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 from 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 rapid increase in electrolyte loss from susceptible leaves of Emperor mandarin, at 2 X 10(-8) M, and from susceptible Japanese pear cv. Nijisseiki (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 caused plasma-membrane invaginations in susceptible Emperor mandarin and Japanese pear but not in resistant genotypes. Infection hyphae of an avirulent isolate of A. alternata were produced in leaf tissues when their spores were inoculated with a small amount of the toxin solution. These results suggest that ACT-toxin Ib rapidly affects plasma-membrane integrity of susceptible genotypes and plays a critical role in the infection process of the pathogen. A minor ACT-toxin, Ic, had selective toxicity, but Ic might be an artifact of ACT-toxin Ib exposed to light during purification.