AAL-toxin induced physiological changes in Lycopersicon esculentum Mill: roles for ethylene and pyrimidine intermediates in necrosis

Treatment of F9 near-isogenic asc/asc tomato leaflets with Alternaria alternata f.sp. lycopersici (AAL)-toxin (0·015 μm) increased steady state levels of 1-aminocyclopropane-1-carboxylic acid (ACC) after 6 h, followed by ethylene evolution after 12 h, followed by the detection of necrosis after 36 h. No changes in these parameters were detected in comparably treated leaflets of the Asc/Asc isoline or water controls. Co-treatment of asc/asc leaflets with AAL-toxin and ethylene inhibitors, aminoethoxyvinylglycine or silver thiosulphate, markedly reduced necrosis and ethylene evolution. Application of exogenous ACC (1 mm) in the presence of AAL-toxin resulted in a two-fold increase in ethylene evolution and interveinal necrosis. However, no necrosis resulted when 1 mm ACC enhanced ethylene levels to 108 nl g−1 h−1 in control tissues without AAL-toxin, even though the ethylene level was similar to that induced by AAL-toxin. Dihydroorotic acid, an intermediate in pyrimidine biosynthesis, at 0·3 μm abolished both the AAL-toxin increase in ACC and necrosis. Addition of N-(phosphonacetyl)-l-aspartate, a specific inhibitor of pyrimidine biosynthesis, elicited interveinal necrosis resembling AAL-toxin treatment. These results indicate an integral but not causal role for ethylene in AAL-toxin-induced necrosis and suggest a metabolic interaction involving a negative regulation of the orotic acid on the ethylene biosynthetic pathways.