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Host selective toxins (HSTs) are positive agents of virulence produced by plant pathogenic fungi. Approximately 20 HSTs have been documented. They are, with one exception, low molecular weight compounds with diverse structures (Fig.1, Fig. 2).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
C. carbonum is a pathogen of maize. 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). 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.
Cochliobolus carbonum Tox2+ infecting maize of genotype hm1/hm1 in the field.
Robert Scheffer (1920-1996), former professor of Botany and Plant Pathology at MSU, discovered HC-toxin in 1965. This picture was taken in his "Cochliobolus carbonum nursery" at MSU in 1987.
Hm1 encodes an enzyme called HC-toxin reductase that 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 (Fig. 3). These genes are HTS1, TOXA, TOXC, TOXD, TOXE, TOXF, and TOXG. HTS1 is a gene with a 16-kb open reading frame and no introns. It encodes a 570-kDa non-ribosomal peptide synthetase with four 'modules', one for activation of each amino acid in HC-toxin (Fig. 3A). Based on peptides derived from the enzyme, we postulate that module 1 activates and epimerizes L-proline, and modules 2, 3, and 4 activate L-Ala, D-Ala, and L-Aeo, respectively. We most recently showed that TOXG encodes 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. 4).
The site of action of HC-toxin is histone deacetylase (HDAC).
STRUCTURE OF NON-RIBOSOMAL PEPTIDE SYNTHETASES
Representative non-ribosomal peptide synthetases (NRPSs). NRPSs are large, modular enzymes. Hts1 is a 570-kDa polypeptide and cyclosporin synthetase is a ~1.5 MDa polypeptide. Each module is responsible for activating one of the constituent amino acids and catalyzing peptide bond formation. The order of modules is the same as the the order of amino acids in the peptide product. Some NRPS modules 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.
Bibliography of HST research from the Walton lab:
Reviews:
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, J.D., C.R. Bronson, D.G. Panaccione, E.J. Braun, and 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, R.P. and J.D. 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.Walton, J.D. (1996) Host-selective toxins: agents of compatibility. Plant Cell 8: 1723-1733.
Walton, J.D., R. Ransom, and J.W. Pitkin (1997) Northern corn leaf spot of maize: chemistry,
enzymology, and molecular genetics of a host-selective phytotoxin. In: G. Stacey and N. T. Keen,
eds., Plant-Microbe Interactions, Vol. 3, Chapman and Hall, pp. 94-123.Original Research Papers:
Structure:
Walton J.D., E.D. Earle, and B.W. Gibson (1982) Purification and structure of the host-specific toxin
from Helminthosporium 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.Walton, J.D. and E.D. Earle (1984) Characterization of the host-specific phytotoxin victorin by high
pressure liquid chromatography. Plant Sci. Lett. 34:231-238.Walton, J.D. (1985) Chimica della HC-tossina, una gene-specifica fitotossina. Phytopath. Medit.
24:319-321.Gloer, J.B., J. Meinwald, J.D. Walton and E.D. Earle (1985) Studies on the fungal phytotoxin
victorin: structures of three novel amino acids from the acid hydrolyzate. Experientia 41:1370-1374.Cheng, Y.-Q., L. D. Le, J.D. Walton, and K. D. Bishop (1999) 13C labelling indicates that the
epoxide-containing amino acid of HC-toxin is biosynthesized by head-to-tail condensation of acetate.
J. Nat. Prod. 62:143-145.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:
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 carbonum. Plant 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.
Biosynthesis:
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 the TOXA gene of the plant pathogenic fungus Cochliobolus carbonum. Microbiology
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.Itoh, Y., R. Kiyohara, Y. Kawamoto, M. Kodama, H. Otani, J.D. Walton, and K. Kohmoto (1998)
A catalytic domain of a cyclic peptide synthetase that is specific for the apple pathotype of Alternaria
alternata and its possible involvement in host-specific AM-toxin production. In: K. Kohmoto and
O.C. Yoder, eds., Molecular Genetics of Host-specific Toxins in Plant Disease, Kluwer
Academic, Dordrecht, pp. 53-61.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 carbonum. Plant 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 carbonum. Mol.
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.
updated Nov. 8, 2004 JW
