Phenolic compounds which do not exist in the plant but are formed as a result of interactions between the host and the pathogen are called phytoalexin.
The literal meaning of the word “alexin” is “to ward off”. Therefore, substances produced as a result of host-pathogen interactions that could ward off the pathogens were called phytoalexins.
Phytoalexin may be defined as “a substance which inhibits the development of a fungus on hypersensitive tissue, formed mainly when host plant cells come in contact with the parasite” (Agrios, 1978).
The term phytoalexin was coined by Muller and Borger in 1940 while studying the “late blight” of potato (by Phytophthora infestans). they found that a toxic chemical compound, a phytoalexin, was produced during necrobiosis by the living host cells as a result of interaction between the metabolic system of the host and the fungal parasite.
Phytoalexins are produced in certain amounts in plants only after stimulation by fungi and other micro-organisms or chemical and mechanical injury, and inhibit the growth of plant-pathogenic micro-organisms. Phytoalexins have now been detected even in healthy and unstimulated tissues. They are more toxic to non-pathogens than to the pathogen.
Types of Phytoalexins
Nowadays, photoalexins are defined as antibiotics produced in host-pathogen interactions or as a response to injury or other physiological stimuli.
Several such toxic substances i.e. phytoalexins have been isolated, some of which are: ipomeamarone from Ipomoea batatas, orchinal from Orchis militaris, pisatin from Pisum sativum, isocoummarin from Daucus carota var. sativa, phaseolin from Phaseolus vulgaris, rishitin and lyubimin from cultivated potato, etc.
Phytoalexins are produced in lower concentrations by pathogenic fungi and other micro-organisms than non-pathogenic.
Besides, pathogenic fungi and other micro-organisms are less sensitive to the toxicity of the phytoalexin produced by their host-plant than non-pathogonic micro-organisms.
Phytoalexins are broad-based antifungal compounds. however bacteriostatic property has also been detected in a few cases.
Phytoalexins are not determined by stimulants, moreover their production is the characteristic property of hosts. The biosynthesis of phytoalexin is influenced by several factors, such as age of the host, inoculum concentration, environmental temperature, and aeration, etc.
In nature, the micro-organisms serve as the main inducers while in the laboratory some chemicals (heavy metal ions, some amino acids, metabolic inhibitors, etc.) induce the phytoalexin synthesis.
In general, phytoalexins are not translocated and rarely persistent. Phytoalexins accumulate more rapidly and in greater amounts in resistant than in susceptible reactions. The role of phytoalexin as a protectant is doubtful.