Phytoalexins processes

Numerous catechols and hydroquinones in both glycoside-masked and -unmasked forms are useful metabolites in plant chemical defense. Many such metabolites are present in concentrations that can prove detrimental due to oxygenation of the tissue accompanying wounding of the plant in the infection process or in other direct physical injury. Some agents are also synthesized subsequent to enzyme induction in association with infection to mediate chemical defense, as in the broad class of defensive substances known as phytoalexins.12 Some of these induced substances are oxidizable polyphenols, while others are not (Figure 8.1). 

They act as antipathogenic agents and thus affect the process of pathogenesis. They may act on the host through the Induction of plant resistance mechanisms such as stimulation of lignification or enhancement of phytoalexin production. (Please refer to the chapter by Salt and Kuc in this volume for further discussion of this type of compound.) They may act on the pathogen to accentuate elicitor release or to prevent infection (host penetration), colonization (inhibition of phytotoxin synthesis, extracellular enzyme production and action, or phytoalexin degradation) or reproduction. 

Immunization of cucumbers by (L lagenarium, C. cucumerinum, P. 1achrymans or TNV generates a systemic increase in peroxidase activities (. TJ, ] 9, 8U) > Like 1 i gni f ic a t ion and phytoalexin induction, peroxidase activities also rise more quickly in response to infection in leaves of immunized plants, even though total activity eventually may be highest in infected susceptible leaves (77). Several other stimuli can induce local (mechanical and chemical injury) or systemic (senescence, ethylene) peroxidase increases that are not accompanied by increased disease resistance. Thus, enhanced peroxidase activity per se may not be a defense mechanism, but may be a necessary adjunct with appropriate chemical substrates for processes important in disease resistance, e.g., lignification, suberization, and me 1anization. 

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. 

We can distinguish between secondary metabolites that are already present prior to an attack or wounding, so-called constitutive compounds, and others that are induced by these processes and made de novo. Inducing agents, which have been termed elicitors by phytopathologists, can be cell wall fragments of microbes, the plant itself, or many other chemical constituents (4,17,22-24). The induced compounds are called phytoalexins, which is merely a functional term, since these compounds often do not differ in structure from constitutive natural products. In another way this term is misleading, since it implies that the induced compound is only active in plant-microbe interactions, whereas in reality it often has multiple functions that include antimicrobial and antiherbivoral properties (see below). 

The reactions leading to the induction and accumulation of phytoalexins with phenolic structures have been studied in molecular detail (4,17,22-24). These studies revealed that plants can detect and react rapidly to environmental problems, such as wounding or infection Within 20 min of elicitation, mRNAs coding for enzymes that catalyze the reactions leading to the respective defense compounds are increasingly generated, leading to the accumulation of the respective enzymes and consequently the production of the secondary metabolites (4,17,22-24). Similar processes are likely for alkaloids, but so far the mechanisms have not been elucidated. 

The addition of a second, reducing end acceptor and a thiophilic promoter (NIS) resulted in trisaccharide formation. Yields as high as 84% for the overall process were reahzed, showcasing the efficiency of this strategy as well as the promoter compatibility, which can be a problem (Fig. 3). The method has been extended to branched structures as well (34). Takahashi has advanced the orthogonal OPG strategy in an impressive synthesis of the heptasaccharide phytoalexin elicitor (HPE) in 24% overall yield (35). 

Albersheim has developed a concept of phytoalexin production being promoted by specific "elicitors" - that is, by molecules produced by certain fungi, which can sometimes be found in fungal culture media (9). Such elicitors have been isolated and their role in the defensive process has been Investigated (10). 

Cline and Albershelm (35) suggested that the primary event in activating the biochemical processes leading to increased concentrations of phytoalexins in plants is the binding of ellcltors to receptors in the plasma membrane. Initial evidence for this was reported by Yoshlkawa et al. 36) when they found that soybean membrane preparations bound HC-laminaran (a 8-1,3-glucan elicitor from Phytophthora spp.). The receptor, while not yet characterized, appears to be a protein or glycoprotein. 

Minimal processing of carrots induces phenylalanine ammonia lyase (PAL) activity and phenolics accumulation. In shredded carrots, chlorogenic acid, which is rapidly accumulated, represents 60% of the total phenolics. In addition traces of 3 -caffeoylquinic and 4 -caffeoylquinic acids are biosynthesised, and 3, 4 -dicaffeoylquinic and 3, 5 -dicaffeoyl quinic acids also accumulate [44]. p-Hydroxybenzoic acid derivatives are also biosynthesised but more slowly, and are related to defence against microbial attack (phytoalexin response), the degree and speed of which depends on the cultivar [45],... 

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