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Gibberellins - promotes internode (between nodes) elongation
Gibberellins Physiological Effects
- promotes internode elongation (creating taller plants vs rosette/dwarf plants - cabbage, dandelions lack GA)
- promotes juvenile to mature stages eg. cone buds in conifers
- sex determination eg. +GA prevent anther development in corn
- promotes seed germination via reserve mobilization (alpha-amylase production) and phytochrome induced transcription of genes
- anatagonistic to ABA (Abscisic acid)
- GA biosynthesis occurs throughout the entire life cycle
Forms:
- GA1 (active form, has COOH at C6 and beta-OH at C3) encodes CPS, the first enzyme in GA biosynthesis
- GA3 commercial form
- mostly inactive form
- GA20 + OH + GA-3beta-hydrolase => GA1
Reserve mobilization by GA
1. GA secreted in embryo
2. GA reaches aleurone layer via scutellum
3. Aleurone layer secretes alpha-amylase (starch hydrolase)
4. Starch is broken down to simple sugars and is transported back to the embryo
Experiment:
- 5'GA1-GUS reporter show sites of GA biosynthesis
- GA1 encodes CPS, the first committed enzyme in GA biosynthesis.
Commercial applications of GA:
- increases stalk length that offsets fruit compaction
- promotes fruit development without pollination (parthenocarpy = virgin/seedless fruit)
- beer, GA promotes malting (start germination by water then quickly halt it by heat then development of color and flavour is produced by kilning) of barley
- sugar - GA increase sugar yield and internode length
- hasten cone production
Commercial applications of GA Inhibitors:
- ornaments - crysanthemums (sprayed with GA inhibitor, reduces internode elongation so they are small)
- prevents lodging (bending of stems to ground) by reducing stem length
GA induces alpha-amylase production via Ca2+ independent pathway.
1. GA binds to receptor
2. GA-receptor binds to G-protein
3. G-protein activates F-box protein
4. F-box protein binds to DELLA-domain repressor (which is degraded by SCF-ubiquitin ligase)
5. GAMYB gene expression is activated
6. GAMYB activates alpha-amylase expression
1. (1 mark) Reproduction in conifers
GA:
Influence the transition from juvenile to
mature stages e.g. induction of cone-
buds in conifers
2. (2 marks) Seed germination
GA:
- Promotes post-germinative mobilization of reserves in cereal
grains
- promotes light-sensitive germination mediated by phytochromes which are induced by transcription of genes encoding GA biosynthetic enzymes
3. (3 marks) Control of root growth
CK:
- CK suppresses root growth so plants that are CK deficient show root growth compared to wild-types
- CK suppresses the size and rate of cell division activity of roots
- bell-shaped curve (X axis = CK signaling, Y axis = Root growth rate):
a) wild-type shows supraoptimal levels of CK
b) Over-expression of CK oxidase decreases CK signaling concentration to optimum level and root growth occurs
c) Further decreasing CK signaling below the optimum amount causes decrease in root growth
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