1 Introduction

144 H. Vierheilig and B. Bago
Apart from plant signals perceived by the fungus, more and more data
indicate that signal exchange between plants and AM fungi is not unidirectional,
but that fungal signals are perceived by plants before the hyphae
are in direct contact with the root (Gadkar et al. 2001; Vierheilig and Piché
2002; Vierheilig 2004).
3.1
Plant-to-Fungus Signals
In this section, we will distinguish between two classes of compounds
released by roots which might act as signals for AM fungi: gaseous compounds
which below we will name root volatiles, and liquid compounds
which below we will name root exudates. Different approaches have been
takentostudyapossiblesignallingeffectofcompoundsreleasedbyroots
on AM fungi. A more generalistic approach is to study the effect of root
volatiles and root exudates on spore germination and hyphal growth. This
is usually done under in vitro conditions (see reviews by Fortin et al. 2002;
Vierheilig et al. 1998). Depending on the experimental system, different
fractions of root volatiles/exudates are studied.
Co-culture of roots and AM fungi in the same agar medium of a closed
Petri plate system means exposing the AM fungi to a combination of root
volatiles and exudates. To study the exclusive effect of volatiles on AM
fungi, monoxenic closed experimental systems are used where AM fungi are
exposed to volatiles, but are physically separated from root exudates (pre-
SMC). To study the exclusive effect of root exudates, exudates are collected
and thereafter applied to AM fungi under axenic conditions (Vierheilig
et al. 1998).
A more specific approach is to test the effect of compounds isolated
and identified from root exudates (e.g. in studies on the rhizobial–legume
interaction) on the above-described AM fungal parameters.
3.1.1
Signals from AM Host Plants Towards the AM
More than 15 years ago, first reports on the stimulatory effect of collected
root exudates from whole citrus and white clover plants on axenic AM
spore germination and hyphal growth were published (Graham 1982; Elias
and Safir 1987). Thereafter, this effect of root exudates was confirmed with
a wide range of host plants (see review by Vierheilig et al. 1998), such
as clover (Gianinazzi-Pearson et al. 1989), alfalfa (El-Atrach et al. 1989),
onion (Tawaraya et al. 1996) and carrot (Poulin et al. 1993), and with root