Auxin-Induced Cell Elongation in Grass Coleoptiles 2.cdr

Auxin-Induced Cell Elongation in Grass Coleoptiles:
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A Phytohormone in Action
U. Kutschera, Institut für Biologie, Universität Kassel, Heinrich-Plett-Str. 40, D-34109 Kassel, Germany. E-mail: kut@ui-kassel.de
INTRODUCTION
Seventy years ago, F. Went and K.V. Thimann postulated that cell
elongation in grass coleoptiles is regulated by auxin (indole-3-acetic acid,
IAA) provided by the organ tip: "without auxin, no growth" (Fig. 1). This
classical concept, which implies that IAA is the growth-controlling factor
in the coleoptile, has been questioned. However, recent analyses of the
IAA-growth-relationships in maize coleoptiles have corroborated the
classical view. The basic results can be recapitulated as follows: 1. In the
intact organ, the endogenous auxin concentration is maintained at a level
that is approximately 1/2 saturation; 2. Experiments with a specific auxin
transport inhibitor have shown that cell elongation is limited and hence
controlled by IAA supplied by the coleoptile tip.
In spite of these unequivocal results, the precise mechanism by
which the phytohormone auxin causes the mechanical loosening of the
growth-controlling outer epidermal wall is unknown (Fig. 2).
Fig. 1. Time-course of IAA-induced growth of rye
coleoptile segments incubated in water (± IAA, 10
µmol/l) that was either aerated ( + O2) (A) or treated
with nitrogen gas ( N2) (B). Arrows indicate addition
of IAA at time zero. - 1 h: cutting of the segments.
Fig. 2. The growth-controlling outer epidermal wall
(OEW) of a maize coleoptile (left) and diagram of a model
for wall extension in vivo (right). Cell-wall loosening
(relaxation of the wall stress Pw) and elongation
(deformation) are dependent on turgor pressure (Pv) and
cellular respiration (O2-dependent metabolism, i. e.,
supply of ATP).
THE PROTEIN SECRETION THEORY
Fig. 3. Dark-grown 3-day-old rye seedling (left) and
ultrastructure of a representative epidermal cell
(transmission electron micrograph of a cross section)
(right). Two osmiophilic particles are present at the outer
epidermal wall (arrow heads). Note that osmiophilic
material is also detectable in a Golgi-vesicle within the
cytoplasm (arrow). C = cytoplasm, D = dictyosome,
P = plasma membrane, V = vacuole, W = outer epidermal
wall. Bar = 300 nm.
Fig. 4. Relationship between the rate of growth and the
average number of osmiophilic particles per cell and
cross section during IAA-induced elongation of rye
coleoptile segments. Time zero: 1 h after cutting. The
arrow denotes the addition of IAA (10 µmol/l). (Data of
H. G. Edelmann, M. Fröhlich and U. Kutschera).
It is obvious that the putative 'wall-loosening factor', secreted by the epidermal
cells into the walls they surround, must fulfil three criteria: 1. It should operate
in the intact, growing coleoptile, i. e. , the metabolically controlled process
must be detectable or measurable in situ; 2. Upon excision and depletion of
endogenous auxin, the 'wall loosening factor' should disappear in parallel with
the decline in coleoptile elongation; 3. After incubation of excised segments in
exogenous IAA, a wall-relaxation process is initiated in temporal correlation
with the growth response: the 'loosening agent' should rapidly reappear upon
addition of auxin. The ultrastructural evidence summarized here (Figs. 3 - 5)
supports the 'protein secretion theory of auxin action' proposed several years
ago: auxin causes cell-wall loosening by rapid stimulation of Golgi-secretion
and the incorporation of glycoproteins. One major weakness of this concept
was the lack of a quantitative relationship between the number of
proteinaceous osmiophilic particles at the OEW and the IAA-induced growth
response. The recent results by H. G. Edelmann and U. Kutschera,
summarized here in a supplemented version (Fig. 4), fill this gap in our
knowledge. Osmiophilic secretion products accumulate at the OEW in the
intact (growing) coleoptile (Fig. 3); they largely disappear in auxin-depleted
segments and reappear after addition of IAA in parallel with the elongation
response. Moreover, the intensity of Golgi-secretion (i. e., number of particles
per cell and cross section) was significantly lower in situ compared with the
optimum reached in IAA (3.3 versus 5.2 granules, respectively, see Figs. 3 and
4). The corresponding growth rates showed a similar quantitative relationship
(in situ: 0.6 mm/h; + IAA: 1.1 mm/h). This finding accords with (qualitative)
observations with maize coleoptiles and the fact that, in the intact organ, the
endogenous IAA concentration corresponds to about one-half saturation.
CONCLUSIONS
The data summarized here demonstrate that the protein secretion model is no
longer a mere hypothesis: it has developed into a theory of IAA action that can be
described as follows (Fig. 5). Under the influence of a growth-promoting
concentration of endogenous or exogenous (applied) IAA a secretion process is
initiated. Proteinaceous wall material is carried to the outer cell surface by secretory
vesicle traffic via the Golgi-apparatus. After infiltration into the peripheral organ
wall (exocytosis/intussusception), this 'intramural lubricant' causes wall loosening
and hence permits the slippage of load-bearing polymers. As a result, an extensible
OEW is created. Upon turgor-driven deformation, the elongated wall is fixed in its
new position (Fig. 2). The next loosening/deformation-cycle is initiated in an
metabolically controlled, ATP-dependent way, causing the growth of the coleoptile.
The exact content of the osmiophilic particles, which represent the
proteinaceous 'wall loosening factor' of the coleoptile, has not yet been identified.
Finally, the wall stiffening reaction (i. e., fixation of the extended polymeric
network in its new position) is still a matter of debate. In other words, the question as
to the biochemical basis of secretion-mediated wall loosening is open: how does the
proteinaceous 'intramural lubricant' mediate the slippage of load-bearing polymers?
However, based on recent experimental evidence, it is reasonable to suggest
.
that hydroxyl radicals ( OH) may be in some way involved (wall-loosening), and
peroxidase-dependent cross-linking reactions follow this weakening of the
polymeric network (fixation of the extendend wall). Both 'ingredients' occur in the
OEW of auxin-treated coleoptiles. In conclusion, this analysis and summary of the
most recent findings on the mechanism of auxin-induced coleoptile elongation
shows that the cytological basis of this physiological process has now been
elucidated (Fig. 5).
Fig. 5. Cytological model of auxin (IAA)
action in the grass coleoptile, illustrated
in a scheme of a longitudinal section
through the peripheral cells. In the
cytoplasm of the epidermis glycoproteins
are synthesized, secreted via the Golgiapparatus and incorporated into the
peripheral organ wall. The infiltration of
this proteinaceous material ('intramural
lubricant') causes stress relaxation and
hence mechanical loosening of the wall.
Cyto = cytoplasm, OEW = outer
epidermal wall, Pv = turgor pressure,
Pw = wall stress.
References
Kutschera, U. (1994) The current status of the acid-growth hypothesis. New Phytol. 126, 549 - 569.
Kutschera, U. (2000) Cell expansion in plant development. Rev. Brasil. Fisiol. Veg. 12, 65 - 95.
Kutschera, U. (2001) Stem elongation and cell wall proteins in flowering plants. Plant Biol. 3, 466 - 480.
Kutschera, U. (2003) Auxin-induced cell elongation in grass coleoptiles: a phytohormone in action. Curr.
Topics Plant Biol. 4, 27 - 46.
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Abbreviated version of the Review Article Kutschera (2003)

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