Elicitation of phytoalexins

Fig. (1). Interaction between invading fungi and host plants in the elicitation of phytoalexin production...

The successful elicitation of phytoalexins in plant cell culture also opens the way to large scale production. Plant culture may incidentally provide additional metabolites to those formed in intact plants (42). 

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

Based on the previous examples, the elicitation of phytoalexins and other inducible compounds in plants by various elicitors might be a successful tactic for controlling insect pests on important crops. This elicitation would need to occur only when the insect feeds, to prevent unnecessary damage to the plant by accumulation of compounds toxic to plant cells as well. Based on presently known carbohydrate elicitors, it seems plausible that plant surfaces could be treated in such a way that the elicitor is carried into the plant upon insect feeding [21]. These carbohydrate elicitors should be stable and should pose no environmental threat since they are most likely non-toxic themselves. 

A ROLE FOR CALCIUM IN THE ELICITATION OF PHYTOALEXIN ACCUMULATION IN POTATO21... 

In light of the role of Ca in transduction of external stimulus to cellular response in a wide variety of animal and plant cells, >23 it was reasonable to investigate whether Ca is involved in the elicitation of phytoalexin accumulation. The stimulatory effects of Ca2+ and the inhibitory effects of Ca antagonists on rishitin and lubimin accumulation suggest that Ca mobilization is an essential part of the elicitation of phytoalexin accumulation in potato tuber tissue. 

Co-mediation of oligosaccharides and MJ has been demonstrated in the rice system in the induction of phytoalexins [100]. Exogenously applied MJ to elicited cells increased production of momilactone A to levels higher than those elicited with AT-acetylchitoheptaose alone. In suspension-cultured cells of parsley the influence of MJ on elicitation using cell walls of Phytophtora megasperma (Pmg elicitor) and chitosan was demonstrated [101]. These results suggested MJ conditioned the parsley suspension cells in a time-dependent manner to become more responsive to elicitation. 

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.

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. 

Hormonal action of oligosaccharides (10 examples). Some free oligosaccharides occur in some plants as a response to contamination, and elicit the production of phytoalexines, plant antibiotics against fungi. 

Evaluation of Phytoalexin-Producing Potential. Six g of kernels (x3) from each maturity stage were sliced 1-2 mm thick, distributed in open 60 mm tissue culture dishes, and dusted with spores of a non-aflatoxin producing strain of A. parasiticus (CP 461 SRRC 2043) to elicit phytoalexin production. The open dishes were incubated in the dark at 25 1.0°C for four days in sealed dessicators over unsaturated NaCl solutions of aw corresponding to that determined for each maturity stage. This was to maintain all peanuts at their preharvest aw during the incubation. 

The use of plant tissue culture as a tool for the elicitation and subsequent characterization of phytoalexins has been outlined (Section V. A) however, the technique has been widely... 

Ref. 6) established that potato-cell suspension cultures synthesize and accumulate sesqulterpenold phytoalexins. This ability to elicit terpenoid phytoalexin formation In cell suspension culture should speed the exploitation of tissue culture techniques In the studies of terpenoid phytoalexin biosynthesis. 

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. 

Furze, J. M., Rhodes, M. J. C., Parr, A. J., Robins, R. J., Withehead, I. M., and Threlfall, D. R. (1991) Abiotic factors elicit sesquiterpenoid phytoalexin production but not alkaloid production in transformed root cultures of Datura stramonium. Plant Cell Reports. 10, 111-114. 

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). 

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