It has been shown that seed development is regulated by a network of transcription factors including LEC1 (leafy cotyledon1), L1L (LEC1-like), and B3 domain factors, LEC2, FUS3 (FUSCA3) and ABI3 (ABA-insensitive3) in Arabidopsis, however, molecular and genetic regulation of seed development in cereals is poorly understood. To understand the seed development and seed germination in cereals, a large-scale screen was carried out with our T-DNA mutant population and a mutant germination-defective1 (gd1) was identified. Except for the severe germination defect, gd1 mutant also shows dwarf and abnormal flower development. Molecular and biochemical analyses reveal that GD1 encodes a B3 domain-containing transcription factor with repression activity. Consistent with dwarf phenotype of gd1, the expression of GA inactivation gene OsGA2ox3 is increased dramatically, accompanied with reduced expression of GA biosynthetic genes including OsGA20ox1, OsGA20ox2, and OsGA3ox2 in gd1, resulting in the decreased endogenous GA₄ level. Exogenous application of GA not only induces GD1 expression, but also partially rescues the dwarf phenotype of gd1. Furthermore, GD1 binds to the promoter of OsLFL1, a LEC2/FUS3-like gene of rice via RY element, leads to the significant upregulation of OsLFL1 and a large subset of seed maturation genes in gd1 mutant. The plants overexpressing OsLFL1 partly mimic gd1 mutant. In addition, the expression of GD1 was induced under sugar treatment and the contents of starch and soluble sugar are changed in gd1 mutant. These data indicate that GD1 participates directly or indirectly in regulating GA and carbohydrate homeostasis, and further regulates rice seed and seedling development.

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Keywords:
germination-defective1 ;B3 domain transcription factor;Seed germination;Gibberellin;Rice