Chemical Nature: N6-adenine derivatives, phenyl urea compounds. Zeatin is the most common cytokinin in plants.
Sites of Biosynthesis: Primarily in root tips.
Transport: Cytokinins are transported via the xylem from roots to shoots.
Effects: Cell division; promotion of shoot formation in tissue cultures; delay of leaf senescence; application of cytokinin can cause release of lateral buds from apical dominance.
CYTOKININS PRECEIVED BY HISTIDINE KINASES AND TRANSDUCED BY A TWO-COMPONENT SYSTEM
Two component systems have two proteins: the histidine kinase and the response regulator
Histidine kinases are transmembrane receptors with a signal sensing domain positioned in the extracellular space and a signal- transducing domain positioned in the cytoplasm.
Response regulators have a characteristic receiver domain, and in addition have an output (effector) domain that regulates downstream events (some RRs only consist of a receiver domain)
Output domains are DNA-binding transcriptional regulators (can also have other functions).
When Histidine kinase senses a signal, the conserved His residue in the transmitter domain is phosphorylated. Histidine kinases are dimers and one subunit of the kinase phosphorylates a conserved His residue of the other subunit by using ATP as the phospho-donor. The phosphoryl group is then transferred to the conserved Asp residue of the receiver domain. Phosphorylation of the receiver domain modulates the activity of the attached output domain, or a separate protein if the response regulator consists solely of the receiver domain. The sensor in eukaryotic systems is usually a hybrid histidine kinase and additionally carries a receiver domain. Another domain HPt, which contains a phosphorylatable His residue also participates in the phosphotransfer. The phosphoryl group is relayed alternatively between conserved His residues and conserved Asp residues. The phosphoryl group is linked by a high energy bond and the phosphotransfer is bidirectional.
CYTOKININ RECEPTORS AS HISTIDINE KINASES
CKI1: first indication that the cytokinin receptor is a histidine kinase BUT CKI1 is not the cytokinin receptor.
Cytokinin receptors found via reverse and forward genetics: CRE1/WOL/AHK4, AHK2 and AHK3 (complementation experiments conducted with the HPt domain protein YPD1).
Extracellular domains at the N-terminal regions of the cytokinin receptors are the cyclase/Histidine Kinase-Associated Sensing Extracellular (CHASE) domain
CHASE domains senses cytokinins
3 Cytokinin receptors have specific functions but also overlapping functions, as seen with their expression.
Cytokinin receptors possess five conserved blocks usually found in Histidine Kinase activity.
All cytokinin receptors have putative membrane-spanning segments.
RESPONSE REGULATORS IN ARABIDOPSIS
Arabidopsis has 22 genes for response regulators, which are grouped into 2 types: type-A ARR and type-B ARR.
Type-B ARRs are key regulators of cytokinin signalling and directly activate type-A ARR and Cytokinins activate type B ARRs. Type B ARR each also possess a DNA-binding domain called GARP and a variable extension. The GARP domain binds to DNA in a sequence specific manner.
Nine type-A ARR genes have been examined for cytokinin inducibility.
Unlike type-A ARRs, type-B ARRs are not induced by cytokinins.
Type-B ARRs: transcriptional activators
Type-A ARRs: transcriptional repressors
HPt PROTEINS AND PHOSPHOTRANSFER
Arabidopsis has 5 genes that code for HPt domain proteins. The phosphyl group is transferred between the side chains of His and Asp residues. AHPs function as shuttles that carry the phosphoryl group from the cytoplasm to the nucleus.
Cytokinin signalling is similar to that of auxin signalling.
Protein degradation can also play an important role in cytokinin signalling by destroying negative regulators.
Which type of ARR is linked to downstream events? 2 scenarios
1) Type-B ARRs regulate unidentified target genes, which in turn bring about cytokinin responses.
2) Type-A ARRs are induced by cytokinins and interact with diverse target proteins, which in turn regulate cytokinin responses
ROLE OF CYTOKININS:
not known very well, inferred mostly from the effects of cytokinin application.
1) Cytokinins are positive regulators of cell division in the shoot apical meristem and negative regulators of cell division in the root apical meristem.
2) Cytokinins are both local and systemic mediators.
3) Cytokinin are also involved in cell division: regulation of both G1-S and G2-M transitions.