The synthesis of ethylene in melon [Cucumis melo] fruit during the early stage of ripening

The levels of mRNA and polypeptide for a 1-aminocyclopropane-1-carboxylate (ACC) oxidase were studied to identify the tissues in which the synthesis of ethylene first occurs during the initial stage of ripening. RNA and immunoblot analysis showed that the levels of the mRNA and polypeptide for ACC oxidase were very low in unripe fruit. They first became detectable in the placental tissue at the pre-climacteric stage, and then their levels increased in the mesocarp tissue during the climacteric increase in the production of ethylene. Two mRNAs for ACC synthase (transcribed from ME-ACS1 and ME-ACS2) were detected in the placental tissue and seeds at the pre-climacteric stage, but only the level of ME-ACS1 mRNA, which has been characterized as the mRNA for a wound-inducible ACC synthase, increased in mesocarp, placental tissues and seeds during ripening. The level of ME-ACS2 mRNA that was isolated from etiolated seedlings of melon, did not change markedly during ripening. These results suggest that the central region of melon fruit (placental tissue and seeds) plays a major role in the production of ethylene during the early stage of ripening.