Cytokinins (CK)

Cytokinins - promotes controlled cell division

Physiological effects:
- occurs to repair leaf abscission and wounds
- leaf senescence signals expression of CKs, and so leaf senescence is prevented and plant remains green
- promotes growth of shoots (lateral bud growth), some regulation of cell division in shoot apical meristem (SAM) (pg 25)
- suppresses growth of roots
- enhance de-etiolation (greening of plants), thylakoid formation, cotyledon expansion
- limited duration

Forms:
- zeatin (found in coconut milk), some kinetin (amino-purine), biosynthetic ipt gene

Experiment:
- excised root grow indefinitely
- excised shoot grow after adding coconut milk and herring sperm DNA with auxin
- so this means, the hormone CK suppresses root growth and is needed for shoot growth
- Infection by Agrobacterium tumefaciens -> crown gall tumours

Infection by Agrobacterium tumefaciens leads to uncontrolled cell division and tumour formation 'crown gall tumours'
- baterium's cell contain a circular Ti-DNA that contains T-DNA which encodes for cytokinin and auxin, this T-DNA integrates with the host DNA during transformation and starts infecting the wound

Cytokinin oxidase metabolizes cytokinin (irreversible degradation, produces adenine + alcohol)

When CK oxidase is over-expressed
- you get REDUCED SAM (shoot apical meristem)
- and INCREASED ROOT growth

Morphogenesis in cultured plants - AUXIN / CK flux:
- low auxin, high CK => formation of shoots
- high auxin, low CK => formation of roots
- med auxin, med CK => undifferentiated

4. (2 marks) Morphogenesis of cultured plant cells
CK:
- High auxin and low CK promotes root growth
- Low auxin and high CK promotes shoot growth
- Equal concentration of auxin and CK shows no differentiation

5. (1 mark) De-etiolation of seedlings (greening)
CK:
- CK enhances de-etiolation of seedlings
- CK promotes chloroplasts development by converting etioplasts in dark-growing seedlings to thylakoids
- CK promotes cotyledon expansion

6. (5 marks) Induction of alpha-amylase production in cereal aleurone layer cells
GA: In Ca2+ independent pathway
1. GA1 binds to membrane receptor in aluerone cell
2. GA-receptor complex binds to heterotrimeric G protein initiating Ca2+ independent and Ca2+ dependent signaling pathway
3. In Ca2+ independent pathway, the Ca2+ activates the F-protein, the F-protein diffuses to the nucleus
4. the SCF-ubiquitin ligase complex degrades the DELLA repressor
5. GAMYB gene is expressed and it binds to the alpha-amylase hydrolytic enzyme promoter
6. alpha-amylase enzymes are expressed and secreted from the aleurone cell for starch degradation in endosperm