Phytoalexin response stimulation

Elicitors of the Phytoalexin Response. The production of phytoalexins can be elicited not only by living organisms, but also by many chemical compounds and stress situations (27,46,47). Hg" (48) and Cu (49) ions, fungicides (50,51), and polyamines (52) are some of the different chemical elicitors of phytoalexins. Physical stresses such as cold (53) and UV light (54,55) also stimulate the phytoalexin response. Even though these different stresses may act at different sites and in different ways, the result is always a dramatic change in metabolism of the susceptible plants. 

In Tabernaemontana divaricata treatment of plant cell suspension cultures with an elicitor cause inhibition of CS activity [24,25]. This response is accompanied by stimulation of activity of constitutive enzyme activities of the isoprenoid pathway leading to 2,3-oxidosqualene (squalene synthase and squalene oxidase), and induction of enzymes required for biosynthesis of pentacyclic triterpenoid phytoalexins (/lAS and aAS). Thus inhibition of the branchpoint enzyme CS results in increased flux through the triterpenoid pathway. 

Secondly, the deacetylated form of chitin, chitosan, does not induce phytoalexin formation in the rice system but is active in other plant culture systems [99]. Glucan elicitors induce phytoalexins in legumes (soybean, chickpea, bean, alfalfa, pea) and solanaceous sp. (potato, sweet pepper) [100]. However, anthraquinone biosynthesis was stimulated in Morinda citrifolia by both chitin and chitosan. The degree of acetylation was found to be important in inducing defense responses. During the first few days of incubation after adding elicitor,... 

Chemical factors are also involved in the resistance of plants to disease and in the competitive ability of a plant to survive within a community of plants. Plant stress may also generate a chemical response giving rise to compounds known as the phytoalexins, the nature of which will depend on the chemistry of the host plant (18, 19). Such response to injury or infection is of great Interest because it has stimulated investigations of the nature of the bloregulatory processes involved. 

These concepts of information transduction by membranes may be used to integrate the scattered observations on the biological effects of interactions of 3,6-3-glticans with plant cells. The observation that chemically different groups of compounds may elicit the production of the same response such as wilting or formation of a phytoalexin, may be explained in terms of multiple receptors coupled to a single effector system which initiates the intracellular response. This would be analogous to the adenyl cyclase of the adipocyte which is stimulated by seven different hormones and inhibited by at least three others (124). In the... 

In 1975 Keen (21) reported that native microflora stimulated production of two antifungal compounds by peanut seeds that were soaked in water, sliced into sections, and incubated for 3-5 days. These compounds were judged to be phytoalexins and were subsequently identified as cis- and trans-isomers of 4-(3-methyl-but-2-enyl)-3,5,4 -trihydroxystilbene [1)(5)(Figure 1). Simultaneously, Ingham (6) reported the isolation of cis- and trans-resveratrol(3,5,4 trihydroxy stilbene[2] ) from peanut hypocotyls. Additional stilbenes have been shown to be produced by peanut seeds in response to wounding, and these include 4- (3-methyl-but-1-enyl) -3,5,3, 4 -tetra-hydroxystilbene [3](7), 4-(3-methyl-but-l-enyl)-3,5,4 -trihydroxy-... 

The production of phytoalexins in response to microbes requires a mechanism which enables the plant to recognize the invading organism and subsequently leads to the stimulation of phytoalexin synthesis at the site of microbial penetration. Substances of microbial origin which mediate phytoalexin accumulation in plants have been called elicitors(Ref. 2) and it has been shown in several cases that the culture medium of pathogens contained components which elicited phytoalexin accumulation in the pathogen s host (Ref. 3). Studies on the nature of elicitors have resulted in the partial purification and characterization of a number of molecules from different microbial sources. Elicitors have been reported to have a dependence for activity on a polypeptide or protein (Ref. 4), a glycoprotein (Ref. 5), or polysaccharide (Ref. 6) component. 

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