Symbiotic Fungi: Principles and Practice (Soil Biology)

200 R. Hampp, M.T. Tarkka
suboptimal medium composition, continuous production of at least some secondary
metabolites can be expected. Among others we have observed that changes in
media composition lead to different outcomes in the interactions between organisms.
For example, the mutualistic interaction between Streptomyces AcH 505 and Amanita
muscaria, changes to an antagonistic one if the organisms are cultivated
on ISP2 medium instead of the MMN medium. AcH 505 grows extremely well
on ISP2 and sporulates much more strongly than on MMN, indicating that the
concentrations of antifungal metabolites dominate on ISP2 but not on MMN. The
fungal response is also related to the growth rate of the mycelium. We and others
have observed that if the growth rate of a fungus is high due to optimal growth
conditions, the growth-promoting influence by a bacterium may no longer be
apparent. In line with this, the strain of Amanita muscaria growing most slowly
on MMN was most responsive to the mycorrhization helper streptomycetes, AcH
505 and AcH 1003 (Schrey et al. 2005). This suggests that bacteria may compensate
for poor growth, but may not increase the maximum growth of faster growing
fungal isolates under optimal growth conditions.
Further crucial factors influencing the bacterium–fungus interactions are the
quality and the extent of bacterial or fungal inocula. Both the bacteria and the
fungi may lose some of their characteristics over time. This includes alterations in
secondary metabolite production in bacteria and decreased mycorrhization capacity
in the fungi. Therefore it is advisable to have a set of glycerol stocks of the bacteria
at 80 C and a good stock of fungal cultures at +4 C. As far as the mycorrhiza
helper bacteria are concerned, a fine balance exists between too small (no effect),
adequate (fungal growth promotion), and excess (no influence or antagonism)
amounts of bacterial inocula. It is thus necessary to inoculate equal amounts of
equally fresh bacteria from the same culture density, holding on to the same
culturing conditions. Growth conditions may also influence the interactions.
Deveau et al. (2007) used 10 C to maximize the reproducibility of Laccaria
bicolor–Pseudomonas fluorescens dual cultures for analyses of the fungal transcriptome.
Correctly they argued that instead of artificial temperatures used normally
for dual cultures (around 20–25 C), 10 C is a normal temperature in temperate or
boreal forest soils, and should thus be more commonly used for interaction studies.
12.3 Rapid Fungal Responses to Bacteria and Their
Metabolites: Bacterium–Fungus Suspension Cultures
Detailed information of interaction mechanisms between microbes may be acquired
by the treatment of one microbe with culture supernatant or growth regulator of
another microbe (Melin et al. 1999; Agarwal et al. 2003). The tip regions of fungal
hyphae respond most strongly to interacting organisms (Schrey et al. 2007). As only
the extreme front of the mycelium in solid agar cultures is metabolically active,
harvesting the entire mycelium from solid agar cultures leads to the risk of masking
the active responses occurring in the extending hyphal tips. One option to prevent this