The Mechanism of Action of a Bacterial Toxin on Plant Cells

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Abstract
The mode of action of the toxin produced by Pseudomonas tabaci was studied by growing Chlorella vulgaris in 50 cc. Erlenmeyer flasks of basic medium supplemented by various concns. of toxin. The toxin was prepared by incubation of the bacteria in a liquid medium for 4 days at 25 C, followed by centrifugation, adsorption onto norite A and separation with methanol. The toxin-containing soln. was standardized to produce a halo 5 mm. in diam. on tobacco leaves at a dilution of 1-100. Growth of Chlorella in a dilution of 1-5 was 1/3 as rapid as in the control medium after a 12-day incubation period. High concns. of toxin completely inhibited growth, but inhibition was removed by 0.5% liver extract, which did not inactivate the toxin. Of a large no. of compounds known to be in liver extract and tested singly, only DL-methionine completely neutralized the inhibiting effect of the toxin, although certain other amino acids were partially effective. Tests with an impure prepn. of toxin and a range of concns. of L-methionine indicated that L-methionine was synthesized by Chlorella in the presence of the toxin but that its utilization was impaired by the toxin. A reversal of toxic effect was not obtained with tobacco, but a similarity in the mechanism of action in tobacco and Chlorella was suggested by the fact that the known methionine antagonist, methionine sulfoximine, produced chlorotic halos in tobacco leaves indistinguishable from those produced by wildfire toxin.

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