Down regulation of ethylene production and softening in 'McIntosh' apples

Most 'Mclntosh' apples are produced for the fresh market and have to be stored under controlled atmosphere conditions to avoid rapid softening. The softening of fruits is the result of structural changes in the cell walls, caused by the activity of hydrolytic enzymes that are under the control of the ripening hormone ethylene. Ethylene is synthesized in plants from S-adenosyl-L-methionine by a short pathway that consists of two enzymes: 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and 1-amino-cyclopropane-1-carboxylic acid oxidase (ACO). To interfere with ethylene synthesis in plants we have cloned two ACS genes from ripening 'Mclntosh' apples. The gene showing the closest similarity to the ripening-related ACS gene in other fruits was used to make antisense constructs. 'Mclntosh' plants were transformed using an Agrobacterium-mediated transformation system. Transgenic plants were propagated on antibiotic-containing agar, transferred to the greenhouse for conditioning, and later to the field. Transgenic 'McIntosh' fruits which developed on these trees were evaluated for morphological characteristics, ethylene production, and ripening parameters. Down regulation of ethylene production that resulted in significant delay in softening was achieved in some transgenic lines resulting in improved storability. The data indicate that structural integrity of this fruit may be maintained simply under refrigerated storage and controlled humidity.