plant surface microbiology.pdf

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Lukas Schreiber, Ursula Krimm and Daniel Knoll
the needle and the leaf surface wetting properties quantified by contact angle
measurements (Fig. 4C, D) was always in parallel with a significant increase in
the colonisation of the needle and leaf surfaces with epiphyllic microorganisms
as seen in scanning electron microscopy. However, changes in the qualitative
and quantitative wax composition, measured by gas chromatography,
were not at all correlated with the changes in the wetting properties of the leaf
surfaces (Schreiber 1996; Knoll and Schreiber 1998).
From this, it is evident that leaf surface microorganisms have the ability to
significantly change leaf wettability by altering the physico-chemical properties
of leaf surfaces. This is probably an important ecological strategy of epiphyllic
microorganisms improving the living conditions in their environment.
Increased wetting will increase the water availability in the leaf surface,
which in turn is highly favourable for the microorganisms living there. Furthermore,
increased wetting will also more easily lead to the formation of thin
water films, which is necessary in order to dissolve substances leaching from
the apoplast to the leaf surface. As a consequence, the availability and the
amount of nutrients in the phyllosphere will increase as well, which again is
favourable for epiphyllic microorganisms.
5 Interaction of Bacteria with Isolated Plant Cuticles
It is generally believed that plant cuticles form more or less impermeable
mechanical barriers for bacteria (Agrios 1995). Whereas fungi may have the
ability to penetrate the cuticle using extracellular enzymes (Schäfer 1998), for
bacteria an infection of the leaf tissue only seems to be possible via stomates
or hydathodes forming natural openings or via artificial openings like cracks
caused by injuries. In order to test this hypothesis, isolated cuticular membranes
from different plant species were mounted in transpiration chambers
and cuticular water permeability was quantified as a measure of the effect of
microorganisms on leaf surface barrier properties.
Cuticular water permeability of selected species (Vinca major L., Hedera
helix canariensis L. and Prunus laurocerasus L.) was measured before and
after inoculation with Pseudomonas fluorescens, which was chosen as a characteristic
and representative epiphyllic microorganism. With all three investigated
species, cuticular water permeability significantly increased by factors
between 40 to 60 % after inoculation with P. fluorescens for 10–12 days (Fig. 5).
In parallel to the observed increase in cuticular water permeability, it was
always observed that the bacteria had successfully penetrated the cuticle,
since bacteria were growing on the inner side of the isolated cuticle, which
was sterile at the beginning of the experiment. From this observation, it must
be concluded that the bacteria had induced additional defects to the transport
barrier of the cuticle, leading to increased rates of water permeability as well
as paths for penetrating the cuticle (Knoll 1998).