Symbiotic Fungi: Principles and Practice (Soil Biology)

9 Role of Root Exudates and Rhizosphere Microflora in the Arbuscular 151
mycorrhizal roots to access resources, counterbalancing weight loss caused by the
pathogen. Increasing inorganic phosphate supply to tomato plants inoculated with
P. nicotianae, Trotta et al. (1996) did not observe a reduction of disease symptoms
similar to that induced by G. mosseae inoculation, showing that the increase
in phosphorus nutrition by mycorrhizal colonization, often suggested as being
importantly implicated in the biocontrol induced by AMF, would not be involved
in the biocontrol in this system.
9.8 Conclusions
G. mosseae but also G. intraradices (DAOM 181 602) have been shown to induce
biocontrol on tomato plants inoculated with P. nicotianae locally (within root
tissues colonized with AMF), and also systemically (in the case of G. mosseae).
Local and systemic plant defense reactions are obviously important factors which
hamper the pathogen proliferation within host tissues and decrease the extent of
disease symptoms. As our understanding of signalling pathways and of the roles of
molecules such as jasmonic acid increases, we should be able to unravel the
complex cascade of events leading to the AMF-mediated biocontrol. The quicker
and upper activation of host defense pathways does not however allow us to explain
every characteristic of biocontrol. Changes in root exudates after mycorrhizal
colonization would not directly or indirectly (by modification of the rhizosphere
bacterial community structure) diminish the pathogen ability to infect roots. However,
bacteria identified within or onto the surface of mycorrhizal structures may
help to reduce the pathogen proliferation not only within the soil but also within
roots. These bacteria could contribute to the biocontrol conferred by AMF through
several mechanisms such as the formation of a suppressive area near the mycorrhizal
network and/or the stimulation of the plant defense reactions, but also to the
overall plant health through other beneficial effects on plant nutrition, growth and
metabolism.
References
Addepalli MK, Fujita Y (2001) Serological detection of red rot disease initiation stages of
microbial pathogen, Pythium porphyrae (Oomycota) on Porphyra yezoensis. J Appl Phycol
13:221–227
Allen RN, Newhook FJ (1973) Chemotaxis of zoospores of Phytophthora cinnamomi to ethanol in
capillaires of soil pore dimensions. Trans Br Mycol Soc 61:287–302
Andrade G, Mihara KL, Linderman RG, Bethlenfalvay GJ (1997) Bacteria from rhizosphere and
hyphosphere soils of different arbuscular–mycorrhizal fungi. Plant Soil 192:71–79
Artursson V, Jansson JK (2003) Use of bromodeoxyuridine immunocapture to identify active
bacteria associated with arbuscular mycorrhizal hyphae. Appl Environ Microbiol 69:6208–6215