Pathogens, phytoalexin elicitation

The most studied aspect of phytoalexin elicitation is that generated by fungal pathogens of plants. Karban et al.[22] found induced resistance and interspecific competition between spider mites and a vascular wilt fungus. McIntyre et al. [2A] demonstrated that 7 days after tobacco plants were infected with tobacco mosaic virus, reproduction of the green peach aphid, Myzus persicae, was significantly reduced on these infected plants. Inoculation of plants with tobacco mosaic virus induced resistance to several pathogens, however the mechanism for induced resistance was not characterized. 

Figure 10.5 Plant cell cultures have proven to be very useful for studying plant-pathogen interactions and isoprenoid metabolism. Tobacco cell cultures respond rapidly to the addition of fungal elicitors (0.5 pg cellulase/ml of culture) by browning (A) (analogous to a hypersensitive response) and the production of phytoalexins (B). Media was collected from elicited cell cultures at the indicated times, partitioned against an organic solvent, and concentrated aliquots run on a silica TLC plate. The plates were then sprayed with a suspension of Cladosporium cucumerinum spores and incubated in a humid environment for 5 days before viewing (B). The compound released from the elicitor-treated tobacco cells that inhibits spore germination is capsidiol, a sesquiterpene.

This phytoalexin helps the plant resist attack by gray mold (Botrytis cinerea). Another possibility is to use a phytoalexin made by a chemical synthesis apart from the plant. Novartis sells a benzothiadiazole (Actigard) that elicits systemic resistance to bacterial and fungal pathogens.189... 

Several naturally occurring indoles possessing sulfur at C-2 have been isolated, such as the phytoalexin sinalexin (11), which was identified as a product formed in white mustard Sinapis alba) under elicitation by e.g. the blackspot fungal pathogen Allemaria brassicae <97P(46)833>. A recent synthetic study provided the first route to 11, also including an... 

A general term for compounds whose biosynthesis is elicited or induced in plants as the result of attack by a pest is "phytoalexin". These higher plant metabolites are antibiotic to certain potential plant pathogens and also on occasion to insects (Chapter 13 of this volume). The phytoalexins typically are biosynthesized in greater concentrations when the plant is subjected to stress. Therefore, the attacking agent (fungi, bacteria, or viruses in most work to date) elicits the initiation or increased synthesis of phytoalexins (antibiotic compounds). 

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. 

Hahn, M. G., A. G. Darvill, and P. Albersheim, Host-pathogen interactions. XIX. The endogenous elicitor, a fragment of a plant cell wall polysaccharide that elicits phytoalexin accumulation in soybeans. Plant Physiol., 68, 1161-1169 (1981). 

A common response of plants, often associated with disease resistance, is the localized and rapid death of plant cells at the infection site. This reaction, referred to as a hypersensitive response (HR), Includes the accumulation of phytoalexins (low molecular weight compounds which Inhibit microbial development). In some plant-parasite interactions, different races of a pathogen can be distinguished by the type of response they elicit in different cultivars. Incompatible races of Phytophthora infestans elicit the accumulation of sesquiterpenoid phytoalexins, principally rishitin and lubimin, in tubers of resistant (R-gene) cultivars faster to higher levels following infection than do... 

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