the water loss of plant leaves through transpiration and evaporation. Most of the plants of the legume family are showing this nyctinastic behaviour to manage and relocate the resources.

Plants like Kathuru Murunga (Sesbania Grandiflora), Black Siris (Albizia odoratissima) are some examples for some plants showing nyctnasty.

Figure 06 : Kathuru MurungaPlant - Sesbania Grandiflora (Source : Sesbania Grandiflora) — *Figure 06 : Kathuru MurungaPlant – Sesbania Grandiflora (Source : Sesbania Grandiflora)*

Touch sensitivity is an interesting adaptation that helps plants do a variety of tasks, such as enabling pollination, protecting against herbivores, maintaining support and stability, maximising resource allocation, and assisting with seed dissemination. Plants can react in ways that improve their survival and reproduction to physical contact and external stimuli through intricate systems including thigmotropism, thigmonasty, and thigmomorphogenesis. We may appreciate the extraordinary richness and diversity of the plant world to a greater extent by comprehending these mechanisms. Further investigation into this field may uncover even more astounding ways that plants perceive and react to their surroundings, further studying of this amazing interactions between plants and the external environment.