ISOLATION AND BIOLOGICAL-ACTIVITIES OF 2 HOST-SPECIFIC TOXINS FROM THE TANGERINE PATHOTYPE OF ALTERNARIA-ALTERNATA

Two host-specific toxins (HSTs), ACT-toxin Ib and Ic, were isolated fr om culture filtrates of Alternaria alternata affecting tangerines and mandarins. The biological characteristics of ACT-toxin Ib matched the criteria for HST. ACT-toxin Ib was detected by HPLC (high-performance liquid chromatography) analysis and was one of the major components in the ethyl acetate extracts of culture filtrates and spore-germinating fluids of the pathogen. ACT-toxin Ib induced veinal necrosis and a ra pid increase in electrolyte loss from susceptible leaves of Emperor ma ndarin, at 2 X 10(-8) M, and from susceptible Japanese pear cv. Nijiss eiki (a potential host), at 1 X 10(-6) M, but did not affect resistant citrus and Japanese pear, at 2 X 10(-4) M. Furthermore, the toxin cau sed plasma-membrane invaginations in susceptible Emperor mandarin and Japanese pear but not in resistant genotypes. Infection hyphae of an a virulent isolate of A. alternata were produced in leaf tissues when th eir spores were inoculated with a small amount of the toxin solution. These results suggest that ACT-toxin Ib rapidly affects plasma-membran e integrity of susceptible genotypes and plays a critical role in the infection process of the pathogen. A minor ACT-toxin, Ic, had selectiv e toxicity, but Ic might be an artifact of ACT-toxin Ib exposed to lig ht during purification.