The regulation of seed dormancy by HONSU and HYPNOS in Arabidopsis thaliana

Abstract

Seed dormancy, a seed status that prohibits germination even in the presence of inductive germination signals, is one of the poorly understood processes. In Arabidopsis, seed dormancy is established during seed maturation and is released after the harvest. To identify molecular components that regulate seed dormancy, we screened T-DNA insertion lines and identified two mutants named as honsu (hon) and hypnos (hyp). Both mutants require a prolonged period of after-ripening.

The HON loss-of-function mutants display deep seed dormancy, whereas HON-overexpressing lines display shallow seed dormancy. HON encodes a seed-specific group A protein phosphatase 2C (PP2C) and is one of major negative regulators of seed dormancy among group A PP2Cs, while other members including AHG1 and AHG3 are minor regulators that redundantly inhibit seed dormancy. Like other PP2C family members, abscisic acid (ABA) receptor Pyrabactin resistant 1/Regulatory components of ABA receptor 11 (PYR1/RCAR11) interacts with HON and inhibits PP2C activity of HON in the presence of ABA. Our analysis indicates that HON inhibits ABA signaling and activates gibberillin (GA) signaling, and both of these conditions must be satisfied to promote the release of seed dormancy. However, HON mRNA levels are increased in mutants displaying deep seed dormancy or under conditions that deepen seed dormancy, and decreased in mutants displaying shallow seed dormancy or under conditions that promote the release of seed dormancy. Our results indicate that the expression of HON mRNA is homeostatically regulated by seed dormancy.

The HYP loss-of-function mutants also display deep seed dormancy. Germination frequencies of hyp-1 and hyp-2 increased during seed after-ripening but the kinetics of germination was slower than that of WT. In addition, 1 day cold-imbibition is not enough to break the seed dormancy of hyp mutant. However, ectopic expression of HYP didn’t show the reduced seed dormancy. HYP mRNA is expresse...