Previously on The plants strike back:
Episode 1: Enemies and thickening cell wall
Episode 2: Antibiotics
One set of early defense against pathogen is the de novo production of small secondary metabolites with antimicrobial activity, collectively known as phytoalexins. Phytoalexins can be of very different types: alkaloids, flavonoids, stilbenes, terpenes, and so on… They are species-specific, meaning that different plants synthesize different phytoalexins.
A same plant can also produce several different phytoalexins in different organs. However, members of a same plant family normally produce similar types of phytoalexins. Their production is generally regulated at the transcription level of key genes of their respective metabolism pathway.
Because of the diversity of their structure, it is difficult to judge about the mechanisms of the antimicrobial activity of phytoalexins. Isolated phytoalexins have rather week antifungal activity so to play a role in disease resistance, they accumulate at the infection site to very high levels which are then toxic for fungi.
Besides their antimicrobial activity, phytoalexins are well known for their health promoting effects.
In addition to phytoalexins, small molecules toxic for pathogens are present in plant cells before infection under inactive precursors (e.g. glycoside conjugate), most frequently accumulating in the vacuole. They are called phytoanticipins. The enzymatic activity required for their activation is often induced after the pathogen attack (e.g. plant glycosidase), or may originate from the pathogen itself.
Phytoalexins are not specific means of defense directed towards a specific microbe, but their effect is non-specific and directed against numerous microbes. For example, phytoalexins are also produced during compatible interactions (development of disease), but their synthesis is induced later, stops earlier and their concentration at the site of infection is lower than in incompatible interactions.
Besides their antimicrobial activity, phytoalexins are well known for their health promoting effects in humans with, for example, numerous demonstrated antioxidant, anticancer or anti-inflammatory activities. To mention only one example, resveratrol and its derivatives are well-studied stilbene phytoalexins of grape and peanut.