Characterization of the ToxB gene from North African and Canadian isolates of Pyrenophora tritici- repentis

Abstract

Pyrenophora tritici-repentis, causal agent of tan spot of wheat, produces Ptr ToxB, a host-specific protein toxin encoded by the ToxB gene. Three of an estimated 8–10 copies of ToxB from an Algerian race 5 isolate (Ptr ToxB+) of the pathogen were cloned and characterized, as was a ToxB homolog identified in a race 3 isolate from Canada that lacks Ptr ToxB toxin activity. All three loci from the race 5 isolate possessed identical 261-bp open reading frames (ORFs) and were associated with retrotransposon-like sequences; the ToxB ORF from the race 3 isolate was nearly identical, but the upstream flanking region was unique to this locus. The presence of ToxB homologs was also confirmed in Canadian isolates of races 4 and 5 showing reduced or no toxin activity. ToxB was found to be expressed in mycelia and conidia of the Algerian race 5 isolate, but only in conidia of the Canadian races 3, 4 and 5 isolates, indicating differences in transcriptional regulation. Comparison of the sequence information obtained in this study with previous reports suggested that the ToxB gene is highly conserved across a geographically-diverse set of isolates.