ETHYLENE INDUCES DE-NOVO SYNTHESIS OF CHLOROPHYLLASE, A CHLOROPHYLL DEGRADING ENZYME, IN CITRUS-FRUIT PEEL

Chlorophyllase (Chlase; EC 3.1.1.14) was extracted from plastid fractions of ethylene-treated orange fruit peel and purified 400-fold to homogeneity by gel filtration, hydrophobic chromatography, and preparative SDS/PAGE of nonheated protein. SDS/PAGE of nonheated purified enzyme indicated that Chlase activity is associated with a single protein band migrating at an apparent molecular mass of 25 kDa whereas the heated purified enzyme had a molecular mass of 35 kDa. The N-terminal sequence of the purified protein was determined. The purified enzyme was used as an immunogen for raising antibodies in rabbits. The antiserum was highly specific and on Western blots recognized both the heated and the nonheated form of Chlase. The antibodies also recognized the solubilized enzyme, as shown by an immunoprecipitation assay and by antigen-antibody capture assays in microtiter plates. Treatment with ethylene, which enhances degreening, increased Chlase activity 12-fold. Immunoblot analyses of crude extracts from ethylene-treated fruit detected a strong signal of the Chlase protein, while only a trace level of the enzyme protein could be detected in air. Gibberellin A3 and N6-benzyladenine partly counteracted the ethylene-induced increase in Chlase activity as well as the immunodetected upsurge of the Chlase protein. Ethylene appears to enhance the degreening of citrus fruit through de novo synthesis of the Chlase protein, which in turn is inhibited by the senescence-delaying regulators, gibberellin A3 and N6-benzyladenine. The Chlase enzyme protein may, therefore, serve as a model system for studying the hormonal molecular regulation of fruit ripening and senescence.