Physiological mechanisms and components in thigmotropism.
Responses to touch are not only seen in tendrils but also in roots and other plant organs. Thigmotropic responses are complex and cannot simply be explained as contact –induced responses to physical pressure or by hormones. Turgor pressure as a mechanical force may regulate and control enlargement and growth of cells, but there is still much uncertainty about mechanisms of perception and responses of non-specialized plants to their response behaviors (Braam, 2004). In the whole process of thigmotropism, perception of a stimulus, signal transduction, amplification of a signal, and plant organ response through differential growth all contribute to the plant response.
In 1990, a comprehensive screen identifying role-playing parts of the mechanosensory response system in Arabidopsis was performed. As a result, a small group of five genes were isolated. The genes encode proteins, one named calmodulin (CaM), which under influence of calcium ions modulates target enzymes. So it is believed that Ca 2+ may also have a significant job as a messenger in thigmotropism. (Esmon, Pedmale, & Liscum, 2005).
Sensing of stimuli.
It is believed that touch stimuli are first recognized at the location of the cell wall where contact is made. However the uncertainty is how that signal is transmitted to the intracellular level (Jaffe, Leopold, & Staples, 2002).
Mechanosensors also play an important part in animals and microbes and it is understood that channels and ionic changes are used to transduce external stimuli to a biochemical signal at the intracellular level. There is evidence that these processes and ionic signals are also very relevant in plant responses to touch and gravity (Gilroy & Masson, 2008). Mechanosensors are not specialized cells or organs as sensory receptors in animals, but may be molecules or structures related to the cell membrane functions either on the cell surface or inside the cell (Hart, 1992). Calcium (Ca 2+) ion level changes in the plasma membrane are known to occur throughout the plant cells and are commonly documented as the initial changes in response to stimuli. This has been determined and measured by using plants with the luminescent Ca2+ protein aquorin or dye in the plants. Whether this calcium fluctuation occurs across the plasma membrane or originates from intracellular locations is uncertain, but changes in Ca and pH both appear to be important signals in mechanical stimulation. A possible explanation is that the increase in Ca ions is alerting the cell that something has happened and then other signaling systems determine the response (Gilroy & Masson, 2008). “Thus, the ionic signaling associated with both touch and gravity signaling may well form a nexus at which information from both systems is incorporated to control pH and auxin flow and so generate an integrated tropic response” (Gilroy & Masson, 2008, p.113). The sensors for touch stimuli at the molecular level are still undetermined and once defined will give a greater understanding to those studying touch and thigmotropic responses. Touch responsive genes identified in some plants appear to have a relationship with the calcium ions, but some genes have also been expressed that are not related to Ca yet are also responsive to touch.
Signal transduction and amplification.
A signal produced by the mechanosensor results from the energy generated by the stimulus. This energy is then transduced into a biochemical form of energy (Hart, 1992). A small stimulus eventually creates wide spread biological response using amplification mechanisms. The amplification may be produced by cellular use of enzymes, regulatory chemicals or hormones, and changes in cell membrane properties but further research is necessary in order to make definitive conclusions. The transduced signal may also need to be transmitted to the area where the tropic response is executed. This concept was initially presented by Darwin and was explained by the growth regulator, auxin (Barrett & Freeman, 1989). Now, other regulating chemicals and electrical signals are believed to transmit information in the plant. These action potentials, special voltage dependent ion channels, are well-known in the Mimosa turgor movements occur rapidly when stimulation is present (www.ncbi.nlm.nih.gov). The cell membrane is often the location of these processes from signaling to transmitting (Hart, 1992).
Plant organ responses.
Scanning electron microscopy has shown the cells on non-contact side of mechano-stimulated tendrils are fully turgid, but the cells on the contact side are flaccid and folded as if they are less turgid (Vartanian, 1997). Potassion ions (K+) were also higher on the contact side. The decrease in turgidity causing cell contraction is believed to be induced by the action potential in tendril tissues.
Turgor movements and differential growth are both implemented in thigmotropism resulting in curvature of the plant organs. Genetic factors play an important role in growth responses such as that of touch. Dr. Janet Braam has isolated four “touch”genes in her research of thigmomorphogenic plants (Vartanian,1997). These genes which increase responses to touch encode calmodulin and calmodulin-related proteins. She discovered that touch induces TCH genes resulting in instant rise in the cytoplasmic Ca2+ levels and thus is a major contributor in the signal transduction pathways upon touch stimulation. Because higher cytoplasmic Ca2+ levels initiate expression of TCH2, TCH3, and TCH4, it is suggested that Ca2+ may be a regulator for expression of these TCH genes. Further research by Dr. Bramm showed the cell wall structure’s strength and size can be changed by a product of one of these genes. This could explain differential cell growth. It has been concluded that mechanical perturbation may be involved both at the signal transduction and the growth response level since both the route for calcium and calmodulin transduction is significant in both thigmotropic and thigmomorphogenic plants (Takahasi & Jaffe, 1990).
Other studies by Dr. Weiler and his colleagues have suggested that compounds known as jasmonates are relevant to the tendril coiling response of Bryonia dioica (Vartanian, 1997). These researchers propose that two acids mediate the reaction for the tendrils to touch. Studies with B. dioic showed exposure to these acid compounds caused a similar tendril response to that of touch. One of the acids, jasmonic acid, levels increased intracellularly upon tendril coiling. This study relates these acids and jasmonates closely to the central transduction mechanism which parallels growth response.
The plant hormone, auxin, is believed to be significant in stimulating cell elongation on one side of the plant, changing the direction of plant growth, in phototropic and gravitropic plants (McIntosh, 2012). The role of auxin and ethylene is uncertain in thigmotropic plant responses but studies have shown that auxin can cause cell turgor changes in some thigmotropic plant tissues (Scorza & Dornelius, 2011).
Summary
The mechanism of thigmotropism explains the movements of vining plants as their tendrils touch obstacles in their surroundings, attaching to gain support. Likewise, plant roots in the soil behave in response to objects in their path by changing their growth direction. These and other touch responses are essential to the survival and existence of plants. It is clear that touch signals received at the cell wall are transmitted through the plasma membrane resulting in changes in both membrane and cytoskeletal structures, culminating in changes in gene expression (Jaffe et al., 2002). The TCH genes which encoded proteins are involved in various cellular processes including calcium sensing, cell wall modifications, and defense (Braam et al., 2008). Additionally, a variety of signaling molecules and phytohormones, including intracellular calcium, jasmonates, ethylene, auxin, and other compounds have been identified in touch responses. The uses of all these physiological components contribute to the ultimate plant organ movements which enable the plant to respond to its environment.
