Figure 1. Example of the molecular structure of a gibberellin, GA₁, presented in 2D and 3D view. One-hundred and thirty-six naturally occurring GAs have been found in plants and fungi so far.

Figure 2. Schematic of a germinating cereal grain. Gibberellin produced in the germinating embryo stimulates production of α-amylase and other hydrolytic enzymes in the aleurone layer. These enzymes break down starch in the endosperm providing nutrition for the emerging seedling.

Figure 3. Early GA biosynthetic pathway showing conversions from geranylgeranyl diphosphate (GGPP) to GA₁₂ and GA₅₃. Numbering of the C-atoms is shown for ent-kaurene.

Figure 4. Late GA biosynthetic and catabolic pathways. The bioactive GAs, GA₄ and GA₁, are synthesized from GA₁₂ and GA₅₃, respectively. Subscripted numbers before the slash indicate the non-13-hydroxylated GA (R=H) and after the slash indicate the 13-hydroxylated equivalent (R=OH).

Figure 5. Gibberellin signaling in plants. (A) Regulation of DELLA proteins by the ubiquitin-proteasome pathway is mediated by GA-dependent phosphorylation. In the absence of GA, DELLA inhibits GA responses. Gibberellin-binding by the GA receptor stimulates a kinase to phosphorylate the DELLA protein. The phosphorylated DELLA is recognized by the SCF^SLY1/GID2 E3 ubiquitin ligase complex (F-box protein, Skp1 homologue, cullin, and ring finger protein Rbx). The SCF complex catalyzes the transfer of ubiquitin from Rbx to the target protein. Formation of a polyubiquitin chain targets the DELLA for degradation by the 26S proteasome. (B) Genetic model for GA signaling. In the absence of GA, DELLA proteins inhibit expression of GA-responsive genes either directly or indirectly through inhibition of transcription factors like GAMYB. SPINDLY may negatively regulate GA response by stabilizing the DELLA protein by O-Glc-NAc modification. In the presence of GA, DELLA is negatively regulated by the SCF^SLY1/GID2 and possibly by the U-box protein PHOR1. DELLA destruction allows activation of GA-responsive gene expression possibly via GAMYB or other transcription factor.

Figure 6. DELLA protein structure. The DELLA protein family consists of a number of conserved domains. This schematic is drawn approximately to scale based on an alignment of Arabidopsis and rice DELLA proteins. The DELLA protein domain consists of two conserved elements, DELLA and VHYNP. Deletions within this domain lead to loss of GA regulation. The GRAS superfamily domain contains two LHR and one SH2-like domain. These domains are found in STAT transcription factors of metazoans.

GA Signaling Genes