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Two Polyketide Synthase-Encoding Genes Are Required for Biosynthesis of the Polyketide Virulence Factor, T-toxin, by Cochliobolus heterostrophus

February 2006 , Volume 19 , Number  2
Pages  139 - 149

Scott E. Baker , 1 Scott Kroken , 1 Patrik Inderbitzin , 2 Thipa Asvarak , 2 Bi-Yu Li , 1 Liang Shi , 1 O. C. Yoder , 1 and B. Gillian Turgeon 1 , 2

1Torrey Mesa Research Institute (former)/Syngenta, 3115 Merryfield Row, San Diego, CA 92121 U.S.A.; 2Department of Plant Pathology, 334 Plant Science Bldg., Cornell University, Ithaca, NY 14853 U.S.A.

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Accepted 2 October 2005.

Cochliobolus heterostrophus race T, causal agent of southern corn leaf blight, requires T-toxin (a family of C35 to C49 polyketides) for high virulence on T-cytoplasm maize. Production of T-toxin is controlled by two unlinked loci, Tox1A and Tox1B, carried on 1.2 Mb of DNA not found in race O, a mildly virulent form of the fungus that does not produce T-toxin, or in any other Cochliobolus spp. or closely related fungus. PKS1, a polyketide synthase (PKS)-encoding gene at Tox1A, and DEC1, a decarboxylase-encoding gene at Tox1B, are necessary for T-toxin production. Although there is evidence that additional genes are required for Ttoxin production, efforts to clone them have been frustrated because the genes are located in highly repeated, A+T-rich DNA. To overcome this difficulty, ligation specificity-based expression analysis display (LEAD), a comparative amplified fragment length polymorphism/gel fractionation/capillary sequencing procedure, was applied to cDNAs from a near-isogenic pair of race T (Tox1+) and race O (Tox1-) strains. This led to discovery of PKS2, a second PKS-encoding gene that maps at Tox1A and is required for both Ttoxin biosynthesis and high virulence to maize. Thus, the carbon chain of each T-toxin family member likely is assembled by action of two PKSs, which produce two polyketides, one of which may act as the starter unit for biosynthesis of the mature T-toxin molecule.

Additional keywords: ascomycete, Didymella zeae-maydis, Mycosphaerella zeae-maydis, Phyllosticta maydis.

© 2006 The American Phytopathological Society