Introduction to Gibberellin Biosynthesis
The gibberellin biosynthesis pathway is crucial for plant growth and development. This pathway involves a series of enzymatic reactions that convert precursor compounds into bioactive gibberellins, which play a significant role in processes such as seed germination, stem elongation, and flowering.
Gibberellin Biosynthesis Pathway
| Stage | Key Enzymes | Location | Key Intermediate Compounds | Description |
|---|---|---|---|---|
| 1. Precursor Formation | Geranylgeranyl diphosphate synthase (GGPS) | Plastids (Chloroplasts) | Geranylgeranyl diphosphate (GGPP) | GGPP, derived from the MEP pathway, acts as the starting point for gibberellin biosynthesis. |
| 2. Terpene Cyclization | Ent-copalyl diphosphate synthase (CPS) \nEnt-kaurene synthase (KS) | Plastids | Ent-Kaurene | GGPP is converted to ent-kaurene through cyclization reactions. |
| 3. Oxidation Reactions | Ent-kaurene oxidase (KO) \nEnt-kaurenoic acid oxidase (KAO) | Endoplasmic Reticulum (ER) | Ent-kaurenoic acid → GA12 → GA53 | Sequential oxidation reactions convert ent-kaurene into GA12 (a key precursor for active GAs). |
| 4. Formation of Bioactive GAs | GA 20-oxidase (GA20ox) \nGA 3-oxidase (GA3ox) | Cytoplasm | GA9, GA20, GA1 (Active GA) | GA20ox and GA3ox catalyze the conversion of inactive precursors into bioactive gibberellins like GA1 and GA4. |
| 5. Deactivation | GA 2-oxidase (GA2ox) | Cytoplasm | GA8, GA34 (Inactive forms) | GA2ox enzymes degrade bioactive GAs to regulate growth and maintain homeostasis. |
Key Points in Regulation
- Light, temperature, and stress conditions influence enzyme activity in the pathway.
- The balance between GA biosynthesis and GA degradation controls plant growth stages like seed germination, stem elongation, and flowering.
Key Stages and Enzymes in the Pathway
The biosynthesis pathway begins with the formation of geranylgeranyl diphosphate (GGPP) in the plastids. This compound is synthesized by the enzyme geranylgeranyl diphosphate synthase (GGPS). Subsequently, GGPP undergoes terpene cyclization, where enzymes like ent-copalyl diphosphate synthase (CPS) and ent-kaurene synthase (KS) convert GGPP to ent-kaurene.
Oxidation Reactions and Regulation
The pathway progresses with oxidation reactions facilitated by enzymes, notably ent-kaurene oxidase (KO) and ent-kaurenoic acid oxidase (KAO). These enzymes act in the endoplasmic reticulum, converting ent-kaurene into important intermediates such as GA12 and GA53. The final step in gibberellin biosynthesis features GA20-oxidase (GA20ox) and GA3-oxidase (GA3ox), which produce the active gibberellins GA1 and GA4 from inactive precursors.
Regulation of this pathway is complex and influenced by various factors including light, temperature, and stress conditions. The balance between gibberellin biosynthesis and degradation is essential for maintaining plant hormone homeostasis and modulating growth stages effectively.
