Virulence Gene Expression DuringConidial Germination in Cochliobolus carbonum. Margaret J. Jones. U. S. Department of Agriculture, Agricultural Research Service, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907-1155 U.S.A. Larry D. Dunkle. U. S. Department of Agriculture, Agricultural Research Service, Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907-1155 U.S.A. MPMI 8: 476-479. Accepted 7 February 1995. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995.

The fungal pathogen Cochliobolus carbonum race 1 produces a host-selective toxin (HC-toxin) that is responsible for increased virulence on susceptible genotypes of maize. The toxin is synthesized by a pcptide synlhetase, which is a product of the HTS1 gene. Because the toxin is not stored in dormant conidia, early expression of HTS1 is crucial for extensive colonization of susceptible leaf tissue. To delect the HTSl transcript and determine the onset of HTSl gene expression, we analyzed RNA preparations from ungerminated and germinated conidia by reverse transcription-polymerase chain reaction using oligonu-cleotide primers within the 15.7-kb open reading frame of HTSl. With primer pairs near both the 3'- and the 5'-termini, amplified products of the HTSl transcript were detected in RNA prepared from dormant conidia. With all primer pairs used, the quantities of transcript increased substantially during germ tube emergence and elongation, indicating that expression of HTSl is up regulated during spore germination. Digestion with restriction endonucle-ases confirmed the identity of the amplified products. Amplification of the constitutively expressed ?-tubulin transcript, which is processed to remove introns, as well as the absence of amplification products with primers spanning the HTSl coding sequence established that cDNA was amplified and not contaminating genomic DNA.