Ultrastructural changes in host leaf cells caused by host-selective toxin of Alternaria alternata from rough lemon

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Abstract
A pathotype of A. alternata with selective pathogenicity to rough lemon produces 2 or more substances with selective toxicity to the host. The major toxin was extracted from culture fluids with ethyl acetate and purified by several different chromatographic procedures. Toxin (10 ng/ml, 25 .mu.l) was applied to leaves; within 24 h, water congestion and veinal necrosis were evident. The 1st toxin-induced change detected by EM was in the mitochondria; toxin at 1 .mu.g/ml caused swelling, reduction in number and vesiculation of cristea, and decreases in electron density of the matrix. At 1 h after toxin treatment the percentages of affected mitochondria in cells of several tissue were 18 in bundle sheaths, 9 in mesophyll cells adjacent to bundle sheath, and 9% in mesophyll cells remote from sheath. The number of affected mitochondria increased with time, until nearly 100% were affected at 6 h. No effects on other organelles were evident at 6 h. No ultrastructural changes were evident in cells of resistant leaves. The same changes in mitochondria were observed in pathogen-inoculated susceptible but not in resistant leaves by 24 h after incubation. The data indicate that the initial site of toxin action may be in the mitochondria.