External and internal spines in plants insert pathogenic microorganisms into herbivore's tissues for defense

Recently, it has been proposed that plants use pathogenic aerobic and anaerobic bacteria, as well as pathogenic fungi that harbor spines, thorns and prickles, for defense from herbivores, especially vertebrates. These sharp defensive organs introduce various pathogenic agents into the body of the herbivores through the outer defensive layer of the skin in a type of biological warfare. This paper provides a review of our data regarding the various bacterial taxa found on spines, as well as the medical literature regarding infections by bacteria and fungi related to spine injuries. In addition, we propose that many plant species which have internal microscopic defensive sharp structures, such as raphides (calcium oxalate needles) and sharp silica bodies, may also introduce pathogenic microorganisms into the tissues of herbivores through the microscopic wounds that these defensive structures induce, especially in the mouth and digestive system. The internal microscopic spines can wound large herbivores and also insects and other very small herbivores that usually attack plants in between spines and thorns. The large spines and the microscopic sharp structures may introduce not only the microorganisms that harbor them, but also those that pre-exist on the skin or inside the digestive system of the herbivores and on the surface of non-spiny consumed plant parts. A majority of the spiny plants visually advertise their spiny nature, a characteristic known as aposematism (warning coloration). Since the pathogenic microorganisms may sometimes be much more dangerous than the physical wounds inflicted by the spines, we propose that the cooperation with the pathogenic microorganisms contributes to the evolution of aposematism in spiny plants (including those with raphides and silica needles) and possibly also in spiny animals.