A comparative structural analysis of direct and indirect shoot ...

52 M. Volgger et al.
Fig. 1 Zones of a growing
root hair: basal zone close to the
rhizodermis (a), vacuolation
zone (b), tip zone with
clear zone at the very front (c).
Bar 10 μm
pectins, hemicelluloses and polygalacturonic acid (McNeil
et al. 1984; VarnerandLin1989) fuse with the existing
plasma membrane and deposit their content towards the
outside of the growing cell by exocytosis. At the same time,
excess membrane material is recycled by endocytotic
processes (Ovečka et al. 2005). In addition, some callose
and cellulose depositions were observed after staining with
aniline blue (callose) and calco fluor white (cellulose) (Strobl
and Lichtscheidl 2004). The subapex contains larger cell
organelles and compartments like endoplasmatic reticulum
(ER), mitochondria and Golgi stacks. The cell walls on the
flanks of the root hairs are thicker and less plastic than at the
dome. At the sides, cellulose microfibrils form primary and
then secondary cellulose textures that support and restrain the
growing root hair (Schröter and Sievers 1971; Schnepf et al.
1986). (2) In the vacuolation zone, the central vacuole fills
most of the cell and, except for some cytoplasmic strands, the
cytoplasm is pushed towards the side walls. Here, organelles
show a rotational streaming. It is postulated that turgor
pressure of the vacuole is the driving force for cell expansion
in general and tip growth in particular. (3) The basal zone of
the root hair lies close to the rhizodermis. The vacuole
completely fills also this part of the root hair, and the thin
layer of cytoplasm is pushed towards the cell wall.
Cell wall formation under reduced turgor
Turgor pressure of the vacuole can be reduced under osmotic
stress conditions, as induced by freezing, drought or soil
salinity. Are tip growth and cell wall formation still possible in
a turgorless state? Here, this situation is simulated by
plasmolysis (de Vries 1877; Oparka1994). The hypertonic
(plasmolytic) solution osmotically extracts water from the
vacuole up to a point of complete turgor loss and, depending
on the concentration of the solution, can even cause the
detachment of the living protoplast from the cell wall
(Schnepf et al. 1986; Lang-Pauluzzi 2000). Thin, threadlike
connections have been observed between the protoplast and
the cell wall of plasmolysed cells. These connections are
named after Hecht (1912) and are referred to as Hechtian
strands (Sitte 1963; Schnepf et al. 1986; Oparkaetal.1996;
Bachewich and Heath 1997; Lang-Pauluzzi and Gunning
2000;Langetal.2004;DeBoltetal.2007). At the cell wall of
plasmolysed plant cells, Hechtian strands pass into a cortical
network, the Hechtian reticulation (Pont-Lezica et al. 1993)
that reminds of cortical endoplasmatic reticulum. Studies by
Oparka et al. (1993, 1996) on plasmolysed onion epidermal
cells demonstrated that, although the cortical ER remained
strongly attached to the cell wall, it is nonetheless bounded by
the plasma membrane. Together, Hechtian strands and
Hechtian reticulum conserve the surface area of the plasma
membrane in plasmolysis/deplasmolysis cycles (Oparka et al.
1994). Hechtian strands may form the basis for freezing
tolerance in cold-hardened plants: as the temperature of the
cell is lowered, ice grows outside the cell by the withdrawal
of water from inside. Cold-hardened cells have been shown
to form more Hechtian strands than non-hardened cells
(Johnson-Flanagan and Singh 1986; Singh et al. 1987; Buer
et al. 2000). The authors conclude that membrane conservation
by Hechtian strands is a means to successfully retrieve
membrane material during expansion in deplasmolysis.
In the present study, the impact of plasmolytic solutions
on cell wall formation was tested in living root hairs of the
crop plant Triticum aestivum. The adaptation to osmotic
stress situations and the ability to form new root hairs under
these conditions were analysed by long-term culture in
plasmolytic solutions. We provide clear evidences for
continuation of the cell wall formation process in root hairs
under turgorless conditions, although no further tip growth
is possible. Together with formation of new root hairs in
higher osmolarity, it shed more light to the adaptation
abilities of tip-growing root hairs.
Materials and methods
Plant material
Living roots and root hairs of T. aestivum (Poaceae) were
prepared according to the protocol of Ovečka et al. (2005).