plant surface microbiology.pdf

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Uwe Nehls
quite untypical for fungi where good nitrogen nutrition usually results in a
strong repression of ammonium transporter genes.
9 Utilization of Organic Nitrogen
Important for the establishment of forest ecosystems is the capability of ectomycorrhizal
fungi to exploit (in collaboration with other soil organisms)
organic debris (e.g., litter) as a nutrient source (Nasholm and Persson 2001).
10 Proteolytic Activities of Ectomycorrhizal Fungi
Ericoid fungi (Bajwa et al. 1985; Leake and Read 1990), but also some ectomycorrhizal
fungi (Abuzinadah and Read 1986; El-Badaoui and Botton 1989; Zhu
1990; Spägele 1992; Zhu et al. 1994; Bending and Read 1996) are able to utilize
protein not only as a nitrogen, but also as a carbon source (for a review, see
Smith and Read 1997).
Two proteins with proteolytic activities and molecular masses of about
45 kDa (AmProt1) and 100 kDa (AmProt2) are excreted by A. muscaria (Nehls
et al. 2001b). AmProt1 was mainly released at pH-values up to pH 5.4 and
revealed a narrow pH-optimum around 3.0. It resembles thus, proteases
released by H. crustuliniforme (Zhu 1990) and the ericoid fungus Hymenoscyphus
ericea (Leake and Read 1990). AmProt2 was only excreted at pH-values
between 5.4 and 6.3 and reveals a broad pH-optimum between 3 and 6. A.
muscaria is mainly growing in the litter layer of both acidic and less acidic
forest soils. Since forest litter layers are, in addition to fungi, intensively colonized
by biofilm-forming bacteria (Berg et al. 1998), where the microenvironment
is adapted to bacterial growth (e.g., pH 5–6; Fletcher 1996), expression
of a protease that is active at a less acidic pH would favor the mobilization of
bacteria-derived proteins by ectomycorrhizal fungi.
A cDNA presumably encoding AmProt1 was identified in an EST project
(Nehls et al. 2001b). AmProt1 was not only regulated by the external pH, but
also by carbon as well as nitrogen availability. Nitrogen starvation alone
increased AmProt1 expression by a factor of 3 to 4. However, the absence of a
carbon source increased the transcript level of the gene by a factor of approximately
12, independent of the presence or absence of nitrogen. The expression
of AmProt1 reflects thus the nutritional status of fungal hyphae with
respect to carbon (major regulatory effect) and nitrogen (minor regulatory
effect).