Plant Biological Warfare Thorns Inject Pathogenic Bacteria into Herbivores Enhancing the Evolution of Aposematism

The physical defense provided by thorns, spines, prickles, silica needles, and raphids against herbivores might be only the tip of the iceberg in a much more complicated story. All of these sharp plant structures may inject bacteria into herbivores by wounding, enabling the microorganisms to pass the animal's first line of defense (the skin), and in so doing may cause severe infections that are much more dangerous and painful than the mechanical wounding itself (Halpern et al. 2007a, b; Lev-Yadun and Halpern 2008).

Another theoretical aspect is the delay between the thorn's contact and wounding and the microorganism's action. While the pain induced by contact with thorns is immediate, the microorganism's action is delayed. However, the same is true for the delayed action of poisons in aposematic poisonous organisms, and yet there is general agreement that colorful poisonous organisms are aposematic (e.g., Cott 1940; Edmunds 1974; Gittleman and Harvey 1980; Harvey and Paxton 1981; Ruxton et al. 2004). Therefore, there is no reason to view a microorganism's contamination and its delayed action any differently.

Lev-Yadun and Halpern (2008) proposed that thorns, spines, prickles, silica needles and raphid-injected microorganisms play a considerable potential role in antiherbivory, actually serving as a biological warfare agent, and they may have uniquely contributed to the common evolution of aposematism (warning coloration) in thorny plants or on the surfaces of plants that have internal microscopic spines (Halpern et al. 2007a, b; Lev-Yadun and Halpern 2008). While it now seems clear that thorny plants are aposematic, the issue of potential aposematism in plants with microscopic internal spines in the form of raphids and silica needles has not yet been systematically addressed.

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