Effects of the aglycone of ascaulitoxin on amino acid metabolism in Lemna paucicostata

Ascaulitoxin and its aglycone (2,4,7-triamino-5-hydroxyoctanoic acid, CAS 212268-55-8) are potent phytotoxins produced by Ascochyta caulina, a plant pathogen being developed for biocontrol of weeds. The mode of action of this non-protein amino acid was studied on Lemna paucicostata. Ascaulitoxin is a potent growth inhibitor, with an I₅₀ for growth of less than 1 μM, almost completely inhibiting growth at about 3 μM. Its action is slow, starting with growth inhibition, followed by darker green fronds, and then chlorosis and death. Most amino acids, including non-toxic non-protein amino acids, reversed the effect of the toxin when supplemented in the same medium. Supplemental sucrose slightly increased the activity. d-Amino acids were equally good inhibitors of ascaulitoxin activity, indicating the amino acid effects may not be due to inhibition of amino acid synthesis. Oxaloacetate, the immediate precursor of aspartate, also reversed the activity. LC-MS did not detect interaction of the compound with lysine, an amino acid that strongly reversed the effect of the phytotoxin. Metabolite profiling revealed that the toxin caused distinct changes in amino acids. Reduction in alanine, paralleled by enhanced levels of the branched chain amino acids valine, leucine and isoleucine and nearly unchanged levels of pyruvate, might indicate that the conversion of pyruvate to alanine is affected by ascaulitoxin aglycone. In addition, reduced levels of glutamate/glutamine and aspartate/asparagine might suggest that synthesis and interconversion reactions of these amino group donors are affected. However, neither alanine aminotransferase nor alanine: glyoxylate aminotransferase were inhibited by the toxin in vitro. Our observations might be explained by three hypotheses: (1) the toxin inhibits one or more aminotransferases not examined, (2) ascaulitoxin aglycone affects amino acid transporters, (3) ascaulitoxin aglycone is a protoxin that is converted in vivo to an aminotransferase inhibitor.