Symbiotic Fungi: Principles and Practice (Soil Biology)

148 L. Lioussanne et al.
P. fluorescens WCS365 to use sugars, an important exudate constituent, does not
play a major role in tomato root colonization. They showed that the mutant
PCL1083 from WCS365, impaired in the ability to grow on simple sugars, reached
the same population level at the root tip as the wild-type strain, when inoculated on
germinated tomato seeds. Additionally, it was demonstrated that the bioavailability
of some amino acids detected in tomato exudates is too low to support root tip
colonization by auxotrophic mutants of P. fluorescens strain WCS365. The genes
required for amino acid synthesis are therefore necessary for root colonization
(Simons et al. 1997).
On the other hand, it has been shown that roots constitute a physical support
essential for rhizosphere competence of specific soil bacteria. Mutants of P. chlororaphis
strain PCL1391 impaired in the known tomato root colonization traits
(motility and production of the site-specific recombinase) were not able to control
F. oxysporum f. sp. radicis-lycopersici contrarily to the wild-type strain (Chin et al.
2000). Root colonization thus plays a crucial role in biocontrol for bacteria. The
presence of AMF structures may also be essential for bacterial competence within
the mycorrhizosphere and, if so, for their contribution to biocontrol. They may
extend the soil area colonized by an AMF-modified bacterial community to the
mycorrhizosphere which is far larger than the rhizosphere, and in this manner favor
their competition on soilborne pathogens.
Physical interactions between bacteria and AMF have been described. The
capacity of rhizobia and pseudomonads to adhere to Gigaspora margarita spores
and hyphae under sterile conditions has been reported to be strain-dependent
(Bianciotto et al. 1996a). The first stages of attachment (nonreceptor-dependent)
would be governed by general physiochemical parameters, such as electrostatic
attraction, and then later secured by specific bacterial cell surface components.
The capacity to adhere to G. intraradices structures by different bacterial species
was shown to depend on the capacity to form biofilms as mutants affected in the
production of extracellular polysaccharides (EPS) essential for biofilm formation
were strongly impaired in their capacity to attach to both mycorrhizal roots and
AMF mycelium (Bianciotto et al. 2001a). Furthermore, mucoid mutants of the
biocontrol strain P. fluorescens CHAO (with an alginate biosynthesis activation)
adhered more importantly to the surface of this fungus than the wild type strain
(Bianciotto et al. 2001b). Toljander et al. (2006) also reported that the attachment of
different bacteria to AMF extraradical hyphae depended on the bacterial species
and on the fungal species and vitality. Levy et al. (2003) reported the colonization
of both hyphae and spores of G. decipiens by Burkholderia spp. The spore’s outer
layer of G. geosporum was shown to be eroded and covered by mucilaginous
products, which suggests that AMF are directly consumed by bacteria (Roesti
et al. 2005). AMF may specifically favor the proliferation of some bacteria,
serving as substrate or interacting with them, favoring the formation of biofilms.
G. mosseae was shown to increase the population of the biocontrol agent
P. fluorescens within the tomato and leek rhizosphere (Edwards et al. 1998).
Nonetheless, AMF have negative effects on the biomass of some soil bacteria
(Bansal and Mukerji 1994; Cavagnaro et al. 2006; Christensen and Jakobsen