Phytoalexins, discussion

A better understanding of the role of phytoalexins in plant defenses and of the mechanisms of induced resistance may potentially open a powerful new approach to the control of insect pests of cultivated plants. If indeed, in light of the hypothesis of optimal defense strategies (3), a post-attack response is a more efficient line of defense than the attack-independent accumulation of allelochemics, the exploitation of phytoalexin-producing mechanisms may represent a fertile field for future investigations. Several uses of induced resistance may be conceived. Four of these approaches are briefly discussed. 

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. 

Physical. Anecdotal accounts abound of enhanced plant resistance to disease achieved by transient exposure to a wide variety of physical stimuli, e.g., heat, light, microwaves, other electromagnetic radiation, electric current, sound waves, and vibration. In our own laboratory, we have made cucumbers resistant to anthracnose by vibration (Stromberg and Kuc, unpublished). However, these phenomena are poorly understood and may include enhanced resistance resulting from non-specific altered (stress) physiology, nonspecific phytoalexin elicitation, modification of the action of gene products, or sensitization. This interesting but little explored area will not be further discussed in this paper. 

The Importance and complexity of various aspects of host defense dynamics. Including phenolic substances, phytoalexins, and ligniflcation, have been studied for some time, but their precise roles In disease resistance are still not fully understood, as summarized In the present discussion and many reviews (e.g.,... 

Detailed discussion of these areas Is beyond the scope of this chapter. Phytoalexins and their ellcitors, as related to defense and the development of secondary metabolic products as herbicides are discussed elsewhere In this book (chapter by N. Keen). 

Horace Cutler organized the section on controlling plant diseases. These chapters discuss trichothecenes and their role in the expression of plant disease the role of phytoalexins in resistance of peanuts to... 

Upon attack by pathogenic micro-organisms, higher plants are known to produce antimicrobial compounds as defense substances. These allelochemicals are called phytoalexins. I chose several phytoalexins as my synthetic targets. Synthesis of some of the phytoalexins will be discussed in this section. 

We have described the chemistry of some selected sesquiterpenes which have been classified as phytoallexins. We would to apologize for unintentional omission of any important information in this review, which we tried to make as comprehensive as possible. A variety of synthetic reactions has been used to achieve the total synthesis of these phytoalexin sesquiterpenes. The isolation and biological activity of these sesquiterpenes have been discussed. It is necessary to mention that Takasugi et al.81 have reported for the first time costunolide is an antifungal sesquiterpene which inhibited spore germination of Ceratocystis fimbriata at concentration of 32 pg/ml. Many diterpenes and flavonoides have been also listed as phytoallexins. It is hoped that the research workers would find some useful information in this article. It is safe to predict from discussions in this article that many organic chemists would try to isolate antifungal sesquiterpenes. 

Elicitation of Phytoalexins. Phytoalexins (previously discussed briefly with regard to activities against insects) are generally... 

Another compound bordering between preformed and induced defenses is gossypol. This compound accumulates within epidermal glands on the surface of cotton plants and is very toxic to some insects [ .]. Gossypol, and a series of structurally related compounds on the same biosynthetic pathway, accumulate to substantially increased levels in plants that have been challenged by potential pathogens. For this reason they have been considered phytoalexins by some plant pathologists. Phytoalexins are inducible antibiotics and will be discussed later. 

The purposes of this chapter are to review the factors involved in aflatoxin contamination of peanuts, review the chemistry of stilbene phytoalexins from peanuts, discuss evidence supporting the involvement of these stilbenes in the bioregulation of aflatoxin contamination, and explore approaches to exploit or enhance such a bioregulative capacity to reduce or eliminate preharvest aflatoxin contamination of peanuts. 

A number of dihydrophenanthrenes and phenanthrenes possess antifungal activity and are important as phytoalexins. Although many of these and related compounds are derived from pathways discussed in this chapter, others that coincidentally possess phenanthrene structures are derived from isoflavonoid precursors and are discussed under that topic. 

Nicotiana. The comprehensive study mentioned already above (see Sect. 7.2.1.1) demonstrated also the occurrence of Cj VOCs, i.e., sesquiterpenes e.g., p-caryophyllene (Fig. 7.4), a-cedrene, different famesenes (Fig. 7.4), a-humulene and oxygenated sesquiterpenes (e.g., different cis-trans isomers of nerolidol, far-nesal, and famesol, as well as caryophyllene oxide) (Raguso et al. 2003). Further constitutive sesquiterpenoids from Nicotiana will be discussed below in connection with phytoalexins of this genus. 

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