plant surface microbiology.pdf

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Ingrid Kottke
nificant structural differences exist between the two root types and may be
even more important for mycorrhiza formation or failure.
Fast growing long root tips are covered by a conspicuous root cap consisting
of non-suberized, rapidly decaying cells (Fig. 2a). Dormant long roots and
short roots have in common that the root cap cells are few and become suberized
(Fig. 2b, c). This type of root cap cells is also found in root buds even
when emerging from below a hyphal sheath (Fig. 2d). The process was termed
metacutization (“Metakutinisierung” Müller 1906) and was found to occur in
a multitude of gymnosperm and angiosperm perennial species irrespectively
of the epidermal or cortical cell type on the root surface of these two taxonomic
groups (Plaut 1918). It was described in detail from light microscope
studies of Fagus sylvatica L. (Clowes 1954), Betula alleghaniensis Britt., Alnus
crispa (Ait.) Pursh, Eucalyptus pilularis Smith (Massicotte et al. 1986,
1987a,b), Abies procera Rehder (Wilcox 1954), Picea abies [L.] Karst. (Kottke et
al. 1986) and Pinus spp. (Hatch and Doak 1933).
3 A Cuticle-Like Layer on the Surface of Short Roots
Ultrastructural investigations yielded further details on the fate of the suberized
root cap cells of short roots. Young root cap cells become suberized by a
lamellar layer imposed on the inner side of the cell walls (Figs. 3a, b, 4a). As
suberin is only weakly stained by osmium and lead, a suberin layer appears
electron-translucent (Sitte 1975; Kottke and Oberwinkler 1990). Lamellae are
visible in the suberin layer if waxes are present additionally (Fig. 4a; Sitte
1975). The suberin layer progressively increases with ageing of the cells.
Finally, these cells accumulate phenolic substances, become impermeable and
moribund (Fig. 3a, b). Short roots proliferate slowly under the root cap cells
(Fig. 2 c) and remain covered by their residues (Fig. 4b, c). The dead root cap
cells progressively detach from the root (Figs. 3b, 4b, c), but the innermost,
suberized root cap cell walls remain tightly connected to the root cortical cell
layer (Figs. 3 c, 4b, c). Thus, the suberin layer of the innermost root cap cells
covers the whole surface of short roots, similar to a fine cuticle. During the
elongation of the root the suberin layer is thinned out (Fig. 4e). It fades away
on the root hairs (Fig. 3d) covering only the root hair base (Fig. 4d). At the
most proximal parts of the rootlets, the suberin layer may also fade away on
the surface of cortical cells (Fig. 5a, c), but the cell junctions remain tightly
covered by the suberin layer all along the rootlet (Fig. 5a, b). The whole situation
is illustrated by a scheme (Fig. 6).
The suberin layer is covered by a thin layer of electron-dense material. Phenols
are strongly stained by osmium and lead and thus appear electron-dense.
It is not always easy to discern if this material originates from insoluble phenolic
residues of the former vacuole or from cell walls of the deceased root cap
cells as vacuoles and moribund cell walls of root cap cells may contain high