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Researchers at the Max Planck Institute for Plant Breeding Research (MPIZ), Cologne, report on the role of physical pressure exerted by an invading fungus. This pressure serves as a signal to trigger plant defense responses (PNAS Vol. 95, No 14, 7 July 1998).
Plants serve as food not only for animals and
insects, but also for a variety of microbes. That's why
microbes, such as bacteria, nematodes, and fungi constantly
attack plants. To protect themselves from being devour plants
have developed broad-acting defense mechanisms. To identify
such threats, plants must be able to perceive signals from a
large variety of potential pathogens to trigger this general
"nonhost" resistance. In recent years, a set of
biological signal molecules (referred to as elicitors) from
different pathogens, have been identified and characterized
with respect to the defense responses induced in plant cells.
Basic knowledge of plant-defense mechanisms can help to
improve crop plants so that they will be better protected
against pathogens.
Scientists in the MPIZ Department of Biochemistry, headed by
Klaus Hahlbrock, use cell-suspension cultures of parsley
(Petroselinum crispum) as a system for studying the nonhost
resistance responses to Phytophthora sojae and Phytophthora
infestans, two important fungal pathogens. Recently,
coworkers of this department purified a glycoprotein from the
mycelium cell wall of P. sojae that was shown to act as an
elicitor in this nonhost system. Incubation of parsley cells
with the elicitor resulted in dramatic biochemical changes in
the affected plant cells, e.g., the generation of reactive
oxygen intermediates (ROI), increased ion fluxes across the
plasma membrane, and changes in gene activity. Researchers
observed the same processes upon infecting parsley cells with
P. infestans.
However, these changes to the physiological status of the
cells do not reflect the entirety of defense responses
induced by fungal infection. Morphological changes in
infected cells can also be observed. At very early stages of
the infection process, before the fungal hypha (a thin tube
formed by the fungus to grow and to invade plant material)
has completely penetrated the cell wall and formed
intracellular structures, increased motion of cell contents
(cytoplasm) towards the penetration site can be detected in
plant cells, with the nucleus moving towards this site. The
cell deposits cell-wall material beneath the penetration site
as a physical barrier against penetration. If all these
defense reactions are not sufficient to prevent further
penetration of the fungus, the plant cell commits suicide, a
process known as hypersensitive cell death. This can be
observed under the microscope as a rapid and sudden collapse
of the cell contents around the intracellular fungal
structure. Furthermore, the plant cell releases toxic
compounds that possibly kill both the plant cell and the
fungus. However, in the parsley/P. infestans system,
morphological changes have never been observed when the cells
are treated with the elicitor molecule alone.
The question therefore arises as to whether additional
signals other than the elicitor are required to induce of the
complete defense response. The scientists speculated that
attempted penetration by the pathogen generates not only
chemical signals represented by the elicitor, but also
mechanical signals resulting from the contact of the fungal
hypha to the plant cell wall and that a combination of both
these signals must be perceived by the plant cell for it to
display the full complement of defense reactions.
Dr. Sabine Gus-Mayer, a postdoctoral fellow of the Deutsche
Forschungsgemeinschaft at the MPIZ, tested this hypothesis.
In her study, she replaced the penetrating fungus with gentle
local mechanical stimulation of the cells using a tungsten
needle of the same diameter as a fungal hypha (2-5 µm). As
with the fungal infection hypha, by itself this local
mechanical stimulus induced the translocation of cytoplasm
and nucleus to the site of stimulation. Interestingly, some
of the biochemical defense responses were also observed. Only
a few minutes of local mechanical stimulation were required
to induce the intracellular generation of ROI (see figure),
in the same way as was previously demonstrated to occur by
fungal infection or elicitor treatment. Surprisingly, some,
but not all, elicitor-responsive plant genes were expressed
upon mechanical stimulation.
These results provide evidence, that a fungal hypha need not
necessarily penetrate a cell to trigger the plant's defense
machinery. The physical touch exerted by an invading fungus
is already sufficient to be sensed by the plant cell as a
signal to instigate its own defense response. Concerning the
signaling involved in the nonhost resistance response of
parsley to P. sojae or P. infestans , these results lead to
the following conclusions. At an initial stage of attempted
fungal colonization, probably prior to penetration of the
plant cell wall, the perception of a local mechanical signal
from the emerging infection tube is sufficient to induce the
generation of ROI, cytoplasmic rearrangements, and the
expression of some infection-related genes. To trigger local
cell-wall thickening, the fungal infection tube may have to
penetrate the plant cell wall. As soon as the fungal
infection tube is in contact the plasma membrane of the plant
cell, the elicitor binds to its specific plant cell receptor,
thereby activating additional biochemical defense reactions.
Thus, the combination of physical and chemical stimuli are
likely to be responsible for triggering intracellular
rearrangements as well as the various biochemical changes
involved in gene activation and product accumulation.
However, other signals are apparently required for induction
of cell-wall thickening and the hypersensitive cell death.
Journal
Proceedings of the National Academy of Sciences