In touch: plant responses to mechanical stimuli
Janet Braam
Biochemistry and Cell Biology, Rice University
New Phytologist (2005) 165: 373–389
This paper addresses plants perceptions and responses to mechanical stimuli. It provides description on different touch-responses in higher plants. Aspects of genomic responses to mechanical responses are explored as well as potentiality of genetic approaches to this intriguing area of biology.
Understanding the mechanisms of perception and response are likely fundamental to understanding plant biology. [Janet Braam]
Rapid responses to touch in plants are referred to as thigmonastic (in directions independent of stimuli) and thigmotropic (in direction of stimuli). Carnivorous plant, Venus’ Flytrap – Dionaea muscipula – displays thigmonastic response when the insects crawls on its leaf surface and bumps into the trigger hairs resulting in intracellular signals that marks the closure of trap within seconds. Interestingly, this carnivory provides nitrogen to the Venus’ Flytrap allowing them to live in nitrogen poor environments. [This nitrogen supplement exchange was also mentioned in Dr Gilles talk on bacteria/plant interaction and communication at the BA Festival of Science this year.]
Other touch induced carnivorous plants mentioned in this paper included the Sundew – Drosera rotundifolia. This plant exhibited both thigmonastic and thigmotropic responses to the preys presence. The tactile sensitive tentacles (neighbouring tentacles bend towards ones directly stimulated by prey’s presence) are able to detect insect movements and together the tentacles can generate cup like closure that traps the prey. The mechano-sensitivity - able to detect piece of human hair less than migrogram (Darwin, 1880) yet fail to respond to water droplets of even the force of heavy rains (Darwin 1889; Darwin 1893) - exhibited in this plant opens questions of how the plant distinguishes between stimuli’s unrelated to food (rain/wind) from the perturbations of the insects.
The Bladderwort – Utricularia – has the most forceful prey capture mechanism amongst plants (Lloyd, 1942). Within 30 ms of tactile detection the trap door burst open and there is a tremendous suction of water into bladder with creatures swept in current. The trap door closes and the meal takes place.
Sensitive plants such as Mimosa pudica are thigmonastic plants where upon touch a rapid folding up of the small leaflets occurs – this touch response is spread to all neighbouring leaflets of the leaf. This may serve to both scare away potential predators and give the appearance of less voluminous meal. The leaf movements may be a mechanism to expose protective thorns (Eisner, 1981). Related species include; Samanea saman.
Plants utilizing touch senses as means to reach vertical height for sun exposure (thereby eliminating the expenditure of developing a supporting trunk) are able to reach on sturdy objects and use them as supports by coiling around them. (Drosera, Bryonia dioica, Ficus costaricensis, Monstera, Pisum)
Many flowers have evolved touch-sensitive organs to prevent self pollination or enable crosspollination (with examples of both thigmonastic and thigmotropic stamen filaments, petals, and pistils). (Catasetum)
Transgenic plants expressing the Aequoria jellyfish gene encoding aequorin, a Ca2+-dependent luminescent protein, have proven to be valuable tools for monitoring Ca2+ fluctuations in response to many different kinds of stimuli (Knight, 2000). In particular, aequorin transgenic plants have demonstrated a rapid intracellular Ca2+ increase in response to touch or wind stimuli (Knight et al., 1991) and have provided evidence that mechano-responsive Ca2+ increases are derived from internal stores (Knight et al., 1992).
Touch-inducible genes in plants (Braam & Davis, 1990) are called TCH genes and isolated by differential cDNA library screening, are strongly and rapidly up-regulated in expression in response to touch (Braam & Davis, 1990). A simple mechanical stimulation, administered by touching the rosette leaves and bending them down or back and forth, was found to be sufficient to elicit a dramatic enhancement of TCH expression (Braam & Davis, 1990).
Many genes have been discovered that have mechano-stimulus-inducible expression.