Secondary Metabolites: Killing Pathogens


Plants produce many antimicrobial secondary metabolites. Two major classes with a demonstrated or proposed role in resistance to plant pathogens are phytoanticipins, or preformed inhibitors, which are present constitutively in plants, and phytoalexins, which are synthesized only in response to pathogen attack. Available evidence is consistent with both phytoanticipins and phytoalexins being important in defence in some disease interactions.

Keywords: phytoalexin; phytoanticipin; plant disease; inducible defence; elicitor

Figure 1.

Structures of three phytoanticipins. Avenacin A‐1 is a triterpenoid saponin from oat (Avena sativa). It is biologically active in its native state and is detoxified by some fungi by hydrolysis of the terminal glucose residues. DIMBOA glucoside is a cyclic hydroxamic acid (benzoxazinone) found in maize (Zea mays), wheat (Triticum aestivum), and rye (Secale cereale). Upon hydrolysis of the glucoside, the compound undergoes spontaneous ring contraction to form the more toxic benzoxazolinone. Dhurrin is a cyanogic glucoside made by sorghum (Sorghum vulgare). Upon hydrolysis of the glucoside, the resulting hydroxynitrile undergoes spontaneous or enzymatic conversion to produce hydrogen cyanide. Some fungi can detoxify cyanide by conversion to formamide.

Figure 2.

Structures of six characteristic phytoalexins. Pisatin is an isoflavonoid of the pterocarpan class produced by pea (Pisum sativum). Rishitin is a bicyclic sesquiterpene made by potato (Solanum tuberosum), and gossypol is a dimeric sesquiterpene phytoalexin made by cotton (Gossypium hirsutum). Wyerone acid is an acetylene produced by Vicia faba. Resveratrol is a stilbene found in several plants including peanut (Arachis hypogaea) and grapevine (Vitis vinifera). Camalexin is a tryptophan derivative found in Arabidopsis thaliana and other species in the Brassicaceae.



Bak S, Kahn RA, Nielsen HL, Moller BL and Halkier BA (1998) Cloning of three A‐type cytochromes p450, CYP71E1, CYP98, and CYP99 from Sorghum bicolor (L.) Moench by a PCR approach and identification by expression in Escherichia coli of CYP71E1 as a multifunctional cytochrome p450 in the biosynthesis of the cyanogenic glucoside dhurrin. Plant Molecular Biology 36: 393–405.

Bell JN, Dixon RA, Bailey JA, Rowell PM and Lamb CJ (1984) Differential induction of chalcone synthase mRNA activity at the onset of phytoalexin accumulation in compatible and incompatible plant–pathogen interactions. Proceedings of the National Academy of Sciences of the USA 81: 3384–3388.

Bowyer P, Clarke BR, Lunness P, Daniels MJ and Osbourn AE (1995) Host range of a plant pathogenic fungus determined by a saponin detoxifying enzyme. Science 267: 371–374.

Friebe A, Vilich V, Hennig L, Kluge M and Sicker D (1998) Detoxification of benzoxazolinone allelochemicals from wheat by Gaeumannomyces graminis var. tritici, G. graminis var. graminis, G. graminis var. avenae, and Fusarium culmorum. Applied and Environmental Microbiology 64: 2386–2391.

Glazebrook J and Ausubel FM (1994) Isolation of phytoalexin‐deficient mutants of Arabidopsis thaliana and characterization of their interactions with bacterial pathogens. Proceedings of the National Academy of Sciences of the USA 91: 8955–8959.

Graham TL, Kim JE and Graham MY (1990) Role of constitutive isoflavone conjugates in the accumulation of glyceollin in soybean infected with Phytophthora megasperma. Molecular Plant‐Microbe Interactions 3: 157–166.

Hain R, Reif H‐J, Krause E et al. (1993) Disease resistance results from foreign phytoalexin expression in a novel plant. Nature 361: 153–156.

Pueppke SG and VanEtten HD (1974) Pisatin accumulation and lesion development in peas infected with Aphanomyces euteiches, Fusarium solani f. sp. pisi, or Rhizoctonia solani. Phytopathology 64: 1433–1440.

Snyder B and Nicholson RL (1990) Synthesis of phytoalexins in sorghum as a site‐specific response to fungal ingress. Science 248: 1637–1639.

Wassmann CC and VanEtten HD (1996) Transformation‐mediated chromosome loss and disruption of a gene for pisatin demethylase decrease the virulence of Nectria haematococca on pea. Molecular Plant–Microbe Interactions 9: 793–803.

Yoshikawa M, Yamauchi K and Masago H (1978) Glyceollin: its role in restricting fungal growth in resistant soybean hypocotyls infected with Phytophthora megasperma var. sojae. Physiological Plant Pathology 12: 73–82.

Further Reading

Bailey JA and Mansfield JW (eds) (1982) Phytoalexins. New York: Wiley & Sons.

Kuc J (1995) Phytoalexins, stress metabolism, and disease resistance in plants. Annual Review of Phytopathology 33: 275–297.

Osbourn AE (1996) Preformed antimicrobial compounds and plant defense against fungal attack. Plant Cell 8: 1821–1831.

Smith CJ (1996) Accumulation of phytoalexins: defense mechanism and stimulus response system. New Phytologist 132: 1–45.

VanEtten HD, Matthews DE and Matthews PS (1989) Phytoalexin detoxification: importance for pathogenicity and practical implications. Annual Review of Phytopathology 27: 143–164.

VanEtten HD, Sandrock RW, Wasmann CC et al. (1995) Detoxification of phytoanticipins and phytoalexins by phytopathogenic fungi. Canadian Journal of Botany 73 (supplement): S518–S525.

Zhu Q, DrogeLaser W, Dixon RA and Lamb C (1996) Transcriptional activation of plant defense genes. Current Opinions in Genetics and Development 6: 624–630.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Walton, Jonathan D(Apr 2001) Secondary Metabolites: Killing Pathogens. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0000917]