leky-latky/cytokiny/10-funkce-v-rostlinach-obecny-uvod
- Phytohormones
- Promote plant cell division in the presence of another phytohormone, auxin (Skoog et al. 1965)?
- Development and growth of both root and shoot systems
- Include water and nutrient mobilization
- Apical dominance
- Branching
- Flowering
- Breaking of bud dormancy
- Seed germination (Werner and Schmülling 2009)
- Delay leaf senescence
- Prevent the degradation of chlorophyll
- Prevent outflow of nutrients from the leaf [1]
The cytokinin signal in plants
- In plants, cytokinins are recognized by
- Receptor histidine kinases
- Part of the His-Asp phosphorelay resembling the two-component environmental sensors of bacteria
- No signaling system with similar organization exists in animals
- Plant leaf senescence
- Active and highly regulated process
- Completely different from animal aging
- Stochastic accumulation of damage
- Irreversible growth arrest
- Protects the body against tumor development
- Studies of cytokinins in mammalian cell cultures and animals
- Range of responsive pathways and processes
- Numerous prospective therapeutic applications
- Adenine and adenosine derivatives
- Influence diverse processes in animal cells
- Consequence of their ability to interact with various components of the animal purinome [1]
- Perceived by a His-Asp phosphorelay
- Similar to the two-component systems of bacteria
- 1) recognition of the cytokinin ligand
- By the extracellular domain of the transmembrane cytokinin receptor
- AHK2, AHK3, or AHK4 in Arabidopsis thaliana [1]
- 2) intracellular portion of the receptor phosphorylates histidine phosphotransfer proteins (AHPs)
- 3) transmission of the signal to nuclear response regulators (ARRs)
- 4) activate or repress transcription of the response genes [1]
Anti-senescence activity of cytokinins
- Mediated through the activation of AHK3
- Type-B response regulator ARR2
- Cytokinin response factor CRF6
- Increased cell-wall invertase activity in response to cytokinins
- Necessary and sufficient for the inhibition of senescence (Zwack and Rashotte 2013) [1]
- Substrate specificity differences between individual cytokinin receptors exist
- Cytokinin bases
- Consistently the most active cytokinin form (Mok and Mok 2001; Spíchal et al. 2004) [1]
- The intensity and duration of the signaling is dependent
- Receptor and response regulator composition
- Availability of individual cytokinins
Omezení syntézy cytokininů
- Rate-limiting step in cytokinin biosynthesis
- Catalyzed by isopentenyltransferases (IPTs)
- Synthesize either free cytokinin nucleotides (adenosine phosphate-IPTs)
- Modify adenosine in tRNA (tRNA-IPTs)
- Conversion of cytokinin 5`-monophosphates into their respective free bases
- Catalyzed by phosphoribohydrolase
- Encoded by the gene LONELY GUY (LOG) (Kurakawa et al. 2007) [1]
- Dephosphorylation of riboside-5’-monophosphates
- Precedes the cleavage of the glycoside bond (Chen and Kristopeit 1981) [1]
Deaktivace cytokiniů
- By cytokinin oxidase/dehydrogenases (CKXs)
- Catalyze removal of the side chain
- Cytokinin signal is also attenuated by conversion of:
- Free bases into less active (ribosides, ribotides)
- Free bases into inactive forms (glucosides, conjugates with alanine)
- Cytokinin conjugates can be converted back into free bases
- Seen as transport/storage cytokinin forms
- Kromě N7 a N9-glucosides [1]
Uptake and efflux of cytokinins by cells
- Facilitated by members of the purine permease family (PUPs) of transmembrane channels (Gillissen et al. 2000)
- By equilibrative nucleoside transporters (ENTs) (Hirose et al. 2008) [1]
Lokace
- Cytokinins are present in both phloem and xylem fluid
- Serve as both acropetal and basipetal messengers (Kudo et al. 2010)
- First acropetal transporter was described recently (Zhang et al. 2014)
- ATP-binding cassette transporter in Arabidopsis - AtABCG14
- Essential for the acropetal (root to shoot) translocation of root-synthesized cytokinins [1]