Evidence for Positive Regulation by Gibberellins and Ethylene of ACC Oxidase Expression and Activity During Transition From Dormancy to Germination in Fagus sylvatica L. Seeds

Abstract

1-aminocyclopropane-1-carboxylic acid (ACC) oxidase is a key enzyme in the ethylene biosynthesis pathway. In the present report the changes in ACC content, ACC oxidase activity and ethylene production have been examined in Fagus sylvatica L. dormant seeds after stratification and different treatments that maintain or release dormancy. Additionally, a cDNA encoding an ACC oxidase (ACO) from Fagus sylvatica has been isolated and characterized. This clone, named FsACO1, exhibits high homology to ACC oxidases from several plant species and the corresponding enzyme, expressed in Escherichia coli as a fusion protein, is active in converting ACC into ethylene. The transcript levels of FsACO1 are correlated with the ACC content, the ethylene production and the ACC oxidase activity measured in vitro as well as with the germination percentages observed in the seeds under the different treatments used in this study. There is a drastic increase in all these parameters when seeds are treated with GA3 or ethephon (which releases ethylene in solution), hormones previously proven to be efficient in the breaking of dormancy of beech seeds. The stimulatory effect of ethephon is reverted by paclobutrazol, a well known GA biosynthesis inhibitor. These results indicate that ethylene biosynthesis is positively regulated by both gibberellins and ethylene and suggest a cross-talk regulation by these two hormones of the processes involved in the transition from seed dormancy to germination.