Abstract
Pure samples of chemically characterized AK-toxins I and II from Alternaria alternata Japanese pear pathotype were examined for their biological activities. Both preparations were toxic to Japanese pear cultivars susceptible to the pathogen, but were harmless to resistant pear cultivars and non-host plants tested. AK-toxins I and II at 5×10-9M and 10-7M, respectively, induced characteristic veinal necrosis and rapid K+ loss on susceptible pear leaves. The dose-response relationship revealed that the activity of AK-toxin II was about one-tenth that of AK-toxin I. AK-toxins I and II at 10-4M did not affect resistant pear leaves. In susceptible pear cells treated with AK-toxin I or II, invagination of the plasma membrane and degradation of the cell wall were observed under an electron microscope. When AK-toxin I or II was added to avirulent spores of A. alternata, the spores could invade susceptible tissues, just as did the virulent spores. These results show that pure samples of AK-toxins I and II fall under the criteria of host-specific toxin, and have the same biological activity as our previous semipurified AK-toxin preparation.
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