plant surface microbiology.pdf

374
Uwe Nehls
2 Trehalose Utilization by Ectomycorrhizal Fungi
Trehalose is a nonreducing disaccharide that is found in a wide variety of
organisms including bacteria, fungi, protozoa, nematodes, insects and plants
(Elbein 1974; Mellor 1992, Müller et al. 1994). Trehalose has been shown to be
a good carbon source for a number of ectomycorrhizal fungi (Lewis and
Harley 1965; Palmer and Hacskaylo 1970).
Amanita muscaria hyphae excrete an acid trehalase into the growth
medium to break down external trehalose (Wisser et al. 2000).With its apparent
molecular mass of 165 kDa, a pI of 3.7, a pH optimum of about 4.0 and an
apparent Km value for trehalose of 0.38 mM, it is comparable to that of other
fungi.
Excreted acid trehalases, in general, are very stable. Owing to the low Km
value of the enzyme, and the acidic pH optimum, trehalose hydrolysis in
acidic forest soils should be very efficient. The resulting monosaccharides are
then taken up by the highly efficient monosaccharide import system of A.
muscaria (Chen and Hampp 1993; Nehls et al. 1998, see below).
Carbohydrate-starved mycelia excreted about four times more acid trehalase
into the growth medium than mycelia that were well supported with
sugar (Wisser 2000), indicating an up-regulated expression of acid trehalase
with regard to poor carbon nutrition.
Trehalose is one of the main storage carbohydrates of basidiomycetes, but
is also found in bacteria in large quantities. The utilization of trehalose by
ectomycorrhizal fungi might thus be important for two reasons:
1. The availability of an additional carbon source would improve ectomycorrhizal
fungal growth in soil.
2. By utilization of a soil carbon source that otherwise could be used by other
microorganisms, ectomycorrhizal fungi could reduce the growth of their
putative competitors for other nutrients (e.g., nitrogen or phosphate).
3 Carbohydrate Uptake
A prerequisite for a rapid uptake of monosaccharides are membrane transport
systems. Experiments with suspension-cultured hyphae of ectomycorrhizal
fungi (Salzer and Hager 1991) and with protoplasts (Chen and Hampp
1993) indicated that most basidiomycotic ectomycorrhizal fungi have no system
for sucrose import or hydrolysis, but for the uptake of glucose and fructose
(Palmer and Hacskaylo 1970; Salzer and Hager 1991).
To date, only two hexose transporter genes from A. muscaria (Fig. 1),
AmMst1 (Nehls et al. 1998) encoding a protein of 520 amino acids and
AmMst2 encoding a protein of 519 amino acids, and one hexose transporter
gene from Tuber borchii (Agostini and Stocchi, pers. comm.) have been identified.
While AmMst1 reveals the highest sequence homology with Hxt1 from