plant surface microbiology.pdf

22 Nitrogen Transport and Metabolism in Mycorrhizal Fungi and Mycorrhizas 403
meases which plays a unique role as a nitrogen sensor in the transduction
pathway of pseudohyphal differentiation in S. cerevisiae not shared with the
related Mep1 and Mep3. Interestingly, in the ectomycorrhizal fungus H. cylindrosporum,
two ammonium transporters (Amt1 and Amt2) are able to complement
the pseudohyphal growth defect of a homozygotous mep2D yeast
mutant, whereas the third ammonium transporter (Amt3) is unable to do so
(Javelle et al. 2001, 2003b).According to the classification of the transport system
available at http://www-biology.ucsd.edu/~msaier/transport/ (TC system),
the HcAmts can be divided into two groups. HcAmt1, HcAmt2 and
Mep2 belong to the high affinity ammonium transporter and sensor family
(TC 2A 49 3 2), whereas HcAmt3 belongs to the low affinity ammonium transporter
family (TC 2A 49 3 1; Fig. 3).
We have recently hypothesized that high affinity ammonium transporters
from mycorrhizal fungi sense the environment and induce via signal transduction
cascades a switch of the fungal growth mode observed during mycorrhiza
formation. Upon entering the root depletion zone, mycorrhizal fungi
may receive a signal through this sensing mechanism which induces hyphal
proliferation around roots, corresponding to the primary events in ectomycorrhiza
formation (Javelle et al. 2003a).
4 Amino Acid Transport
4.1 Utilization of Amino Acids by Ectomycorrhizal Partners
It has been well established that ectomycorrhizal fungi can use amino acids as
nitrogen and carbon sources (Abuzinadah and Read 1988; Näsholm et al.
1998). Using 14 C-labelled compounds, Wallenda and Read (1999) determined
the kinetics of uptake of amino acids by excised ectomycorrhizal roots from
beech, spruce, and pine. All mycorrhizal types took up amino acids via highaffinity
transport systems with Km values ranging from 19 to 233 mM.A comparative
analysis for the uptake of amino acids and the ammonium analogue
methylammonium showed that ectomycorrhizal roots have similar or even
higher affinities for the amino acids, indicating that absorption of these N
organic forms can contribute significantly to total N uptake by ectomycorrhizal
plants.
Transport of amino acids was investigated in the mycorrhizal fungi Paxillus
involutus (Chalot et al. 1996), and Amanita muscaria (Nehls et al. 1999),
which demonstrated their ability to take up a variety of amino acids. In the
latter fungus, the uptake characteristics of the encoded transporter protein, as
analysed by heterologous expression in yeast, identified the protein as a highaffinity,
general amino acid permease (Km: 22 mM for histidine and up to
100 mM for proline). The uptake of amino acids showed characteristic features
of active transport.