Question
Hi, I have read an article stating that plants can react to damage by releasing specific hormones.

http://www.sciencedaily.com/releases/1998/08/980806090010.htm

But what I do not understand is : how can a plant react defensively to damage without being aware of it like humans and animals when they are hurt? Is it a kind of automatic reaction like when your computer gets a virus or something? Thank you a lot and have a nice day !

Answer
Hi Jack,

Just because you don't see a physical reaction, don't think that the plant is not reacting biochemically.  The plant membranes serve as a complex network of biochemical communications, sending signals when herbivores or pathogens damage cell membranes.

It is quite correct that plants don't have an immune system, but they have developed a way to initiate some non-disease-specific defense mechanisms which produce many biochemical products that induce the deposition of lignin (a substance that thickens/repairs and strengthens the cell walls)throughout the plant, produces a cascade of pathogen related proteins, complex molecules that further strengthen cells and biochemical pathways, such as producing jasmonic acid and terpenoids (foul tasting chemicals that reduce herbivory).  The plant is not "aware" of the damage in a sensory context, but is very much aware biochemically.  These pathogen related proteins are produced only upon a disturbance, either a pathogen or a physical damage.  (The plant species with which I work is so sensitive that a subtle change in light can induce the expression of the pathogen-related genes.)

One way to think of these reactions in plants is to compare it to your own body's response when the human immune system successfully mounts a defense against a viral pathogen to which you have immunized.  You are not aware of that biochemical and cellular battle going on in your body.

Plants are susceptible to bacterial, fungal, viral, and parasitic worm pathogens.  Each of these microbes (or worms) must enter into the plant cell to cause or initiate disease.  The plants have evolved mechanisms which protect from those invaders.  The damage caused by a broken branch induce the same type of reactions that a bacterium or germinating fungal spore would cause upon breaking the cell wall or cell membrane.

Interesting topic.  Even more interesting is the fact that there are common (shared) pathways in plants and animals.  That is why the aspirin is capable of relieving a headache in humans--we have similar biochemical receptors.

Check out this site for a diagram and some information about the plant defense mechanisms.

http://dmd.nihs.go.jp/latex/defense-e.html