plant surface microbiology.pdf

20 Mycorrhizal Fungi and Plant Growth Promoting Rhizobacteria 355
al. 2001). The main activity of these bacteria is associated with the production
of auxin-type phytohormones (Dobbelaere et al. 1999). The production and
significance of auxins have been investigated at the molecular level (van de
Broek et al. 1999; Lambrecht et al. 2000).
Several systems for studying rhizosphere colonization by PGPR have been
proposed, the one described by Simons et al. (1996) appears interesting, not
only for testing the PGPR ability, but also to investigate PGPR/plant root interactions.
It is known that several bacterial cell surface properties could be involved
in the adhesion of PGPR to roots that may be either nonspecific based on electrostatic
forces, or involve specific recognition between the surfaces. In this
context, root surface glyco-proteins and several different bacterial exo-polysaccharides
could be involved (Weller and Thomashow 1994). In some PGPR,
fimbriae (pili) may function in the adherence of cells to roots, but the contribution
of flagella to the colonization process apparently depends on the PGPR
strain, plant species and type of soil (Kloepper 1994).A sugar-binding protein
system has been described in Azospirillum related to chemostaxis and root
colonization (van Bastelaere et al. 1999).
Novel techniques for microbial community fingerprinting are being developed
(Kozdroj and van Elsas, 2000), and these are being adapted for the
genetic characterization of PGPR. For example, the approach used by Zinniel
et al. (2002), based on the 16S rRNA gene amplification and sequencing, has
been proposed for the genetic characterization of endophytic bacteria, while
Bertrand et al. (2001) identify PGPR 16S rDNA sequence analysis. The molecular
bases of rhizosphere colonization have recently been reviewed (Lugtenberg
et al. 2001). Siciliano and Germida (1998) proposed the use of BIOLOG
analysis and fatty acid methyl ester profiles to study PGPR behavior after
inoculation, particularly the effects on root-associated microbial populations.
Confocal laser scanning microscopy is being used for studying the
microorganisms/plant root interactions, for example, to detect a currently
used marker such as the green fluorescent protein (Lagopodi et al. 2002) or to
localize colonizing bacteria by fluorescence and in situ hybridization, after
staining with the fluorescent Live/Dead BacLight dye (Bianciotto et al. 2000,
2001).
The molecular bases of the biocontrol ability of these rhizobacteria have
been investigated in the last few years (Keel et al. 1992; O’Gara et al. 1994;
Cook et al. 1995; Tomashow and Weller 1995; Chin-A-Woeng et al. 2001;
Moenne-Loccos et al. 2001), and systemic induced resistance has been argued
as a mechanism of disease suppression by endophytes (Duijff et al. 1998), or
other PGPR (Defago and Keel 1995; Chin-A-Woeng et al. 2001).