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.