plant surface microbiology.pdf

plant roots from the sand, these can be directly fixed and prepared according
to standard protocols for TEM or SEM analyses. A prominent feature
observed with these techniques is the mucilage or biofilm which surrounds
the root and in which microorganisms develop. This biofilm is believed to
provide a contact between soil and roots for diffusion of nutrients and may
give some protection from other microorganisms. Although the film is also
produced by the plant under axenic conditions, it appears to be thicker in
non-sterile roots, where bacterial capsular material such as exopolysaccharides
(EPS) may contribute significantly to this layer. The biofilm can also be
visualised using confocal laser scanning microscopy combined with fluorescently
marked bacteria. The encapsulation of bacteria in a mucigel may have
considerable consequences for the action of certain diffusible compounds
such as autoinducer molecules involved in quorum sensing. This phenomenon
also complicates proper visualisation of marked bacteria that have penetrated
deeper into the surface layers of the root. CLSM usually can cope with
these difficulties since the system can focus on multiple planes of the specimen.
An in-depth study of the stages of root colonisation by CLSM has shown
that P. fluorescens WCS365 microcolonies on the root surface are usually
formed from one single cell, since mature microcolonies that have been visualised
on the root surface usually consist of one type of bacterium. The lognormal
distribution of bacteria on the root tip indicates that most bacteria
remain close to the inoculation site after seed inoculation. It is believed that
occasionally, single cells detach from older parts of the root and travel along
the growing root tip to establish new colonies. In later stages, mixed microcolonies
can be observed with CLSM, indicating that other bacteria can join at
some stage of microcolony formation.
5.2 Competitive Colonisation Studies
2 Root Colonisation Following Seed Inoculation 23
For a long-lasting effect, biocontrol bacteria must compete with the native
microflora and establish themselves for several months at a high level in the
rhizosphere. Successful colonisation of the plant root is often considered to be
important for the success of various applications for beneficial purposes and
for suppression of plant diseases. When studying colonisation traits in our
laboratory, we therefore determine competitive root colonisation of two or
more strains on the root.
It was assumed that various bacterial traits contribute to the ability of a
bacterial strain to colonise the rhizosphere and that loss of such a trait
reduces the ability to establish itself effectively in the rhizosphere and, hence,
also reduces its beneficial effects. Using initially competitive root tip colonisation
in the gnotobiotic system as the assay, various competitive colonisation
genes and traits were identified. One of the identified traits involved in coloni-