plant surface microbiology.pdf

18 Mycorrhizal Development and Cytoskeleton 299
ferentiated plant parenchyma cell with full turgor pressure could cause
mechanical stress (Ko and McCulloch 2000; Stamatas and McIntire 2001) at
the plant cell membrane, perhaps associated with chemical signals from the
fungus to the plant cell. These signals could cause the depolymerization of
MTs at the plasma membrane surrounding the cell and direct MT repolymerization
at the expanding plasma membrane surrounding the growing hyphae,
together with altered gene expression of the invaded parenchyma cells. In line
with this idea is the observation that in transgenic tobacco the GUS expression
under the promoter of the maize tubulin gene Tuba3 increased in differentiated
cortical cells infected by endomycorrhizal fungus (Bonfante et al.
1996). The accumulation of Tuba3 transcripts was similarly observed to
increase in maize cortical cells during the invasion of the cells by an endomycorrhizal
fungus. Cell wall material is deposited into the extracellular space
between the plant cell membrane and fungal hyphae (Peterson et al. 1996),
which could require the presence of MTs as these are required for cell plate
formation in meristems (Lloyd and Hussey 2001), primary wall formation in
elongating cells (Wasteneys 2000), and secondary wall thickenings in differentiating
cells (Chaffey et al. 2000). In addition, proteins necessary for the
nutrient exchange between the symbionts probably are synthesized and
transported to the perifungal membrane (Rosewarne et al.1999; Hahn and
Mendgen 2001), which could also require transport along the MTs.
The distribution of MFs has also been studied in endomycorrhiza formed
in tobacco roots (Genre and Bonfante 1998) and in protocorm cells (Uetake
and Peterson 1997). In noninfected cells, MFs appeared to have the distribution
reported for parenchyma cells in a large number of plants investigated
(Staiger 2000), with thin and thick MF cables crossing the cell cytoplasm. At
fungal invasion, reorganization of MFs were observed in cortical cells of
tobacco, in which the cables disappeared and MFs seemed to become tightly
associated with the plasma membrane surrounding the arbuscular branches
(Genre and Bonfante 1998). In protocorm cells, no clear reorganization of
MFs was observed, but the distribution of MFs was comparable to that in
uninfected cells (Uetake and Peterson 1997), which was a result quite different
from the reorganization of MTs in the protocorm cells at fungal invasion. The
different behavior of MFs in tobacco and protocorm cells at fungal invasion is
suggested to be due to the physiological difference between the endomycorrhizal
and orchid endosymbiotic fungus (Genre and Bonfante 1997), or it
could result from differences in the processing of the cells for confocal microscopic
investigation (Uetake and Peterson 1997). In tobacco root cells, the
accumulation of MFs in close association with the plasma membrane surrounding
the fungus is suggested to be due to the involvement of the actin
cytoskeleton in localization of proteins necessary for membrane transport
and signal transduction between symbionts (Genre and Bonfante 1997). Noteworthy
is that at the invasion of the plant cell by the endomycorrhizal fungus,
the plant cell nucleus moves from the periphery of the cell to the center and