Elicitor compounds

The processes connected with the hypersensitive response give rise to a localized necrosis which may be regarded as a special kind of programmed cell death. In this way the spreading of the pathogen to other parts of the host plant is prevented. The hypersensitive response is initiated by a number of compounds which are collectively designated as elicitors. Compounds of different chemical classes such as proteins, carbohydrates, lipids and certain polyenoic fatty acids have been identified as elicitors. 

Fig. 4. Comparison of the two signal-reaction chains leading either to the UV light-induced formation of flavonoids or to the elicitor-induced formation of furanocoumarins and related compounds with antimicrobial activity. From Hahlbrock et al. (1985). PR -proteins are pathogenesis-related proteins.

A four-year study of field-grown commercial carrot roots revealed that recently harvested, unprocessed carrot roots contained 24 ppm falcarinol and 65 ppm falcarindiol (8). 6-Methoxymellein (6-MM) had not been identified by Yates al (8) at that time, and was not measured in that study. Reexamination of data revealed that 6-MM was absent from most samples, but present in a few at concentrations of 2 to 8 ppm. Myristicin, 1 ppm, was detected in only one sample. Wulf et 1978, reported that myristicin was present in supermarket carrots. Other studies have shown that certain brands of supermarket carrots contain myristicin while others do not (Yates, unpub.). The presence of myristicin in some samples from the supermarket and its absence in unprocessed carrots analyzed as soon after harvest as possible suggests that myristicin formation is induced during some stage of processing. Since light is known to be an elicitor of a plant system that results in the synthesis of phenylpropanoid compounds, a study of the effect of light on harvested carrot roots was undertaken. 

Compounds (I) through (IV) were assayed for their cytotoxic and allergenic potential on guinea pigs, which are effective indicators of contact allergenicity in humans (10). The same compounds were also tested on Tenebrlo sp. (mealworm beetles), an experimental Insect used to test the potential of insecticides. Geranyl-benzoquinone proved to be a very potent elicitor of allergic skin reactions as well as a potent insecticide (10). 

Common elicitors like JA and SA and knowledge about the biochemical pathways that they induce are used to identify the plant genes that are involved in the induction process (Reymond et al., 2000). Various genes that are induced by JA and related compounds have been identified (Reymond and Farmer, 1998 Stinzi etal.,... 

Based on 13C-NMR analyses, it has been assumed [62] that 6-methoxymellein, a polyketide compound, is synthesized by head-to-tail condensation of one acetyl-CoA and four malonyl-CoA to form 3,4-dehydro-6-hydroxymellein as an intermediate. This isocoumarin derivative is then reduced to its dihydro-form, 6-hydroxymellein, which then accepts a methyl-unit via Omethyltransferase. We have shown [63] that cell extracts prepared from elicitor-treated carrot root disks catalyzes... 

Randhir R, Lin Y-T, Shetty K. 2004. Stimulation of phenolic compounds, antioxidant and antimicrobial activities in dark germinated mung bean sprouts in response to peptide and phytochemical elicitors. Process Biochem 39 637-646. 

These two constitutive antifungal compounds were exhibited in both Chandler and Sweet Charlie plants but Sweet Charlie plants produced approximately 15 times more antifungal activity than Chandler plants. Fungal growth inhibition associated with extracts from Chandler plants appeared to be temporary. A third compound, detected exclusively in Sweet Charlie plants, was produced only after young leaves were sprayed with a commercially available elicitor of antifungal compounds (Vincent et al., 1999). 

On the other hand, the impact of ethylene in the composition of headspace gases fed to Taxus sp. cultures was very evident. The positive involvement of ethylene was also noted when cultures were elicited with both dissolved and volatilized methyl jasmonate and with chitin- and chitosan-derived oligosaccharides. The effect of the latter compounds on biosynthesis of ethylene by the plant cell cultures brings us to a better understanding of the interdependence of elicitor and hormone concentrations and of cross-talk signal transduction in plants. 

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.

To date, progress achieved clearly demonstrates the potential of cultured plant cells for secondary metabolite production. Use of concurrent immobilization/permeabilization procedures, as well as precursor and elicitor treatments, may open new avenues of increasing product yields and will consequently affect the economic aspects of plant cell culture in a positive manner. However, our understanding of the many biosynthetic pathways of desired secondary metabolites is incomplete and successful industrial scale plant cell culture processes are still limited. Results of research in the area of plant cell culture will increase our understanding of the biosynthesis of plant metabolites, enhance our knowledge of plant-microorganism or plant-plant interactions and can lead to entirely new products or product lines of desirable compounds currently not available to use. Such work can also lead to development of industrial scale production processes for products now produced and recovered by conventional methods. Also, the genetic variety of the 250,000 to 750,000 plant species available remains to be explored. Presently only 5 to 15% of these species have been subject to even... 

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