ALTERNARIA TOXINS AND THEIR EFFECTS ON HOST PLANTS

There are now nine or more Alternaria pathogens that produce host-specific toxins, and the structures of most of the toxins have been elucidated. Alternaria host-specific toxins are classified in three groups in terms of the primary site action. ACT-, AF-, and AK-toxins have in common an epoxydecatrienoic acid structure and exert their primary effect on the plasma membrane of susceptible cells. A rapid increase in electrolyte loss from tissues and invaginations in the plasma membranes are common effects of these toxins. The second group is represented by ACR(L)-toxin, which induces changes in mitochondria, including swelling, vesiculation of cristae, decrease in the electron density of the matrix, increase in the rate of NADH oxidation, and inhibition of malate oxidation. The third group consists of AM-toxin, which appears to exert an early effect on both chloroplasts and plasma membranes. AM-toxin induces vesiculation of grana lamellae, inhibition of CO2 fixation, invagination of plasma membranes, and electrolyte loss. The roles of host-specific toxins in pathogenesis are discussed.