Host-selective toxins and their role in plant diseases.

Toxins with unusual characteristics are involved in some destructive diseases of plants. Certain parasitic fungi produce toxins of low molecular weight that selectively affect the host plant; nonhosts are tolerant. These toxins have diverse structures, including cyclic peptides and linear polyketols. Genetic and other data show that resistance to each fungus is based on tolerance to its toxin. The same fungal genes control toxin production and ability to cause disease. Little is known about toxic action, although one toxin selectively affects mitochondria. Plant cell membranes are affected; this may allow the fungus to colonize tissues. Resistant cells may lack toxin receptor sites.