In an effort towards understanding the biochemical properties and physiological functions of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase homologues, we have isolated three ACC oxidase clones from sunflower (Helianthus annuus) seedlings. ACCO1 is a cDNA clone while ACCO2 and ACCO3 and reverse transcriptase-polymerase chain reaction clones. Southern analysis indicated the existence of at least three members in the sunflower ACC oxidase gene family. Expression studies showed that ACCO3 was equally expressed in leaves, hypocotyl, and roots of sunflower seedlings, but it constituted only a small amount of the total ACC oxidase transcripts. In contrast, ACCO1 and ACCO2 were differentially expressed in these organs. ACCO1 mRNA was most abundant in roots, whereas ACCO2 was the major homologue in leaves and in hypocotyl. The levels of total ACC oxidase transcripts in these organs were also determined. High ACC oxidase transcript levels were associated with tissue containing rapidly dividing cells. Wounding and silver ion treatments of hypocotyls increased ACC oxidase mRNA levels and ACC oxidase activity; these events being consistent with the increases in ethylene production. In contrast, ACC oxidase protein levels were not affected by these treatments, suggesting that either a translational regulation and/or a rapid turn-over of the protein is involved in both wound- and silver ion-induced gene expression. Contrary to data in the literature, we found that auxins, ethephon and ACC did not affect ACC oxidase mRNA levels in sunflower hypocotyls. The complexity of ACC oxidase regulation and the significance of organ differential expression of ACC oxidase genes are discussed.