Because
there is no cell movement during plant development, cell expansion
is one of the key parameters that determine the ultimate form of plant
organs. The organization of the cellulose microfibrils in the cell
wall plays an important role in differential expansion. Mutations
in the
Cobra (COB) gene, which we previously showed affects the
orientation of cell expansion in the root, also reduce the amount
of crystalline cellulose in cell walls in the root growth zone. We
identified the COB gene by map-based cloning and found that it contains
a sequence motif found in proteins that are anchored to the extracellular
surface of the plasma membrane through a glycosylphosphatidylinositol
(GPI) linkage
(Schindelman et al., 2001). In animal cells, this lipid
linkage is known to confer polar localization to proteins. By immunolocalization
we detected the COB protein predominately on the longitudinal sides
of root cells in the zone of rapid elongation. Moreover, COB RNA levels
are dramatically upregulated in cells entering the zone of rapid elongation.
Based on these results, it appears that
COB plays an important role in the transition from radial to longitudinal
expansion. |
