Cytokinin Flashcards
The discovery of Cytokinin
Discovered through identifying compounds that increase the growth of plant cells in culture
Kinetin
Autoclaved herring sperm that is structurally similar to cytokinin/ aminopterin derivative. It is not a natural cytokinin.
Cytokinins exist in sugar conjugated forms. What is the active and inactive part?
Glucosides are the inactive storage form and the Free base is the active form
Inactivation of cytokinin
Reversible conjugation or irreversible degradation.
Most common cytokinins
Zeatin, isolated from corn endosperm
Plant pathogen Agrobacterium tumefaciens
Natural plant pathogen causes crown gall disease and tumor-like growth by inducing auxin and cytokinin production.
Transfer T-DNA mobilized into the plant nucleus.
T-DNA, Auxin, and Cytokinin
T-DNA includes genes for the biosynthesis of auxin and cytokinin.
Normal T-DNA= Auxin and CK produces
No tmr gene= Auxon produced (rooty tumor)
No tms gene= CK produced (shoot tumor)
Cytokinin Synthesis
Synthesized primarily in the meristematic region of the roots, it is also produced in developing embryos and young leaves. Moves upward via the xylem, and there are high levels of CK in shootless plants. Zeatin ribosides coverts leave to free CK.
Cytokinin biological roles: Morphogenesis
The ratio of auxin to CK determines the difference between root and shoot development.
CK promotes cell division and stem cell fate at the SAM.
CK inhibits root meristem size and cell division and promotes cell differentiation at the root apical meristem.
Cytokinin biological roles: Delay of senescence
Cytokinin works by inhibiting the genes responsible for initiating senescence, preventing the breakdown of chlorophyll and other important components within the leaf
Soy beans
Cytokinin biological roles: Apical dominance
The growing apical bud inhibits the growth of the lateral buds. Removal of the apex results in developing one or more lateral shoots; IAA can substitute for the apex in maintaining the inhibitor.
Cytokinin biological roles: Promote nutrient mobilization
CK causes preferential transport to tissues with higher CK levels.
Cytokinin biological roles: Promote chloroplast maturation
Partial development towards light-grown plastid elicited with CK application.
Cytokinin biological roles: Root vascular tissue development
In the root vascular cylinder, some cells in the procambium differentiate into protoxylem, metaxylem, and phloem cells.
wol mutant all vascular cells differentiate to protoxylem causing severe inhibition of CK signaling
WOL CK receptor
CK prevents cells from differentiating into protoxylem
Isopentyle transferase
IPT overexpression (increases CK levels) causes reduced apical dominance, root growth, and delayed leaf senescence.
Cytokinins in cell division regulation
In the absence of either auxin
and cytokinin, cell cultures remain in the G1 or G2 phases of interphase.
Cytokinins are needed to activate cyclin-dependent kinases (CDK).
The activation of CDK by cytokinins allows cells to transition from the G2 phase to mitosis.
Cytokinin 2-component system
- Receptors: Hybrid Histidine Kinases (HKs), Cytokinin receptors
the cytokinin binding to the receptor initiates autophosphorylation and activates the signaling cascade. - Phosphotransfer Proteins (AHPs)
The phosphorylated histidine in the receptor transfers its phosphate group to a conserved aspartate residue on a response regulator (RR) through intermediary Histidine Phosphotransfer Proteins (AHPs).
AHPs shuttle between the cytoplasm and nucleus, enabling them to bridge the receptor’s signal to nuclear responses. - Response Regulators (ARRs)
ARRs are classified into three types:
Type-A ARRs: Negative regulators; act as feedback inhibitors of the signaling pathway.
Type-B ARRs: Positive regulators; act as transcription factors and activate cytokinin-responsive genes.
Type-C ARRs: Additional negative regulators with lesser characterization.
