1 Introduction

Carbon Metabolism, Lipid Composition and Metabolism 161
isolated to date range from nutritionally to morphologically important
genes (review in Harrier 2001; Golotte et al. 2002). More recently, nontargeted
approaches have been used to isolate AM fungal genes. They
include differential display, differential screening, and large-scale EST
sequencing (Gollotte et al. 2002). The use of mycorrhizal defective mutants
and the application of high-density array technology are promising
tools to isolate genes involved in the different steps of plant–AM interactions.
Concerning genes involved in carbon metabolism and transport, the
nature of the plant transporters or fungal transporters involved in the
movement of glucose from the cortical cells to the interface compartment
and to arbuscules and hyphae are still unknown. A gene encoding
a transmembrane sugar transporter (Mtst1) was cloned from mycorrhizal
roots of Medicago truncatula. This transporter was designed as a hexose
transporter by activity measured in yeast (Harrison 1996). In situ hybridization
showed that the sugar transporter transcripts were induced
in cells containing arbuscules and in adjacent cells, suggesting that the
genemaybeactiveintheprocessofhexosetransporttowardscolonized
plant cells. A phosphoglycerate kinase (PGK) cDNA was isolated from
tomato mycorrhizas by differential display (Harrier et al. 1998). Quantitative
immunoblotting using a polyclonal antibody specific for the G. mossae
PGK protein revealed a significantly higher accumulation of the protein
during symbiosis compared with presymbiotic development (Harrier and
Sawczak 2000). These results suggested that there is a differential regulation
of fungal genes during symbiosis. Analysis of the Gmpgk fungal promoter
showed several regulatory elements homologous to carbon sourcecontrolled
upstream activating elements from Saccharomyces cerevisiae
(Harrier 2001).
AM fungi could cause changes in the regulation of plant genes in roots.
Ravnsknov et al. (2003) demonstrated a higher gene expression of both
Sus1 and Sh1, the two isoforms of sucrose synthase cleaving sucrose into
hexoses, in maize roots colonized by different AM fungal isolates. Higher
sucrose synthase gene expression was not related to the concentrations
of sucrose, reducing sugars or starch in the root tissue whereas increasing
soil phosphorus concentrations decreased this gene expression. The
higher gene expression of gene coding for sucrose synthase in AM roots
was measured during the earliest phase of root colonization by fungal
isolates.