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Auxin - structure
Cycle of the
tobacco cell culture VBI-0
Synchronization of cell division by auxin
Auxin controls the organization of actin filaments
Auxin transport during gravi-tropism: A coleoptile releases at
the basal end auxin that can be collected and later measured in
small agar blocks. As long as it stands upright, equal amounts of
auxin are collected in both blocks. When the coleoptile is tilted,
twice as much of auxin will arrive in the lower as compared
to the upper block. This result was first predicted by Cholodny (University
of Kazan, Russia) and a few years later confirmed experimentally
by Dolk.
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Auxin - an ambiguous
molecule
Auxin ist probably one of the famous molecules in plant biology.
There is almost no event in the life of a plant without auxin
being involved. However, this molecule is very simple. How can
such a simple signal control events that are so complex and
diverse? One century of research has been dedicated to this
molecule - this produced more new questions than answers to old
questions. There is almost no aspect of auxin research that is not
under dispute. An exciting field, that quite recently has become
very dynamic.
Why we are interested in auxin
Plants
must respond very flexibly to their environment. Nevertheless they
are able to coordinate organs and cells. This ability is called
pattern formation and auxin seems to play a central role as
coordinating signal in this context. For instance, it could be
shown recently that phyllotaxis, i.e. the ordered pattern of
leaves and flower organs, is regulated through competition of leaf
primordia for free auxin.
The same
is true for the pattern of leaf veins that is individual for each
given leaf, but at the same time "typical" for the respective
species. Auxin is transported directionally and this auxin
transport defines, what is "top" and "down" in a plant and how
a pattern is established.
However,
reality is more complex than that - whereas a given cell can
respond strongly to auxin, its sister cell can respond weakly or
does not respond at all (so called competence). This auxin
competence is not a constant, but is regulated differentially
depending on development, tissue and environment.
Patterning and Auxin Transport
Since it
is very difficult to study the formation of leaf veins in real
leaves, we work with tobacco cell cultures that can be maintained
in liquid culture. These cultures derive from cells that are able
to produce leaf veins. The decisive signal is auxin that is
transported through the cell.
These cell
cultures can divide after addition of auxin and develop into cell
files consisting of 4-10 individual cells. The divisions within a
file does not occur randomly, but is coordinated. We could show
that this coordination is mediated by auxin. A cell that prepares
for division signals to her neighbour and this neighbour
accelerates its progression through the cycle and initiates
division earlier.
This
system thus represents a very simple multicellular organism where
we can study, how inidividual cells are coordinated into an entity
- a basic question of developmental biology. In these cells we can
analyze the cellular and molecular aspects of multicellular
coordination better than in a "real" leaf.
More...
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Lecture (in German) on our work on patterning at the Biologentag
in Karlsruhe (29.09.2006) in
pdf-format
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Publication, where we describe our cell system and auxin-dependent
coordination:
Campanoni
et al. 2003
Patterning and Auxin Competence
Tropism
represents a classical example for patterning through auxin
transport. Plant organs bend in response to a lateral light
stimulus (phototropism) or bend upwards, when they are removed
from the vertical (gravitropism). Already at the beginning of the
last century, this curvature was explained in terms of a growth
substance that is transported across the stimulated organ. This
idea was proposed independently and almost simultaneously by two
researchers (Cholodny in Russia and Went in the Netherlands) and
shortly after culminated in the discovery of auxin. We have
revisited this classical Cholodny-Went Theory and asked, what
happens, when rice coleoptiles are flooded with auxin during
gravitropic stimulation. We expected according to the
Cholodny-Went-Theory that these coleoptiles should not be able to
respond to a gravity stimulus by bending. To our surprise they
bend perfectly. A closer investigation revealed that gravity
induces a differential auxin competence in the upper and the lower
flank of the stimulated organ. Whereas auxin competence is
strongly reduced in the upper flank, it is elevated in the lower
flank. Even when auxin is present equally (as in our flooding
experiments), the cells in the upper flank will grow less than
those in the lower flank. Thus, the gravitropic stimulation
induces a pattern of differential competence. It is still unknown,
how this happens, but we could define the plant hormone jasmonic
acid as a key player in this process.
More...
© 2007 Peter Nick, Botanisches Institut 1, alle
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Letzte Änderung Freitag, 12. Januar 2007 |
