Regulation of Seed Germination by GA signaling genes

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This project examines how the plant hormone gibberellin (GA) regulates seed germination, dormancy, and after-ripening. Besides being a major food source, seeds propagate, store, and disperse plants. Seeds are like lifeboats for embryonic plants. It is essential that seeds germinate only under conditions that will allow the seedlings to thrive and the species to survive. Thus seed germination is exquisitely regulated by the balance between two plant hormones, abscisic acid (ABA) and GA. During embryo maturation ABA triggers a condition called seed dormancy which prevents seed germination, even under favorable conditions. Dormancy is relieved by a period of dry storage or afterripening. This requirement for after-ripening assures seed dispersal over time as well as space. GA stimulates seed germination and has been implicated in after ripening. Plants such as Arabidopsis and tomato have an absolute requirement for GA synthesis to germinate. This project uses Arabidopsis to determine how the GA signal is perceived leading to germination. Germination is repressed by DELLA transcription factors. In Arabidopsis, GA stimulates germination by triggering destruction of the DELLA RGL2 protein. This destruction requires the GA receptor GID1 and the F-box protein SLY1. Interestingly, germination can occur in sly1 mutants that produce high levels of DELLA protein when seeds have been after-ripened a long time or when the GID1 receptor is over-produced. This suggests that a new mechanism controls seed germination and implicates the GA receptor. Three mechanisms will be tested: 1) DELLA-repression is bypassed by a parallel pathway; 2) DELLA is inactivated by interaction with the GID1 receptor; and 3) DELLA is inactivated by phosphorylation or ubiquitination. Elucidating these mechanisms will benefit society by providing molecular genetic methods to improve crop seedling establishment, yield, and resistance to preharvest sprouting in cereals like wheat. Knowledge about seed biology will be disseminated through annual presentations to Washington wheat growers and elementary schools. The project will promote education through training a postdoctoral fellow, an undergraduate and graduate student.