Cytokinin metabolism

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Cytokinin metabolism and signal transduction
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Chapter 4. Cytokonin metabolism and signal
transduction
4.1 Introduction 4.2 Cytokonin metabolism
4.2.1 Cytokinin biosynthesis 4.2.2
Cytokinin interconversion and conjugation
4.2.3 Cytokinin catabolism
4.3 Cytokinin signal transduction 4.3.1
Cytokinin signal perception 4.3.2 Cytokinin
signal transduction 4.3.2.1
Histidine phospho-transfer proteins
4.3.2.2 Response regulators
4.3.2.2.1 B-type response regulators
4.3.2.2.2 A-type response
regulators 4.4 Conclusions
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Cytokinins
Cytokinins (CK) are a class of plant growth
substances (plant hormones) active in promoting
cell division, and are also involved in cell
growth, diff- erentiation, and other
physiological processes. There are two kinds of
cytokinins ? Adenin cytokinins Examples
kinetin, zeatin, benzyl adenine ( same as 6-
Benzylaminopurine). The DNA base adenine is a
structural analogue of cytokinins and have low
cytokinin bioactivity. ? Phenylurea cytokinins
Example N, N'-diphenylurea Although their
chemical compositions differ, there is a
structural correlation between adenine
cytokinins and urea cytokinin, and both show
similar biolo- gical activities.
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Cytokinin signal transduction
via a His-to-Asp phosphorelay
lt Two-component system(TCS) gt 1.Membrane bound
receptor kinase Senses the signal and
autophosphorylates 2.Response regulator
Phosphorylation by the receptor kinase
Activates the transcription of its target
genes Initiates another output reaction
Current Opinion in Plant Biology 2003, 6480488
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Structures of cytokinin receptors
Cytokinin receptors consist of input domain,
histidine kinase, receiver domain.
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AHKs can interact with all five AHPs
Fig. 1 Both the Arabidopsis His-kinase AHK4 and
HPt factor AHP2 function in E. coli. When the
Arabidopsis AHK4 gene is expressed in this
particular mutant, AHK4 is capable of signaling
the downstream YojN  RcsB  csplacZ pathway in
response to external cytokinins. Consequently,
the target csplacZ gene is activated. However,
when the Arabidopsis AHP2 gene is co-expressed in
this E. coli assay system, AHP2 can titrate a
phosphoryl group form AHK4, thereby inhibiting
the AHK4  YojN phosphorelay.
Plant and Cell Physiology, 2002, Vol. 43, No. 1
123-129
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Ligand specificity of cytokinin receptors
Fig 1.
Fig. 1 Comparison of the sensitivities of
CRE1/AHK4 and AHK3 to different cytokinin bases
in the E. coli assay.
Plant Cell Physiol. 45(9) 12991305 (2004)
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Cytokinin receptors have at least
a partial overlap in their function
Figure 1. Shoot Development of ahk Mutant Plants.
The Plant Cell, Vol. 18, 4054, January 2006
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Cytokinin receptors have at least
a partial overlap in their function
Figure 8. Contributions of Different Cytokinin
Receptors and Receptor Combinations to
Cytokinin-Regulated Processes.
The Plant Cell, Vol. 18, 4054, January 2006
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Arabidopsis Histidine phospho-transfer
proteins(AHP)
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Histidine phospho-transfer proteins
The central role of the AHPs as mediators of the
cytokinin signal requires that they can interact
with AHKs and ARRs.
Bioscience, Biotechnology, and Biochemistry Vol. 68 (2004) , No. 2 pp.462-465
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AHP proteins as cytoplasm and nucleus shuttles
Figure 2 AHP acts as a shuttle between the
cytoplasm and nucleus in cytokinin signalling.
NATURE VOL 413 27 SEPTEMBER 2001
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Arabidopsis Response Regulators(ARR)
Current Opinion in Plant Biology 2003, 6480488
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Response regulators
Analysis of the Arabidopsis genome sequence
reveals the existence of 22 predicted response
regulator (ARR) genes. They contain a RR domain
the conserved aspartate of this domain, which is
required for the His-to-Asp phospho-relay. The
ARRs are divided into two major classes, the A-
and the B-type, on the basis of their structure.
? The 11 type-A ARRs consist mainly of the
receiver domain with a short extension at the
N- and C-terminal ends. ? The 11 type-B ARRs
contain a C-terminal output domain in addition to
the receiver domain.
Current Opinion in Plant Biology 2003, 6480488
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B-type response regulators
Figure 7. Overview of Arabidopsis type-B ARR
localization based on GUS analysis.
Plant Physiol. Vol. 135, 2004
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Subcellular localization of B-type ARRs
Localize to nucleus. Predicted role as
transcription factors.
Figure 6. Nuclear localization of type-B ARRs.
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Several studies reporting B-type ARR gene loss
of function mutants did not display any
phenotypical alteration, except for longer
roots. ?Redundant B-type ARR functions Overexpre
ssion of ARR2 did not cause dramatic phenotipic
changes. Substitution of the conserved Asp80
with Glu created a dominant-active form of
ARR2, whose overexpression caused severe
pleiotropic growth aberrations, including
disturbance in the shoot meristem and an aberrant
leaf shape.
The EMBO Journal VOL 23 NO 16 2004
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A-type response regulators
lt Expression patterns of A-type response
regulators gt ARR3, ARR4 vascular tissue of
shoots and roots ARR5, ARR6 meristems of shoots
and roots ARR8, ARR9 roots Several A-type
ARRs have been shown to be negative regulators of
cytokinin signaling in a protoplast
system.(ARR3, ARR6, ARR8, ARR9) Several A- type
ARRs might be positive regulators and thus
enhance the cytokinin response. ?Negative
feedback regulation of cytokinin signaling by
A-type ARRs. For all A-type ARRs tested,
interactions with some or all of the AHPs were
detected. However no direct interaction between
the A- and B- type ARRs was found. ?Negative
regulation of the signaling pathway by A-type
ARRs operates via their interaction with AHPs.
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Conclusions
lt Two different type of cytokinin hormonal
activities exist in the plant gt A
local(paracrine or autocrine) activity, which may
regulate mainly cell division. Long-distance
signaling(endocrine-like), in which cytokinin may
sever primarily as a root-to-shoot signal and
may regulate physiological processes.
lt Future goals gt To find
out which are the individual functions of the
cytokinin metabolism and signaling. To
determine how signal specification is generated
in this partially redundant signaling system.
Integration of this signaling system into other
pathways within the plant cell, thus to
understand the molecular mechanisms that drive
the developmental and physiological processes
related to cytokinin.
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Features of cytokinin receptors and
components of the signalling pathway
Current Opinion in Plant Biology 2003, 6480488