Elevated H₂O₂ production via overexpression of a chloroplastic Cu/ZnSOD gene of lily (Lilium oriental hybrid 'Marco Polo') triggers ethylene synthesis in transgenic potato

Transgenic potato plants (SS2 and SS4) that overexpressed a chloroplastic copper/zinc superoxide dismutase lily gene were utilized as an H₂O₂-inducible system in order to study the role of H₂O₂ as a signaling molecule in the biosynthesis of ethylene. SS2 and SS4 plants grown in vitro under sealed microenvironment (SME) conditions displayed anomalous phenotypes including reduction of stem elongation, radial stem growth, and promotion of root hair formation in the generated root, which were similar to ethylene-induced responses. In addition, SS4 plants showed severe vitrification in developing leaves and elevated ethylene production under SME conditions. After the ethylene action inhibitor AgNO₃, 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO) inhibitor CoCl₂, and ACC synthase inhibitor l-aminoethoxyvinylglycine were added to the growth media, the anomalous phenotypes in SS4 plants reverted to their normal phenotype with a concurrent decrease in ethylene production. Northern blot analysis showed that ACO transcripts in SS4 plants were constantly at high levels under normal and SME conditions, indicating that a high level of H₂O₂ in SS4 plants up-regulates ACO transcripts. Moreover, the direct treatment of H₂O₂ in potato plants confirmed the elevated expression of the ACO gene. Taken together, these data suggest that the high concentration of H₂O₂ in transgenic potato plants stimulates ethylene biosynthesis by activating ACO gene expression.