Hormonal cross-talk between auxin and ethylene differentially regulates the expression of two members of the 1-aminocyclopropane-1-carboxylate oxidase gene family in rice (Oryza sativa L.)

Two cDNA clones, pOS-ACO2 and pOS-ACO3, encoding 1-aminocydopropane-1-carbo xylate (ACC) oxidase were isolated from rice seedling cDNA library. pOS-ACO 3 is a 1,299 bp full-length clone encoding 321 amino acids (M-r = 35.9 kDa) , while pOS-ACO2 is 1,072 bp long and is a partial cDNA clone encoding 314 amino acids. These two deduced amino acid sequences share 70% identity, and display a high degree of sequence identity (72-92%) with previously isolat ed pOS-ACO1 of deepwater rice. The chromosomal location studies show that O S-ACO2 is positioned on the long arm of chromosome 9, while OS-ACO3 on the long arm of chromosome 2 of rice genome. A marked increase in the level of OS-ACO2 transcript was observed in IAA-treated etiolated rice seedlings, wh ereas the OS-A CO3 mRNA was greatly accumulated by ethylene treatment. Resu lts of ethylene inhibitor studies indicated that auxin promotion of the OS- ACO2 transcription was not mediated through the action of auxin-induced eth ylene. Thus, it appears that there are two groups of ACC oxidase transcript s in rice plants, either auxin-induced or ethylene-induced. The auxin-induc ed OS-A CO2 expression was partially inhibited by ethylene, while ethylene induction of OS-ACO3 transcription was completely blocked by auxin. These r esults indicate that the expression of ACC oxidase genes is regulated by co mplex hormonal networks in a gene specific manner in rice seedlings. Okadai c acid, a potent inhibitor of protein phosphatase, effectively suppressed t he IAA induction of OS-ACO2 expression, suggesting that protein dephosphory lation plays a role in the induction of ACC oxidase by auxin. A scheme of t he multiple regulatory pathways for the expression of ACC oxidase gene fami ly by auxin, ethylene and protein phosphatase is presented.