Leakage of sodium ions from plasma membrane modification, associated with permeability change, in host cells treated with a host-specific toxin from a Japanese pear pathotype of Alternaria alternata

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Abstract
Ultrastructural sites for leakage of sodium ions in susceptible leaves of Japanese pear treated with AK-toxin I were examined by a precipitation method for sodium ions and analytical electron microscopy. The earliest effect of the toxin (50 μg/mL) appeared on the plasmodesmatal regions of the susceptible vascular bundle sheath and phloem cells within 5 min of the exposure where two cellular events were found at about the same time: plasma membrane modification and the precipitation of sodium antimonates. The precipitates were first found at the cell walls near plasmodesmata in susceptible cells, prior to the occurrence of the modification. Afterwards, susceptible plasma membrane became invaginated at both ends of plasmodesmata and the invaginations grew larger with the time of exposure. As the modification developed, the precipitates spread from the cell walls near plasmodesmata to the neighbouring or remote areas. These events were observed only in the toxin-treated susceptible cells. This work indicated that plasmodesmata of susceptible cells were the initial sites of toxin attack and sodium ions were leached as a result of the modification of plasma membrane near plasmodesmata. The results showed that the modification apparently was associated with a permeability change in the toxin-treated susceptible cells, and the active sites of toxin might be located on the plasma membrane of plasmodesmata.