1 Introduction

286 A. Schüßler and E. Wolf
Glomeromycota (Schüßler et al. 1994). These bacteria are not enclosed
within a host membrane. They probably are ancient, obligate endosymbionts,
which are horizontally transferred and are likely to have been symbionts
of the fungi for many hundreds of million years.
Little is known about these bacteria, except for those found in the Gigasporaceae
(see Bianciotto et al. 2003 and references therein) which have
been recently described as ‘Candidatus Glomeribacter gigasporarum’. Several
genes were characterized, e.g. vacB, a pst operon, and nif D/K. Particularly
the nif genes are highly interesting, since this means that BLOs could
fix N2. However, these are not the ‘typical’ BLOs for AM fungi, because
they have a different ultrastructure and are enclosed by a host membrane.
Specific primers for Glomeribacter do not amplify SSU rDNA from nongigasporacean
BLOs.
We do not yet know the phylogeny of the Geosiphon BLOs. However,
perhaps in the future their role in the AM can be uncovered by using
Geosiphon.Therefore,weintendtoinvestigatetheirrelationships,inwhich
context FISH could play an important role (Bertaux et al. 2003). Geosiphon
bladders have the advantage that the big cells can easily be cut after fixation,
resulting in direct access to the BLOs in the interior.
7.3
Identification of Differentially Expressed Fungal Genes
When compared to the AM, one of the main advantages of the Geosiphon
symbiosis for gene expression studies is that only one eukaryote is present.
In the symbiotic stage of the AM, the plant is always accompanying and
fungal gene expression is difficult to investigate. By contrast, from the
Geosiphon symbiosis fungal mRNA can be isolated specifically and easily
by means of its poly(A) tail.
Studies on differential gene expression under certain environmental
conditions were initiated. For example, we are trying by subtractive hybridization
methods to gather indications about genes relevant for the
uptake (and metabolism) of photosynthetic products by the fungus. Candidate
genes are sugar transporters, C transfer to the fungal symbiosis
partner being a key factor in the AM. Characterized gene fragments then
could also be used for screening purposes, and the design of primers for
the amplification of corresponding genes in other AM fungi. Other approaches
include the investigation of the reaction to heavy metal stress (see
Sect. 5.3). Methods for microinjection have already been established and, if
project funding were available, these could be used to inject GFP-construct
expression vectors suitable for AM fungi.