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Host Selective Toxins

Host selective toxins (HSTs) are positive agents of virulence produced by plant pathogenic fungi. Approximately 20 HSTs have been documented. Most of the HSTs are secondary metabolites, but in recent years a number of host-selective proteins (i.e., direct gene products) have been discovered.

HSTs were critical factors in two major epidemics of crop plants in the US, including the Southern corn leaf blight epidemic of 1970 that destroyed ~15% of the crop.

Studies of diseases involving HSTs led to the first elucidation of the molecular basis of disease susceptibility in any interaction (Dewey et al., 1988, Science 239:293) and the first cloning and functional characterization of a Mendelian disease resistance gene (Johal and Briggs, 1992, Science 258:985; Meeley et al., 1992, Plant Cell 4:71).

The study of HSTs continues to contribute fundamental knowledge about the processes and regulation of disease susceptibility and resistance, basic plant biochemistry through their use as specific metabolic inhibitors, the structure and organization of secondary metabolite pathways, and the organization of fungal genomes and the evolution of new pathogen races.


Cochliobolus carbonum and HC-toxin

Cochliobolus carbonum Tox2+ infecting maize of genotype hm1/hm1 in the field.

Fig. 1. Cochliobolus carbonum Tox2+ infecting maize of genotype hm1/hm1 in the field.

C. carbonum is a pathogen of maize (Fig. 1). It has two mating types and is crossable and transformable in the laboratory. Transforming DNA integrates at its homologous location ~80% of the time, allowing the rapid construction of specific mutants of any cloned gene of interest.

"Tox2+" (also known as race 1) isolates of C. carbonum produce a cyclic tetrapeptide called HC-toxin (Fig. 2). HC-toxin was discovered by Bob Scheffer, former professor of plant pathology at MSU (Fig. 3). Maize plants that are homozygous recessive at the nuclear Hm1 locus are sensitive to HC-toxin and hence susceptible to Tox2+ isolates of C. carbonum(Fig. 1).

Hm1 encodes HC-toxin reductase, which detoxifies HC-toxin by reducing the 8-carbonyl group of the Aeo sidechain (Fig. 2).

HC-toxin biosynthesis requires at least seven genes distributed over ~600 kb of DNA. These genes areHTS1, TOXA, TOXC, TOXD, TOXE, TOXF, andTOXG.

Structure of HC-toxin

Fig. 2. Structure of HC-toxin

Most of these genes are duplicated in every strain that we have studied. To date, no simiilarly complex fungal secondary metabolite gene cluster has been described in any other fungus, except the HC-toxin cluster in Alternaria jesenskae (Wight and Walton, unpublished results). HTS1is a 16-kb open reading frame with no introns. It encodes a 570-kDa non-ribosomal peptide synthetase with four 'domains', one for activation of each amino acid in HC-toxin. Based on our biochemical studies, domain 1 activates and epimerizes L-proline, and domains 2, 3, and 4 activate L-Ala, D-Ala, and L-Aeo, respectively. TOXGencodes an alanine racemase responsible for making the D-Ala of HC-toxin. In its absence,C. carbonum is still quite pathogenic because it can still produce a minor form of HC-toxin that has glycine in place of D-alanine (Fig. 3) (Cheng and Walton, 2000).

The site of action of HC-toxin is histone deacetylase (HDAC). For information on our research on HDACs, go to this page.

Robert Scheffer (1920-1996), former professor of Botany and Plant Pathology at MSU, discovered HC-toxin in 1965.

Fig. 3. Robert Scheffer (1920-1996), former professor of Botany and Plant Pathology at MSU, discovered HC-toxin in 1965.

Structure of Non-Ribosomal Peptide Synthetases

Domain organization of representative nonribosomal peptide synthetases (NRPSs).

Fig. 4. Domain organization of representative nonribosomal peptide synthetases (NRPSs).

NRPSs are large, modular enzymes (Fig. 4). HTS1 is a 570-kDa polypeptide and cyclosporin synthetase is a ~1.5 MDa polypeptide. Each domain is responsible for activating one of the constituent amino acids and catalyzing peptide bond formation. The order of domains is the same as the the order of amino acids in the peptide product. Some NRPSs also have domains also catalyze epimerization and N-methylation. (ACV synthetase is the first committed step in penicillin biosynthesis.) More about HC-toxin biosynthesis and non-ribosomal peptides.


Walton, J.D. (2006) HC-toxin. Phytochemistry 67:1406-1413.

Walton JD (1996) Host-selective toxins: agents of compatibility. Plant Cell 8: 1723-1733.

Walton, J.D. (1990) Peptide phytotoxins from plant pathogenic fungi. In: H. Kleinkauf and H. von Döhren, eds,Biochemistry of Peptide Antibiotics. de Gruyter, Berlin,pp. 179-203.

Walton, J.D. and D.G. Panaccione (1993) Host-selective toxins: perspectives and progress. Annu. Rev. Phytopathol. 31:275-303.

Walton JD, CR Bronson, DG Panaccione, EJ Braun, K Akimitsu (1995) Cochliobolus. In: Pathogenesis and Host Specificity in Plant Diseases, Volume II: Eukaryotes, pp. 65-81, K. Kohmoto, U.S. Singh, and R.P. Singh, eds., Pergamon (Elsevier).

Scheffer RP, JD Walton (1995) Toxin-producing Cochlioboli as model pathogens of plants: ecological, evolutionary, and genetic considerations. In: J. Chekowski, ed., Helminthosporia: metabolites, biology, plant diseases, Institute of Plant Genetics, Polish Academy of Sciences, Pozna, pp. 61-87.

Structure of HC-toxin:

Walton J.D., E.D. Earle, and B.W. Gibson (1982) Purification and structure of the host-specific toxin fromHelminthosporium carbonum race 1. Biochem. Biophys. Res. Comm.107:785-794.

Walton, J.D. and E.D. Earle (1983) The epoxide in HC-toxin is required for activity against susceptible maize.Physiol. Plant Path. 22:371-376.

Kawai M., D.H. Rich, and J.D. Walton (1983) The structure and conformation of HC-toxin. Biochem. Biophys. Res. Comm. 111:398-403.

Basis of Specificity:

Meeley, R.B. and J.D. Walton (1991) Enzymatic detoxification by maize of the host-selective toxin HC-toxin.Plant Physiol. 97:1080-1086.

Meeley, R.B., G.S. Johal, S.P. Briggs and J.D. Walton (1992) A biochemical phenotype for a disease resistance gene of maize. Plant Cell 4:71-77.

Meeley, R.B. and J.D. Walton (1993) Molecular biology and biochemistry of Hm1, a maize gene for fungal resistance. In: E.W. Nester and D.P.S. Verma, eds., Advances in Molecular Genetics of Plant-Microbe Interactions, Vol. 2, pp. 463-475, Kluwer Academic, Dordrecht.

Mode of Action of HST's:

Walton J.D., E.D. Earle, O.C. Yoder and R.M. Spanswick (1979) Reduction of adenosine triphosphate levels in susceptible maize mesophyll protoplasts by Helminthosporium maydis race T toxin. Plant Physiol. 63:806-810.

Walton, J.D., E.D. Earle, H. Stahelin, A. Grieder, A. Hirota and A. Suzuki (1985) Reciprocal biological activities of the cyclic tetrapeptides chlamydocin and HC-toxin. Experientia 41:348-350.

Walton, J.D. and E.D. Earle (1985) Stimulation by victorin of extracellular polysaccharide synthesis in oat mesophyll protoplasts. Planta 165:407-415.

Akimitsu, K., L.P. Hart and J.D. Walton. (1993) Immunological evidence for a cell surface receptor of victorin using anti-victorin anti-idiotypic polyclonal antibodies. Mol. Plant Microbe Interact. 6:429-433.

Akimitsu, K., L.P Hart and J.D. Walton (1993) Density gradient study of victorin-binding proteins in oat (Avena sativa) cells. Plant Physiol. 103:67-72.

Akimitsu K., L.P. Hart, J.D. Walton, and R. Hollingsworth (1992) Covalent binding sites of victorin detected by anti-victorin polyclonal antibodies. Plant Physiol. 98:121-126.

Brosch, G., R. Ransom, T. Lechner, J.D. Walton, and P. Loidl (1995) Inhibition of maize histone deacetylase by HC-toxin, the host-selective toxin of Cochliobolus carbonum. Plant Cell 7:1941-1950.

Ransom, R.F., and J.D. Walton. (1997) Histone hyperacetylation in maize in response to treatment with HC-toxin or infection by Cochliobolus carbonumPlant Physiol.115:1021-1027.

Lechner, T., A. Lusser, A. Pipal, G. Brosch, A. Loidl, M. Goralik-Schramel, R. Sendra, S. Wegener, J. D. Walton, and P. Loidl (2000) RPD3-type histone deacetylases in maize embryos. Biochemistry 39:1683-1692.

Brosch, G., M. Dangl, S. Graessle, A. Loidl, P. Trojer, E.-M. Brandtner, K. Mair, J.D. Walton, D. Baidyaroy, and P. Loidl (2001) An inhibitor-resistant histone deacetylase in the plant pathogenic fungus Cochliobolus carbonum.Biochemistry 40:12855-12863.


Walton, J.D. (1987) Two enzymes involved in biosynthesis of the host-selective phytotoxin HC-toxin. Proc. Natl. Acad. Sci. USA 84: 8444-8447.

Walton, J.D. and F.R. Holden (1988) Properties of two enzymes involved in the biosynthesis of the fungal pathogenicity factor HC-toxin. Mol. Plant-Microbe Interact. 1:128-134.

Panaccione, D.G., J.S. Scott-Craig, J.A. Pocard and J.D. Walton (1992) A cyclic peptide synthetase gene required for pathogenicity of the fungus Cochliobolus carbonum on maize. Proc. Natl. Acad. Sci. U.S.A. 89:6590-6594.

Scott-Craig, J.S., D.G. Panaccione, J.A. Pocard and J.D. Walton (1992) The multifunctional cyclic peptide synthetase catalyzing HC-toxin production in the filamentous fungus Cochliobolus carbonum is encoded by a 15.7 kb open reading frame. J. Biol. Chem. 67:26044-26049.

Pitkin, J.W., D.G. Panaccione, and J.D. Walton (1996) A putative cyclic peptide efflux pump encoded by theTOXA gene of the plant pathogenic fungus Cochliobolus carbonumMicrobiology 142:1557-1565.

Ahn, J.-H., and J.D. Walton (1997) A fatty acid synthase gene required for production of the cyclic tetrapeptide HC-toxin, cyclo(D-prolyl-L-alanyl-D-alanyl-L-2-amino-9,10-epoxi-8-oxodecanoyl). Mol. Plant-Microbe Interact. 10:207-214.

Ahn, J.-H., and J.D. Walton (1998) Regulation of cyclic peptide biosynthesis and pathogenicity in Cochliobolus carbonum by TOXE, a gene encoding a novel protein with a bZIP basic DNA binding motif and four ankyrin repeats. Mol. Gen. Genet. 260:462-469.

Cheng, Y.-Q., J.-H. Ahn, and J.D. Walton (1999) A putative branched-chain-amino-acid transaminase gene required for HC-toxin biosynthesis and pathogenicity in Cochliobolus carbonum. Microbiology 145:3539-3546.

Cheng, Y.-Q., and J.D. Walton (2000) A eukaryotic alanine racemase involved in cyclic peptide biosynthesis. J. Biol. Chem. 275:4906-5004.

Pedley, K. F. and J. D. Walton (2001) Regulation of cyclic peptide biosynthesis in a plant pathogenic fungus by a novel transcription factor. Proc. Natl. Acad. Sci. U.S.A. 98: 14174-14179. (Commentary PNAS 98:14187-8)

 Genomic Organization and Evolution: 

Nikolskaya, A.N., D.G. Panaccione, and J.D. Walton (1995) Identification of peptide synthetase-encoding genes from filamentous fungi producing host-selective phytotoxins or analogs. Gene 165:207-211.

Ahn, J.-H., and J.D. Walton (1996) Chromosomal organization of TOX2, a complex locus required for host-selective toxin biosynthesis in Cochliobolus carbonumPlant Cell 8:887-897.

Panaccione, D.G., J.W. Pitkin, J.D. Walton, and S.L. Annis (1996) Transposon-like sequences at the TOX2 locus of the plant-pathogenic fungus Cochliobolus carbonum. Gene 176:103-109. 

Pitkin, J.W., A. Nikolskaya, J.-H. Ahn, and J.D. Walton (2000) Reduced virulence caused by meiotic instability of the TOX2 chromosome of the maize pathogen Cochliobolus carbonumMol. Plant-Microbe Interact. 13:80-87.

Walton, J.D. (2000) Horizontal gene transfer and the origin of secondary metabolite gene clusters in fungi: an hypothesis. Fung. Genet. Biol. 30:167-171.

Ahn, J.-H., Y.-Q. Cheng, and J.D. Walton (2001) An extended physical map of the TOX2 locus of Cochliobolus carbonum required for biosynthesis of HC-toxin. Fung. Genet. Biol. 35:31-38

Wight, W.D., R. Labuda, J.D. Walton (2013) Conservation of the genes for HC-toxin biosynthesis in Alternaria jesenskae. BMC Microbiol. 13:165.