Elicitors of phytoalexins

Paxton, J. D., 1988. Fungal elicitors of phytoalexins and their potential use in agriculture. In Cutler, H. G., (Ed.), Biologically Active Natural Products. ACS Symposium Series 380, ACS Books, Washington, DC, 109-119... 

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. 

Glucan Elicitors of Phytoalexin Production. Phytoalexins are post-infection toxins formed de novo by the host. 

The stem and root-infecting pathogen of soyhcdja, Phytophthora megasperma, excretes glycoproteins. They have proved to be poor, non-specific elicitors of phytoalexin accumulation. 

It has recently been shown that attack by insects and nematodes also can elicit the formation of phytoalexins. A number of chemical elicitors of phytoalexins have also been identified (11, 12, 22, 23). 

Sharp et al. (1) reported that the partial hydrolysis of mycelial walls of PhvtoDhthora megasnerma f. sp. glvcinea produced a mixture of soluble substances containing elicitors of phytoalexin production in soybean cotyledons. Reduction of a hepta glucoside fraction with NaBH, followed by purification and structural characterization, showed the presence of one active and seven... 

Scheme 4.21 Mukaiyama s one-pot synthesis of phytoalexin-elicitor active heptasaccharide.

A recent search for general and specific elicitors from L. maculans demonstrated that the phytotoxins sirodesmin PL (1) and deacetylsirodesmin PL (2) are general elicitors since both induced the production of phytoalexins in resistant brown mustard and in susceptible canola [31]. Furthermore, two specific elicitors, a mixture of cerebrosides C (13) and D (14), were reported from mycelia of liquid cultures of L. maculans virulent on canola (Fig. 9.5) [19]. Previously, cerebrosides C (13) and D(14) were reported from a number of phytopathogenic fungi and were reported to induce the production of phytoalexins in rice plants and disease resistance to the rice blast fungus [32]. 

Ryder, T.B., Cramer, C.L., Bell, J.N., Robbins, M.P., Dixon, R.A. Lamb, C.J. (1984). Elicitor rapidly induces chalcone synthase mRNA in Phaseolus vulgaris cells at the onset of phytoalexin defense response. Proceedings of the National Academy of Sciences (USA) 81, 5724-8. 

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

Secondly, the deacetylated form of chitin, chitosan, does not induce phytoalexin formation in the rice system but is active in other plant culture systems [99]. Glucan elicitors induce phytoalexins in legumes (soybean, chickpea, bean, alfalfa, pea) and solanaceous sp. (potato, sweet pepper) [100]. However, anthraquinone biosynthesis was stimulated in Morinda citrifolia by both chitin and chitosan. The degree of acetylation was found to be important in inducing defense responses. During the first few days of incubation after adding elicitor,... 

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. 

HAGMANN, M.-L., HELLER, W., GRISEBACH, H Induction of phytoalexin synthesis in soybean. Stereospecific 3,9-dihydroxypterocarpan 6a-hydroxylase from elicitor-induced soybean cell cultures. Eur. J.Biochem., 1984,142,127-131. 

ZAHRINGER, U EBEL, J., MULHEIRN, L.J., LYNE, R.L., GRISEBACH, H., Induction of phytoalexin synthesis in soybean - dimethylallyl pyrophosphate trihydroxypterocarpan dimethylallyltransferase from elicitor-induced cotyledons. FEBSLett., 1979,101,90-92. 

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.

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. 

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