Symbiotic Fungi: Principles and Practice (Soil Biology)

9 Role of Root Exudates and Rhizosphere Microflora in the Arbuscular 147
impact on the vegetative growth, causing root necrosis, of the P. nicotianae-related
pathogen A. euteiches, but only on its capacity to form reproductive structures.
9.7 Effect of Mycorrhizal Root Exudates on the
Rhizosphere Bacterial Community Structure
Mycorrhizal colonization was shown to modify the bacterial community
structure of the rhizosphere (Marschner and Timonen 2005; Marschner et al.
2001; Wamberg et al. 2003). Among the possible mechanisms involved in this
process, modification in root exudation has been suggested (Bansal and Mukerji
1994; Marschner et al. 1997). Filion et al. (1999) observed in vitro that extracts
from G. intraradices mycelium had differential effects on soil microbes, stimulating
the growth of Pseudomonas chlororaphis and Trichoderma harzianum,
reducing germination of F. oxysporum, and having no effect on the growth of
Clavibacter michiganensis. Sood (2003) also reported that the chemotactic response
of the plant-growth-promoting rhizobacteria Azotobacter chroococcum
and Pseudomonas fluorescens were significantly stronger towards exudates of
tomatoes colonized with G. fasciculatum than towards exudates of nonmycorrhizal
roots. The utilization of the PCR-DGGE technique on the ribosomal gene 16S
permitted Lioussanne et al. (2006a) to efficiently characterize the bacterial community
structure within the rhizosphere of tomato exposed to mycorrhizal root
exudates or direct AMF inoculation. As shown before, root colonization with either
G. mosseae or G. intraradices had a significant impact. However, the bacterial
community structure of rhizosphere supplied with exudates from tomatoes colonized
with the same AMF isolates was not significantly different from the one of
tomatoes supplied with exudates from nonmycorrhizal-tomatoes. These results
suggest that the modification of the rhizosphere bacterial community structure
induced by mycorrhizal colonization would not be mediated by root exudation
modification. If bacterial community structure changes in mycorrhizal rhizosphere
were involved in the AMF-mediated biocontrol, it would not depend on changes in
root exudation pattern.
The high microbial activity present in the rhizosphere in comparison to the bulksoil
was often believed to be due to the supply of nutrients liberated by the roots in
the form of exudates. Soil or rhizosphere enrichment with artificial exudates shifted
the microbial community structure more and more consistently as substrate concentration
load increased (Baudoin et al. 2003; Griffiths et al. 1999; Kozdrój
and van Elsas 2000; Pennanen et al. 2004). The modification of the amount of
root exudate after mycorrhizal colonization would not be important enough to
induce significant bacterial community changes. Lynch and Whipps (1990) calculated
that exudates contained only 9–10% of the amount of substrate required to
explain the quantified microbial biomass in the rhizosphere of barley and maize.
Lugtenberg and colleagues (1999) reported that the ability of the biocontrol bacteria