Nonessentiality of Boron in Fungi and the Nature of its Toxicity

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Abstract
This investigation was undertaken to determine the B requirement of 4 representative spp. of fungi, and to elucidate the mechanism of B toxicity. Aliquots of suspensions of Saccharomyces cerevisiae Mayer -Gebruder strain were inoculated into B-free test tubes containing media varying only in B concentration. Fifty ppm of B were toxic to S. cerevisiae. B did not appear to be essential, since this sp. was capable of growth and reproduction in its absence. Furthermore, growth rates approaching the maximum reported in the literature were obtained in the minus-B as well as the plus-B cultures. Aliquots of conidial suspensions of Neurospora crassa, Aspergillus niger, and Penicillium chrysogenum were similarly inoculated into B-free flasks containing media varying only in B concentration. A dose of 250 ppm of B was toxic to N. crassa, but 1300 ppm were required for toxicity in A. niger, and 4000 ppm for P. chrysogenum. B did not appear to be essential for these fungi, since dry weight yields were similar in the absence and presence of B. Fructose-1, 6-diphosphate, and ADP accumulated in B-toxic cultures of S. cerevisiae and P. chrysogenum, whereas 3-phosphoglyceric acid and ATP levels decreased, as compared to the minus-B cultures. B inhibited 02 uptake and CO2 evolution. This inhibition by B of gas exchange by S. cerevisiae was reversed by glyceraldehyde. It was concluded that the aldolase system of glycolysis was inhibited by toxic B levels, and this was verified with rabbit muscle aldolase. The inhibition of aldolase by toxic levels of B was of an uncompetitive nature.

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