Literature Review About How Plants Respond to Touch Stimuli

Similar to all other organisms' plants will have the ability to detect and respond to different kinds of stimuli present within their environment, as plants are rooted within the soil its main response to this will be too then change the way in which it grows and this will be done using hormones. A plants movement which is under a continuous directional pressure is known as thigmotropism, a thigmonastic response will be a touch response which is independent of the stimulus direction. There is also then thigmorphogenesis which is a slower change in the plants shape which will be in response to a constant mechanical stress. the touch stimuli can come from a number of different sources such as other plants as well as animals in particular insects which can be predators or prey of the plant, these interactions will include the likes of the detection and capture of prey by the carnivorous plant species. Touch can therefore lead to a number of different responses in plants such as a reduction in plants growth, a delay in its flowering time or a change in the direction of growth. These different responses are dependent on the phytohormone jasmonate.

Through evolution plants have developed a number of different mechanisms in order to perceive and react to their surroundings, this is known as plant behavior which is a complex set of informative cues which are received from the neighboring plants. Plants will be exposed to a range of differing mechano stimuli from the likes of their neighbors, leaf movement or from phototropism, an example of a response to touch is canopy shyness in trees in which canopies stop expanding once they touch canopies of other trees. Touch can then be said to induce a strong morphogenetic change in plants over time. plants abilities to detect and respond to these changes will be vital to their survival. Plants have evolved in different manners to one another and thus will respond differently to touch, species like the Venus fly trap will have fast touch responses, here there will be two modified leaves on a hinge with tines on the outer edge, there is then tiny hairs inside the trap and when touched the leaves close trapping any prey. Most other plants will have a more gradual touch response known as thigmorphogenesis. Plants have evolved too have visible morphological modifications over long periods of time such as the inhibition of internode elongation, an increased ration of branch to stem as well as creating stronger and more flexible stems. 

A thigmonastic response is a response to touch which is independent too the stimulus direction. This kind of response can be seen by the Mimosa pudica in which touch will result in the rapid folding of its small leaflets, this can result in the propagation of all the leaflets on the leaf. This type of response can scare away predators as well as exposing its protective thorns. This response results from a loss of turgor in extensor cells which results in flexor cells stretching, the cell volume and shape then changes which allows for the leaflet and petiole movements. The coiling of tendrils is also an example of a thigmonastic response, tendrils will curve by differential growth which will be the stimulation of growth in certain areas of the tendril. The side of the tendril opposite the side of contact will thus grow faster causing the tendril to curve to the side of contact. Some tendrils also show rapid contact coiling which is a result of changes in the cell turgor which causes the tendril to curve, cells on the side of contact contract, and those on the non-contact side expand which causes the rapid contact coiling. The thigomonastic response may help plants survive as it may provide them with nutrients such as with the Venus fly trap or it may make it more difficult for herbivores to eat their leaves such as is seen with Mimosa pudica.

In contrast to a thigmonastic response, Thigmorphogenesis will show morphogenetic alterations over long periods of time, a number of phytohormones and signalling molecules will be used in the response too touch. There will also be a number of genes which will be induced after touch, these will then encode proteins which will then be involved in cellular processes such as calcium sensing. One of the most common responses is a decrease in shoot elongation with an increase in radial expansion, other changes can include the likes of changes in chlorophyll content as well as biotic and abiotic stress resistance. Thigmorphogensis is said to be a adaptive response in which will help plants to withstand continuous force or stimuli. Younger plant tissues are seen to have a higher thigmorphogenetic response as oppose to older ones, this could be caused by the fragility and susceptibility of younger plants meaning they need to respond more dramatically in order to survive. 

The regulation of a plants growth is required for marketable ornamental plants, people will have a preference for stronger more compact plants with greener leaves, the same also applies to young vegetables. These kinds of plants need to be solid, compact and resilient as welll as being able to survive transport and planting with ease, this can be carried out through the use of mechanical stimulation which can then go on too produce the commercially relevant phenotypes. The process of thigmomorphogensisi is therefore of particular importance to produce vegetables as where automated crop transplanters are in use the seeds robustness will be of high importance. The action of brush of stroking seedlings within greenhouses is also shown to improve crop quality as well as counter the problem of them being thinner. Short stakes can also be used when planting trees, the unsupported area of the stem then has the ability to flex thus making it stronger.

Therefore in conclusion plants will have a number of different methods in which they respond to touch and this will be for a number of different reasons such as for defense or to capture prey and these processes can also happen quickly or over the span of many weeks too months. The ways in which plants do respond too different touch stimuli can also then be exploited for human gain and monetary value as this can produce more desirable ornamental house plants as well as creating better and more resilient vegetable crops.