Characterization of a host-specific protein toxin (Ptr ToxB) from Pyrenophora tritici-repentis

Pyrenophora tritici-repentis, the causal agent of tan spot of wheat, differ entially induces tan necrosis and/or chlorosis in wheat. A chlorosis-induci ng, host-specific toxin, termed Ptr ToxB (formerly Ptr chlorosis toxin), wa s purified from the culture filtrates of a race 5 isolate of P. tritici-rep entis. Partial purification was performed by 25 to 80% ammonium sulfate pre cipitation and passage through a carboxymethyl-Sephadex C-25 cation exchang e column. Final purification was performed by fast performance liquid chrom atography, with a Mono S HR 5/5 cation exchanger, followed by size fraction ation on a Superose 12 HR 10/30 column. The toxin was shown to be proteinac eous in nature, and purity was confirmed by sodium dodecyl sulfate-polyacry lamide gel electrophoresis. The molecular mass of Ptr ToxB was determined t o be 6.61 kDa. The amino acid composition and partial N terminus amino acid sequence of the toxin were also obtained. Ptr ToxB was found to be heat st able, maintaining full toxic activity after 1 h at 55 degrees C. Infiltrati on of toxin concentrations as low as 14 nM produced chlorosis on susceptibl e cultivars.