Download The mechanics of cell separation during plant organ abscission Prof

The mechanics of cell separation during plant organ abscission
Prof. Jerry Roberts
School of Biosciences, University of Nottingham
The shedding of plant organs such as leaves, flowers and fruit is the culmination
of a highly co-ordinated sequence of cellular and molecular events that take place
at discrete positions termed abscission zones. The final process results in the
breakdown of the pectin-rich middle lamellar region that cements cells together
and is thought to be mediated by enzymes secreted into the cell wall by the
abscission zone cells. Our studies have shown that in flowers of Arabidopsis the
abscission zone comprises two layers of cells. After shedding one of these is
retained by the parent plant whilst the other remains at the base of the petal,
sepal or anther filament that is shed. We know that both layers of cells express
genes encoding pectin degrading enzymes.
Problem 1
What events are in operation to bring about separation between the two layers of
abscission zone cells so that one layer remains on either side of the fracture
surface? Perhaps the simplest explanation is that shown in Figure 1 where the
abscission zone cells secrete hydrolytic enzymes equally in all directions and that
twice as much enzyme accumulates at the site between the two AZ layers which
precipitates separation at this site. Is this explanation feasible and what would
happen between cells in a layer?
Figure 1
Secretion of wall degrading
enzymes
Cell separation
Another possibility is that the two layers of cells secrete the enzymes in a
directional way towards each other. This would be particularly exciting from a
mechanistic understanding if this hypothesis was correct.
Problem 2
We believe that the enzymes secreted by the abscission zone cells are able to
degrade cellulosic and pectinaceous wall material but that lignified tissues are not
solubilized. Lignin is deposited in the walls of dead cells and is the major
component of woody materials. The degradation of pectin is a particular challenge
for the Bioenergy industry. The vascular tissue of plants comprises lignified xylem
vessels and this strand of dead material has to be severed for organs such as
leaves and fruit to be shed. The accepted wisdom is that lignified tissues are
stretched and ultimately break by the hydraulic expansion of abscission zone cells
once the cell walls that surround them have been softened (Figure 2).
Figure 2
Vascular tissue
Is this possible and what force would need to be established to achieve this?
Would some breakdown of the lignified tissues be necessary to sever the vascular
trace?
Answers to these questions would provide important information that would help
direct research on abscission and in particular would focus efforts on
understanding the mechanisms regulating polarity of secretion in abscission zone
cells and the cocktail of enzymes necessary to bring about cell separation.
References
GONZALEZ-CARRANZA, Z.H., WHITELAW, C.A., SWARUP, R. and ROBERTS, J.A.
(2002) Temporal and spatial expression of a polygalacturonase during leaf and
flower abscission in Brassica napus and Arabidopsis thaliana. Pl. Physiol. 128,
534-543.
ROBERTS, J.A., ELLIOTT, K.A. and GONZALEZ-CARRANZA, Z.H. (2002)
Abscission, dehiscence, and other cell separation processes. Ann. Rev. Plant Biol.
53, 131-58.
SEXTON, R. and REDSHAW, A.J. (1981) The Role of Cell Expansion in the
Abscission of Impatiens sultani leaves Annals of Botany 48, 745-756.