Monday 23 January 2012
CYTOKININ
Cytokinins (CK) are a class of plant growth substances (phytohormones ) that promote cell division , or cytokinesis , in plant roots and shoots. They are involved
primarily in cell growth and differentiation , but also affect apical dominance, axillary bud growth, and leaf senescence. Folke Skoog discovered their effects using coconut milk in the 1940s at the University of Wisconsin–Madison. [1] There are two types of
cytokinins: adenine-type
cytokinins represented by kinetin , zeatin, and 6- benzylaminopurine , and phenylurea-type cytokinins
like diphenylurea and
thidiazuron (TDZ). Most
adenine-type cytokinins are
synthesized in roots. [2]Cambium and other actively dividing tissues also synthesize cytokinins. [3] No phenylurea cytokinins have been found in plants.[4] Cytokinins participate in local
and long-distance signalling,
with the same transport
mechanism as purines and nucleosides.[5] Typically, cytokinins are transported in the xylem .[2] Cytokinins act in concert with auxin , another plant growth hormone.[2] Mode of Action The ratio of auxin to
cytokinin plays an important
role in the effect of cytokinin
on plant growth. Cytokinin
alone has no effect on parenchyma cells. When cultured with auxin but no
cytokinin, they grow large
but do not divide. When
cytokinin is added, the cells
expand and differentiate.
When cytokinin and auxin are present in equal levels, the
parenchyma cells form an
undifferentiated callus. More cytokinin induces growth of shoot buds, while more auxin induces root formation.[2] Cytokinins are involved in
many plant processes,
including cell division and
shoot and root
morphogenesis. They are
known to regulate axillary bud growth and apical
dominance. The "direct
inhibition hypothesis" posits
that these effects result from
the cytokinin to auxin ratio.
This theory states that auxin from apical buds travels down
shoots to inhibit axillary bud
growth. This promotes shoot
growth, and restricts lateral
branching. Cytokinin moves
from the roots into the shoots, eventually signaling
lateral bud growth. Simple
experiments support this
theory. When the apical bud is
removed, the axillary buds
are uninhibited, lateral growth increases, and plants
become bushier. Applying
auxin to the cut stem again inhibits lateral dominance.[2] While cytokinin action in vascular plants is described as pleiotropic, this class of plant hormones specifically induces
the transition from apical
growth to growth via a
three-faced apical cell in moss protonema. This bud induction can be pinpointed to differentiation of a specific single cell, and thus is a very specific effect of cytokinin. [6] Cytokinins have been shown
to slow aging of plant organs
by preventing protein breakdown, activating
protein synthesis, and
assembling nutrients from nearby tissues. [2] A study that regulated leaf senescence
in tobacco leaves found that
wild-type leaves yellowed
while transgenic leaves remained mostly green. It
was hypothesized that
cytokinin may affect
enzymes that regulate protein synthesis and degradation. [7] Biosynthesis Adenosine phosphate-
isopentenyltransferase (IPT) catalyses the first reaction in the biosynthesis of isoprene cytokinins. It may use ATP, ADP, or AMP as substrates and may use dimethylallyl diphosphate (DMAPP) or hydroxymethylbutenyl
diphosphate (HMBDP) as prenyl donors .[8] This reaction is the rate-limiting step in cytokinin biosynthesis.
DMAPP and HMBDP used in
cytokinin biosynthesis are
produced by the
methylerythritol phosphate pathway (MEP). [8] Cytokinins can also be
produced by recycled tRNAs in plants and bacteria.[8][9] tRNAs with anticodons that start with a uridine and carrying an already-
prenylated adenosine adjacent
to the anticodon release on
degradation the adenosine as a cytokinin. [8] The prenylation of these adenines is carried out
by tRNA- isopentenyltransferase .[9] Auxin is known to regulate
the biosynthesis of cytokinin. [10] Uses Because cytokinin promotes
plant cell division and growth,
produce farmers use it to
increase crops. One study
found that applying cytokinin
to cotton seedlings led to a 5– 10% yield increase under drought conditions. [11] Cytokinins have recently been
found to play a role in plant
pathogenesis.
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