Differential expression of the 1-aminocyclopropane-1-carboxylate oxidase gene family of tomato

The tomato ACC oxidase gene family is comprised of three members designated ACO1, ACO2 and ACO3. These are highly homologous throughout the protein coding regions but do show a degree of sequence divergence within the 3' untranslated regions. These regions have been cloned and used as gene-specific probes to analyse the differential expression of the tomato ACC oxidase gene family in various tissues at different stages of development. Results indicate that all three genes are transcriptionally active and display a high degree of inducibility in a number of organs at various stages of the life cycle. Both ACO1 and ACO3 transcripts accumulate during the senescence of leaves, fruit and flowers. In addition, it appears that ACO1 is wound-inducible in leaves. All three ACC oxidase genes are expressed during flower development, with each showing a temporally distinct pattern of accumulation. In addition, the ACC oxidase transcripts are also spatially regulated throughout flower development; ACO1 is predominantly expressed in the petals and the stigma and style, ACO2 expression is mainly restricted to tissues associated with the anther cone whereas ACO3 transcripts accumulate in all of the floral organs examined apart from the sepals. ACC oxidase enzyme assays and Western blot analysis indicate that both enzyme activity and ACC oxidase protein increase with transcript abundance in several tissues. The physiological role of the differential expression of the ACC oxidase gene family, in relation to the regulation of ethylene synthesis, during these various developmental processes is discussed.