Functional Characterization of the Amanita muscaria Monosaccharide Transporter, AmMst1

Abstract

Fungal carbohydrate nutrition is an important aspect of ectomycorrhizal symbiosis. At the plant/fungus interface, fungal and root cortical cells compete for monosaccharides, generated from plant‐derived sucrose. Therefore, the kinetic properties of the monosaccharide uptake systems are decisive for the monosaccharide yield of each partner. For the functional characterization of a hexose transporter (AmMst1) of the ectomycorrhizal fungus Amanita muscaria, the entire cDNA was expressed in a Saccharomyces cerevisiae strain unable to take up hexoses. Uptake experiments with 14C‐labelled monosaccharides resulted in KM values of 0.46 mM for glucose and 4.20 mM for fructose, revealing a strong preference of AmMst1 for glucose as substrate. Glucose uptake by AmMst1 was strongly favoured even in the presence of a large excess of fructose. Comparable affinities of AmMst1 for glucose, 3‐O‐methyl glucose and mannose were obtained. In contrast, AmMst1 imported galactose with a much lower efficiency, revealing that this transporter distinguishes pyranoses by steric hindrance at the C‐4 position. While yeast contains numerous hexose transporter genes, the AmMst1 gene seems to be the main, if not the only, hexose transporter that is expressed in A. muscaria, as concluded from the comparison of hexose import properties of A. muscaria protoplasts and AmMst1 expressed in yeast.

The impact of ectomycorrhiza formation on monosaccharide transporter gene expression in poplar roots

“…Wiese et al, 2000), was not successful (data not shown). The fact that functional expression in heterologous systems does not always work for a number of plant monosaccharide transporters is well known (U. Ludewig, pers.…”

The role of haustoria in sugar supply during infection of broad bean by the rust fungusUromycesfabae

“…Results from both, uncoupling experiments and electrophysiology, demonstrate a proton-substrate symport mechanism for HXT1p. AmMst1, the closest homolog of HXT1p, also seems to operate as a symporter (36 HXT1p are at least to some extent produced in the extrahaustorial matrix, most likely through a combined action of fungal and plant invertases (R.T.V., U. Möll, S. Wirsel, K.M., unpublished data). Fructose, too, would have to be removed efficiently from this compartment.…”

Carbon Metabolism and Transport in Arbuscular Mycorrhizas

“…An alternate manner, or together with such a porter, may be active transport. H ϩ -hexose cotransport by fungi is well known (Sanders, 1988) and such a transporter has been reported in an ectomycorrhizal species (Wiese et al, 2000).…”