The leaves of tobacco protect from parasitic insects by releasing a chemical signal, inducing production in adjacent cells of a compound toxic to invertebrates.

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When cells within a tobacco leaf are touched by an external force, stretch receptors cause them to release calcium ions. The signal spreads into adjacent cells in a propagating wave triggering the release of more calcium ions and the subsequent production of a chemical called GABA. Tobacco leaf parasites that ingest GABA suffer tremors, excitablilty, paralysis, and death.

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"How does a plant sense touch? Ion channels in the plasma membrane are thought to be activated by stretching which is followed by a rapid change in the cytosolic Ca++ signature. Increased cytosolic Ca++ concentrations may trigger further plant signal transduction resulting in induction of genes involved in plant defence or growth." (Hilker 2010:268)

"Bristles and crotchets on tarsi of insect larvae crawling on a leaf may inflict minor scratches on a plant that lead to rapid synthesis of γ-aminobutyric acid (GABA)...Very high levels of dietary GABA may have detrimental effects on growth and survival rates of insect larvae." (Hilker 2010:268)

"Since touch can be sensed by a plant, then walking of herbivorous arthropods on a plant when searching for mates or an oviposition site may inform the plant of a future danger of feeding damage. It is possible that touch could prime the plant to get 'ready for the battle.' (Hilker 2010:273)

"...disruption of GABA signaling in insect larvae causes physiological and developmental damage. Blockage of the peripheral neuronal GABA-gated Cl- channels in invertebrates leads to tremors, hyperexcitability, paralysis and death." (Bown 2006:424)

"Within 5–10 min of tobacco budworm (Heliothis virescens) larvae crawling on plant-attached tobacco (Nicotiana tabacum) leaves the GABA levels increase four- to fivefold. Similarly, crawling of oblique-banded leafroller larvae on soybean leaves raises GABA levels 10- to 12-fold within 10 min. Larval footsteps on the leaf surface can be visualized through superoxide production or transient increases in chlorophyll synthesis" (Bown 2006:425)

"Touch stimulation elicited a relatively prolonged (2-5 min) transient increase in [Ca2+ ] at the cellular level...The touch stimulus led to elevated [Ca2+] that moved away from the site of the stimulus in what may be a propagating wave of Ca2+." (Legue 1997:795)

Journal article

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Journal article
Gamma-aminobutyrate: defense against invertebrate pests?Trends in Plant ScienceAugust 7, 2006
Alan W. Bown, Kennaway B. MacGregor, Barry J. Shelp

Journal article
Cytoplasmic free Ca2+ in Arabidopsis roots changes in response to touch but not gravityPLANT PHYSIOLOGYJuly 27, 2002
V Legue

Journal article
Hilker M; Meiner T

No link available.
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