1 Introduction

Host and Non-Host Impact on the Physiologyof the AM Symbiosis 143
nificant rate of trehalose, a short-term fungal storage carbohydrate, was
13 C-labelled when 13 CO2 was supplied to asymbiotic spores. However, after
deciphering the metabolic pathways involved, these authors concluded
that such 13 C incorporation occurred via dark fixation, a side part of gluconeogenesis
which does not lead to net gain of C. Nevertheless, such dark
fixation seems to have an important role in anaplerotic reactions, thus explaining
the better pre-symbiotic AM fungal development shown in the
presence of CO2.
2.4
Light
In nature, the exposure of soil-borne AM fungi to light is an extremely
unlikely event (see Chap. 6), as underground roots are the colonized plant
organ. Surprisingly, light treatment affects the growth pattern of axenically
growing hyphae. Light induced hyphal branching in developing germ
tubes of Gi. gigantea, Gi. rosea and G. intraradices (Nagahashi et al. 2000).
In a study with Gi. gigantea, wavelengthsinthebluetoUVrangewere
identified as the responsive ones (Nagahashi and Douds 2003). Blue light
has been found to regulate all known differentiation processes in Basidiomycetes
(Kues et al. 1998). The biological relevance of these findings in
nature still remains unclear, and it has been suggested that a general mechanism
of stress response could be implicated (Kues et al. 1998); in any case,
in vitro systems are ideal to further study such intriguing questions.
3
Pre-Symbiotic AM Fungal Growth
More than 80% of all plant species are hosts for AM fungi. Depending on the
host status, plant signals perceived by AM fungi differ. Whereas there are
abundant data on the stimulatory effects of plant signals from host plants
on AM fungal spore germination, hyphal growth and branching, there are
contradictory reports on the effect of signals from non-host plants on AM
fungi (Giovannetti and Sbrana 1998; Vierheilig et al. 1998; Giovannetti
2000). To our knowledge, although AM fungi can grow to some extent in
the presence of AM non-host plants, in general, they do not grow on their
root surface and do not form appressoria on them. As the pre-symbiotic
phase is characterized by the presence of both root and AM fungi, still in the
absence of physical contact between them, it can be concluded that the nonhost
status of plants to AM fungi is determined during the pre-symbiotic
stage of the symbiosis.