plant surface microbiology.pdf

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Ingrid Kottke
suberin digestion is shown here for the first time. It remains to be elucidated
if there is a direct signalling link between the digestion of the suberin and the
change of hyphal growth characters.After digesting the suberin layer and disrupting
the root cap cell wall, the hyphae come into direct contact with the live
cortical cells. There may then be additional signals involved in triggering the
hyphal growth changes at the root surface (Salzer et al. 1997, 2000).
8 Pectins in the Cortical Cell Walls of Nonmycorrhizal Long
and Mycorrhizal Short Roots
Methyl-esterified pectins were localized in the root cell walls of Picea abies
using the monoclonal antibody JIM7 (K. Roberts, John Innes Institute, Norwich
UK; Fig. 7d). No difference in the amount of pectin was found between
cortical cells in contact to hyphae and those lacking hyphal contact when
short roots and mycorrhizas were compared (Fig. 9). The cortical cells of noncolonized,
long roots, however, were significantly more densely marked by the
antibody (Fig. 9). There was no difference between both root types in the
amounts of methyl-esterified pectins in the cell walls of the meristems
(Fig. 9). During differentiation, the amounts of methyl-esterified pectins obviously
increase in cortical cell walls of long roots, but are reduced in cortical
cell walls of short roots. There is no indication for a digestion of pectins by the
hyphae as no changes in the amounts of pectins was found during early stages
of Laccaria amethystea- Picea abies mycorrhiza formation. Balestrini et al.
(1996) could not find any indication for polygalacturonase activity during
ECM development between Coryllus avellana and Tuber magnatum either.
The authors supposed de-esterification of the pectins according to increased
labelling of de-esterified pectins after mycorrhiza formation. In the case of P.
abies, however, immunogold labelling by the monoclonal antibody JIM5
showed low amounts of de-esterified pectins and labelling decreased from
inner cortical cells to outer cortical cells (not shown). High labelling of
methyl-esterified pectins was detected in roots of Daucus carota L. and Avena
sativa L. (Knox et al. 1990). This finding would support the view that fast
growing roots contain high amounts of methyl-esterified pectins in cortical
cells.
It is unclear whether the amounts of methyl-esterified pectins have any
influence on mycorrhiza formation. There is too little knowledge on the
importance of methyl-esterified pectins for stability or plasticity of cell walls
and cell-to-cell adhesion (Liners et al. 1994). Previously, reduction of the cell
wall-bound ferulic acid, linking pectic substances in the cell wall matrix, was
found to occur during mycorrhiza formation of Picea abies, Larix decidua and
Arbutus menziesii (Münzenberger et al. 1990, 1995, Weiss et al. 1999). Less
rigid cortical cell walls were considered a prerequisite for intercellular hyphal
penetration during Hartig net establishment.