Plant Hormones -
Plant Physiology (Biology 327)
- Dr. Stephen G. Saupe; College of St. Benedict/ St.
University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782;
(320) 363 - 3202, fax;
- Called "cytokinins" because they stimulate cell
division (i.e., cytokinesis)
- Haberlandt (1913) noted that non-dividing potato parenchyma
cells would revert to actively dividing ones in the presence of phloem sap.
This observation suggested a soluble material was
responsible for cell division.
- Folke Skoog (1940's) and colleagues at Univ. of Wisconsin
found that cultured tobacco pith tissue explants would proliferate only if they were
supplemented with various substances such as autoclaved herring sperm or coconut milk.
- Miller (1956) identified the first cytokinin, called
kinetin, in the herring sperm.
- Cytokinins occur in most plants including mosses, ferns, conifers,
algae and diatoms
- adenine derivatives (amino purines)
- occur as: (a) the free nitrogenous base; (b) a nucleoside
(base + ribose); (c) a nucleotide (base + ribose + phosphate); or (d)
- The free base is the active form.
- approximately 40 different structures known.
- Zeatin (Z), which was first isolated from maize
is the most common cytokinin.
- Other naturally occurring cytokinins include, dihydrozeatin
(DHZ) and isopentenyladenosine (IPA).
B. Synthetic cytokinins
- kinetin (as above) probably byproduct of zeatin
- there are several other substances with cytokinin activity
such as benzyl adenine (benzylaminopurine; BA).
C. Cytokinins and nucleic acids
- can occur as a modified base in tRNA, but the bases exist
in the cis form, rather than the typical trans form. These modified bases
that are found in all organisms from bacteria to plants to humans.
- The function of the tRNA cytokinins is not clear, but after
hydrolysis of the tRNA the products can act as a cytokinin. The importance of the tRNA
derived cytokinins in overall growth and development is not clear, either.
- Interestingly plants have different sets of tRNAs
with different cytokinins that participate in protein synthesis in the cytoplasm and the
- Site: synthesized primarily in the meristematic region of the
roots. This is known in part because roots can be cultured (grown in
artificial medium in a flask) without added cytokinin, but stem cells
- Cytokinins are also produced in developing embryos and crown gall tissues
- first major precursor to the cytokinin is AMP (adenosine
- side chains of the cytokinins are made by the terpene
pathway (IPP isoprene) and added to the AMP by cytokinin synthase
- there is some speculation that surface and endophytic
bacteria (i.e., Methylobacterium) may be the actual source of plant cytokinins.
Rationale: (1) haven't isolated some of the putative genes for cytokinin
synthesis; (2) remove bacteria show impaired cytokinin production.
- tRNA nucleotides are modified to cytokinins after the tRNA
is transcribed (post-transcriptional processing)
- via xylem (transpiration stream)
- in peas, a signal from the leaves may signal/regulate
transport of cytokinins from the roots
- zeatin ribosides are the main transport form; converted to
the free base or glucosides in the leaves
- some cytokinin also moves in the phloem.
- Callus culture cell proliferation - not used too much
because it takes too long
- Expansion of radish or cocklebur cotyledons
- Inhibition of chlorophyll loss by detached oat leaves
B. Methods of Analysis liquid chromatography, mass
- forms conjugates with glucosides. major storage form of
- cytokinin oxidase may be an important route of disposal,
A. Control morphogenesis
- in plant tissue cultures, cytokinin is required for the
growth of a callus (an undifferentiated, tumor-like mass of cells):
callus + auxin + no cytokinin
|| little growth of callus
callus + auxin + cytokinin
|| callus grows well, undifferentiated
- ratio of cytokinin and auxin are important in determining
the fate of the callus:
callus + low
||callus grows well,
callus + high [cytokinin/auxin]
|| callus grows well, forms meristem
- some tissues become habituated during repeated cell culture
loose the requirement for cytokinin in the growth medium
B. Crown Gall
- tumor-like mass of undifferentiated cells that typically
occurs near the crown (junction of root and stem) of the plant
- caused by the bacterium Agrobacterium tumefaciens
- carries a plasmid (Ti plasmid; a plasmid is a small loop of
non-chromosomal DNA) with loci/genes for auxin production (tms), zeatin production
opines (are nitrogen-containing molecules that provide food for the bacteria).
- upon infection, the plasmid is incorporated into the plant
cell genome which begins to overproduce auxin and cytokinin
- stem forms an undifferentiated tumor (crown gall)
- as predicted, if the tms genes (auxin production) are
deleted from the plasmid, which would increase the cytokinin/auxin ratio, the resultant
crown gall is "shooty". If the tmr genes are deleted the gall is
C. Regulates the cell cycle/cell division (hence, the name
"cytokinins) especially by controlling the transition from G2
à mitosis. This
effect is moderated by cyclin-dependent protein kinases (CDK's) and their subunits,
D. Delay senescence
- senescence is the programmed aging process that occurs in
plants (and other organisms for that matter).
- loss of chlorophyll, RNA, protein and lipids.
- cytokinin application to an intact leaf markedly reduces
the extent and rate of chlorophyll and protein degradation and leaf drop
- correlation between cytokinin levels and senescence.
example, as detached leaves senesce the cytokinin levels drop. And, when these leaves are
treated with auxin to stimulate rooting, when roots form the senescence process stops and
cytokinin levels rise.
- Tobacco plants + senescence promoter sequence
+ ipt gene (cytokinin gene)
causes gene to become active on senescence
The exact mechanism by which this occurs is unclear but
likely involves the ability of cytokinins to mobilize nutrients. Application of cytokinin
to a leaf will cause it to act as a sink and nutrients will be directed towards it.
Promotes the light-induced formation of chlorophyll and
conversion of etioplasts to chloroplasts (greening process).
F. Promote lateral bud development
to dormant buds will cause them to develop. A witches broom is caused by a
pathogen such as the bacterium Corynebacterium fascians (or A. tumefaciens) that produces
cytokinin which, in turn, causes stimulates lateral bud
development (branching). These results suggest that apical dominance may be related to
For example, when tobacco cells are infected with the
Ti-plasmid that has been modified to possess the heat shock promoter, a heat treatment
stimulates the cells to produce increased amounts of cytokinin. These plants exhibit less
apical dominance and remain green longer than non-heat treated controls. Thus, these
results support the conclusion that senescence and apical dominance are related to
G. Promote cell expansion
Cytokinins stimulate the
expansion of cotyledons. This is the basis for the classical bioassay. The mechanism is
associated with increased plasticity of the cell wall, not associated with acidification.
VIIII. Mechanism of action
binding sites (receptor) for cytokinin are known. These may be ribosomal proteins. Thus,
it is not too surprising that cytokinins have been shown to regulate protein synthesis.
01/07/2009 © Copyright by SG