|J.H. Oetiker, S.F. Yang
|fruit ripening, ethylene, ACC synthase, ACC oxidase, gene expression
Ethylene is a plant hormone regulating fruit ripening by coordinating the expression of genes that are responsible for a variety of processes, including a rise in respiration, autocatalytic ethylene production and changes in color, texture, aroma and flavor.
Ethylene is biosynthesized from S-adenosylmethione via 1-aminocyclopropane-1-carboxylic acid (ACC), catalyzed by ACC synthase and ACC oxidase.
Both enzymes are limiting in preclimacteric fruits but are greatly induced during the ripening.
ACC synthase has been purified and characterized from various fruit tissues and its cDNAs cloned.
ACC oxidase was identified by a reverse genetic approach and subsequent identification of gene function by expression in heterologous systems.
ACC synthase and ACC oxidase are encoded by multigene families.
In ripening tomato fruits both LE-ACS2 and LE-ACS4 ACC synthases are induced, but in preclimacteric fruit it is unclear which enzymatic isoforms function.
Experiments using ethylene inhibitors, pulse ethylene treatment and antisense transgenic fruits demonstrated that ethylene synthesis plays a key role in regulating fruit maturation and ripening.
Time course studies of enzyme activity and northern blot analyses indicate that the induction of the ACC oxidase gene precedes that of climacteric ACC synthase.
While ACC oxidase gene expression is a system 2 ethylene-independent process, the climacteric ACC synthase genes are activated by system 2 ethylene.
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