The chronic food shortage that was feared after the rapid expansion of the world population in the 1960s was averted largely by the development of a high-yielding semi-dwarf variety of rice known as IR8, the so-called rice 'green revolution'. The short stature of IR8 is due to a mutation in the plant's sd1 gene, and here we identify this gene as encoding an oxidase enzyme involved in the biosynthesis of gibberellin, a plant growth hormone. Gibberellin is also implicated in green-revolution varieties of wheat, but the reduced height of those crops is conferred by defects in the hormone's signalling pathway.

To enhance our understanding of GA metabolism in rice (Oryza sativa), we intensively screened and identified 29 candidate genes encoding the following GA metabolic enzymes using all available rice DNA databases: ent-copalyl diphosphate synthase (CPS), ent-kaurene synthase (KS), ent-kaurene oxidase (KO), ent-kaurenoic acid oxidase (KAO), GA 20-oxidase (GA20ox), GA 3-oxidase (GA3ox), and GA 2-oxidase (GA2ox). In contrast to the Arabidopsis genome, multiple CPS-like, KS-like, and KO-like genes were identified in the rice genome, most of which are contiguously arranged. We also identified 18 GA-deficient rice mutants at six different loci from rice mutant collections. Based on the mutant and expression analyses, we demonstrated that the enzymes catalyzing the early steps in the GA biosynthetic pathway (i.e. CPS, KS, KO, and KAO) are mainly encoded by single genes, while those for later steps (i.e. GA20ox, GA3ox, and GA2ox) are encoded by gene families. The remaining CPSlike, KS-like, and KO-like genes were likely to be involved in the biosynthesis of diterpene phytoalexins rather than GAs because the expression of two CPS-like and three KS-like genes (OsCPS2, OsCPS4, OsKS4, OsKS7, and OsKS8) were increased by UV irradiation, and four of these genes (OsCPS2, OsCPS4, OsKS4, and OsKS7) were also induced by an elicitor treatment.

ABA is an important phytohormone regulating various plant processes, including stress tolerance, seed development and germination. SRK2D/SnRK2.2, SRK2E/ SnRK2.6/OST1 and SRK2I/SnRK2.3 are redundant ABAactivated SNF1-related protein kinases 2 (SnRK2s) in Arabidopsis thaliana. We examined the role of these protein kinases in seed development and germination. These SnRK2 proteins were mainly expressed in the nucleus during seed development and germination. The triple mutant (srk2d srk2e srk2i) was sensitive to desiccation and showed severe growth defects during seed development. It exhibited a loss of dormancy and elevated seed ABA content relative to wild-type plants. The severity of these phenotypes was far stronger than that of any single or double SRK2D , SRK2E and SRK2I mutants, including the srk2d srk2i mutant. The triple mutant had greatly reduced phosphorylation activity in in-gel kinase experiments using basic leucine zipper (bZIP) transcription factors including ABI5. Microarray experiments revealed that 48 and 30 % of the down-regulated genes in abi5 and abi3 seeds were suppressed in the triple mutant seeds, respectively. Moreover, disruption of the three protein kinases induced global changes in the up-regulation of ABA-repressive gene expression, as well as the downregulation of ABA-inducible gene expression. These alterations in gene expression result in a loss of dormancy and severe growth defects during seed development. Collectively, these results indicate that SRK2D, SRK2E and SRK2I protein kinases involved in ABA signaling are essential for the control of seed development and dormancy through the extensive control of gene expression.

Gibberellin (GA) regulates growth and development in plants. We isolated and characterized a rice GA-insensitive dwarf mutant, gid2. The GID2 gene encodes a putative F-box protein, which interacted with the rice Skp1 homolog in a yeast two-hybrid assay. In gid2, a repressor for GA signaling, SLR1, was highly accumulated in a phosphorylated form and GA increased its concentration, whereas SLR1 was rapidly degraded by GA through ubiquitination in the wild type. We conclude that GID2 is a positive regulator of GA signaling and that regulated degradation of SLR1 is initiated through GA-dependent phosphorylation and finalized by an SCF(GID2)-proteasome pathway.