Symbiotic Fungi: Principles and Practice (Soil Biology)

146 L. Lioussanne et al.
9.6 Effect of Mycorrhizal Root Exudates on Tomato
Infection by P. nicotianae in Soil
Our group recently studied the role of mycorrhizal root exudates in the biocontrol of
P. nicotianae in tomato plants in soil. Tomato plants with different mycorrhizal
inoculation or root exudate treatments were grown individually in a compartmentalized
system and placed equidistant from a central unit inoculated with
P. nicotianae. Half of the plants were filled with root exudates collected from
tomato plants colonized with either G. mosseae or G. intraradices or from nonmycorrhizal
plants. The other half was not supplied with root exudates but was
inoculated with the same two AMF species or left noninoculated. As expected,
direct tomato root colonization significantly reduced the intraradical growth of
P. nicotianae, while the application of mycorrhizal root exudates did not affect
the growth of the pathogen which colonized roots to the same extent as in the
nonmycorrhizal plants (Lioussanne et al. 2006a). While a differential attraction of
zoospores by mycorrhizal root exudates was noted in vitro, root infection was not
affected by the application of mycorrhizal root exudates in a more complex soil
system. Other authors also observed that exudates from AMF structures or from
mycorrhizal roots affected the formation and/or the germination of pathogen
propagules. It was observed that germination and hyphal growth of F. oxysporum
f. sp. chrysanthemi was stimulated when conidia were inoculated directly onto a
G. intraradices mycelium in vitro (St-Arnaud et al. 1995b). On the other hand,
Filion et al. (1999) showed that crude extracts of in vitro-grown G. intraradices
mycelium reduced germination of F. oxysporum conidia. Analogous inhibitive
effects were also reported with exudates liberated by strawberry roots colonized
by G. etunicatum or G. monosporum, on the sporangia production of the pathogen
Phytophthora fragariae in vitro (Norman and Hooker 2000). More recently, microconidia
germination of F. oxysporum f. sp. lycopersici was shown to be more than
doubled in the presence of root exudates from tomatoes colonized with G. mosseae
compared to nonmycorrhizal plants. The more the tomato roots were colonized by
the AMF, the more microconidia germination was increased, suggesting a relation
between the level of root colonization and the alteration of exudation pattern
(Scheffknecht et al. 2006). A similar stimulatory effect was exhibited by root
exudates of twelve F. oxysporum lycopersici nonhost species from eight plant
families, showing that mycorrhization-induced changes in the root exudates were
unrelated to the pathogen sensitivity of the plant (Scheffknecht et al. 2007).
Even if exudates from mycorrhizal plants were shown to affect the formation, the
attraction and/or the germination of various pathogen propagules, the mycorrhizalinduced
modifications would not interfere with the proliferation of P. nicotianae
within host tissues (Lioussanne et al. 2006a). Moreover, it was shown that application
of carbenzamine, a fungicide reducing mycorrhizal colonization, had also the
effect of increasing the Aphanomyces euteiches oospores number in pea roots
(Bødker et al. 2002). In this experiment, mycorrhizal colonization extent was not
correlated with disease severity. From this, the authors suggested that AMF may not