Phytoalexin production

The term elicitor, initially defined as a fungal metabolite capable of inducing phytoalexin production when applied to host plants (122, 123), has since been applied to parasite-derived molecules which induce any facet of resistance in appropriate host plants, including lignification (124). 

De Novo Synthesis of Phytoalexins. Phytoalexins have been studied in great depth by plant pathologists. Excellent review papers are available in Hedin s ACS symposium volume. Host Plant Resistance to Plants (42), and more recently in the book edited by Horsfall and Cowling, Plant Disease (43). The antiherbivory effect of phytoalexins, however, is only now beginning to be fully appreciated. It is apparent that pathogen induced phytoalexins do have a definite effect on insect herbivores. There is mounting evidence that herbivore-inflicted injury may also result in the induction of phytoalexin production and accumulation. 

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

TRANSMEMBRANE SIGNALING MECHANISMS IN STIMULATION OF PHYTOALEXIN PRODUCTION... 

One aspect shared with several other genes of the phenylpropanoid pathway is the transient induction after environmental challenge. This has also been demonstrated for chalcone synthase (Ryder et al., 1984) and chalcone isomerase (Cramer et al., 1985 Mehdy Lamb, 1987), enzymes involved in phytoalexin production, and for cinnamyl alcohol dehydrogenase (CAD) an enzyme of lignin biosynthesis, in response to elicitor treatment of bean tissue culture cells (Grand et al., 1987). 

Cartwright, D. Langcake, P. Ride, J.P. (1980) Phytoalexin Production in Rice and its Enhancement by a Dichlorocyclopropane Fungicide. Physiological Plant Pathology 17, 259-267. 

Nakazato, Y., Tamogami, S., Kawai, H., Hasegawa, M., Kodama, O. Methionine-induced phytoalexin production in rice leaves. Biosci Biotech Biochem 2000 64 577-583. 

Tamogami, S., Rakwal, R., Kodama, O. Phytoalexin production by amino acid conjugates of jasmonic ads through induction of naringenin-7-o-methyltransferase, a key enzyme on phytoalexin biosynthesis in rice (Oryzja sativa L.). FEBS Lett 1997a 401 239-242. 

Keen, N. T. 1975. Specific elicitors of plant phytoalexin production determinants of rice specificity in pathogens Science 187, 74-75... 

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. 

The compound for which the best biochemical evidence has been reported for sensitization of host plant response to pathogens, rather than direct or indirect fungitoxicity or nonspecific phytoalexin induction, is 2,2-d ich 1 oro-3,3-d ime thy lcyc 1 opropane carboxylic acid (108-110). Neither the compound nor any of its metabolites generated after treatment of rice plants are directly inhibitory to the rice blast fungus, Piricularia oryzae. Constitutive phytoalexin production was not induced in rice by the cyclopropane derivative. However, infection of plants treated with the compound results in rapid localized cell death, me lanization, and production of the phytoalexins, momilactones A and B. 

Figure 25. Schematic representation of phytoalexin production following exposure of the plant cell wall to a pathogen. E represents plant cell wall and pathogen enzymes M, plant cell messengers.

NADPH oxidase. This defense mechanism has not received much attention, but has been implicated in some host-pathogen interactions. NADPH oxidase in potato tuber tissue was shown to be activated immediately after invasion by an incompatible race of PhvtoDhthora infestans. causing simultaneous superoxide production, hypersensitive ce11 death, and phytoalexin production (70-72). This enzyme system was not activated by a compatible . infestans race... 

Metabolites have been isolated from pathogens that suppress phytoalexin production by a host plant In the presence of otherwise functional ellcltors (e.g. ). The elaboration of such suppressors... 

Phytoalexin production is not an immediate response of plants to attack of pathogens or mechanical injury. Phytoalexins begin to appear about 6 hours after attack by microorganisms [95] and also exposure to light [96] and accumulate to maximum levels 20-30 hours later [96]. In contrast 10 min after treatment with UV radiation phytoalexin production was observed [97], indicating that the phytoalexin generation is dependent on the kind of elicitor and probably on the extend of... 

In addition to possible phytoalexin production it appears that certain flavonoids and furanocoumarins inhibit the production of trichothecenes. Strong preliminary evidence indicates that when Fusarium sporotrichioides is incubated in the presence of flavones or furanocoumarins the production of... 

The first report of phytoalexin production in peanuts appeared in 1972 when Vidhyasekaran ejt al. (2) reported that they detected phytoalexin production in response to invasion by storage fungi. It was observed that phytoalexin was produced to a greater extent in immature pods and to a lesser extent in mature pods. The chemical nature of these phytoalexins was not determined in this study. 

Recent studies by Wotton and Strange (8) provided circumstantial evidence for phytoalexin involvement in the resistance of peanuts to Aspergillus flavus. Their results indicated that resistance of peanut kernels to invasion by A. flavus was correlated with their capacity to synthesize phytoalexins as an early response to wounding. Also, conditions that promoted invasion of peanuts by A. flavus inhibited phytoalexin production. Thus, kernels of drought stressed plants, which are more susceptible to A. flavus than kernels of nondrought stressed plants, produced less phytoalexin in response to wounding by slicing than kernels from non-stressed plants. 

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. 

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