Fig. 7.

Proposed model of GA metabolism during barley grain germination. The uptake of water into the grain initiates active GA metabolism in the scutellar epithelial layer, where stored precursors ent-kaurene (KAU) and GA₅₃ are converted to GA₁, which is subsequently glycosylated to form GA₁-G. The GA₁-G, KAU, and GA₅₃ are secreted and diffuse to the aleurone layer, where KAU and GA₅₃ again act as precursor metabolites for active GA synthesis. The GA₁-G is de-glycosylated and free GA₁ causes transcription of GA3ox, GA13ox, and GA20x genes. The GA₃ so produced may be glycosylated to form GA₃-G before it is secreted, although it is also possible that free GA₃ is secreted. The GA₃-G, GA₃, and GA₄ from the aleurone layer diffuse to adjacent aleurone cells, where KAU and GA₅₃, and some GA₁-G have also diffused from the scutellum. The process continues from the proximal end towards the distal end of the aleurone layer; the concentration of the GA₁-G is likely to become limiting towards the distal end of the grain, while GA₃-G, GA₃, and GA₄ will be replenished en route. The long-distance transport of the hydrophobic ent-kaurene may involve lipid transport proteins. The red stars indicate gene transcription.