Plants take shape in a
completely different way from animals. Plant cells, unlike animal cells, divide
and grow while encased within a strong cell wall. The cell walls of each cell
are permanently attached to the walls of the cells round about. The growth of a
plant is therefore more or less the growth of its cell-wall network, and that
is the level at which plant growth is controlled. The expansion of each cell is
driven by the osmotic pressure of the cell contents, but the rate and direction
in which it expands is determined by the extensibility of the cell wall.
Plant cell walls are
biocomposites, with a resemblance to fibreglass at the engineering level.
Cellulose fibres provide much of their strength. The direction in which the
cell grows is normally at right angles to the dominant orientation of
cellulose, and the rate of expansion depends on how well the non-cellulosic
matrix allows the cellulose fibres to move apart or slide past one another.
This is dependent on enzyme action as well as the nanometre-scale geometry of
the structure.
We are now beginning to
understand how the geometry works in wood, using a combination of mechanical
experiments, solid-state NMR and vibrational spectroscopy and polymer
modelling. By the time plant cells become recognisable as woody, though, they
have ceased to grow. Only growing cell walls contain the enzyme activity that
allows them to stretch without loss of strength. The aim of this studentship is
to extend these studies to growing cell walls
It is expected that most of
these experiments will be done on seedlings of sunflower, cucumber and the
experimental plant Arabidopsis, although it may be possible to adapt the for
tree species if the student prefers. Many of the experiments will be carried
out on an FTIR microscope and will require some manipulative skill. The
mathematical depth in which the project is developed depends very much on the
student concerned.