Light regulates seed dormancy through FHY3-mediated activation of ACC OXIDASE 1 in Arabidopsis.

Seed dormancy enables plants to delay germination until conditions are favorable for the survival of the next generation. Seed dormancy and germination are controlled by a combination of external and internal signals, in which light and ethylene act as critical regulators. However, how light and ethylene are interlinked to control these two processes remains to be investigated. Here, we show that ethylene and its precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), promote seed germination under light. Light facilitates the conversion of ACC to ethylene, in which phytochrome B (phyB) and FAR-RED ELONGATED HYPOCOTYL3 (FHY3) are functionally required. ACC oxidases (ACOs) catalyze the conversion of ACC to ethylene, among which ACO1 is specifically and predominantly expressed in imbibed seeds. Ethylene induces FHY3 protein accumulation in imbibed seeds, whereby FHY3 directly binds to ACO1 promoter and specifically mediates light-promoted ACO1 expression. Light promotes ACO1 protein accumulation. Overexpression of ACO1 significantly promotes seed germination, and almost completely restores the dormant defect of fhy3 loss-of-function mutants. In summary, this study reveals an ethylene-responsive regulatory cascade of phyB-FHY3-ACO1 that integrates external light input with internal factors to regulate seed dormancy and germination.