Biology Reference
In-Depth Information
FIGURE 6.4
The processes involved in production of cortical microtubules (red) in plant cells. Nascent
microtubules are cut off from their organizing centre and undergo net depolymerization at their '
' ends and net
polymerization (with occasional catastrophic collapse) at their '
' ends, so that they move away from the place of
their birth. They become cross-linked to each other by microtubule-associated proteins and cross-linked to the
membrane by phospholipase D.
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with 1-butanol, on the other hand, disrupts the organization of cortical microtubules so that
they have oblique and longitudinal orientation as well as transverse, in zones of the plant
where elongation is meant to take place.
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This disorganizes the morphology of the resultant
plant.
Linkage via a signalling protein makes functional sense because the alignment of cellulose
microfibrils is known to be responsive to hormonal cues. For example, the growth-associated
hormone giberellic acid, which causes elongation of stems,
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induces microtubules to become
transverse and so to cause cell elongation within the stem.
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The wound-associated Ethylene,
on the other hand, causes microtubules to shift from a transverse to a longitudinal orienta-
tion.
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This presumably allows cells to expand sideways to refill the wound space. Cessa-
tion of cell elongation caused by reduction of turgor induced experimentally by a raise in
environmental concentration of KCl or sorbitol also causes a reorientation of microtubules
from transverse to longitudinal, although in this case reorientation follows cessation of
growth and not the other way round.
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During these reorientations, cells are never without
microtubules, suggesting that the reorientation takes place gradually, rather than by scrap-
ping the existing cytoskeleton and building a new one de novo. Detailed confocal observations
have been made of plant cells that are undergoing a transition from transverse to longitudinal
orientation and have been injected with fluorescent tubulin. These show that, while some
destruction of microtubules does take place, many existing tubules survive to be reorientated
to line up with new microtubules that have formed in the new orientation.
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