Book of Abstracts (PDF) - International Mycological Association

IMC7 Tuesday August 13th Lectures
130 - Mycorrhiza-bacteria interactions and functioning
in boreal forest soils
R. Sen
Division of General Microbiology, Department of
Biosciences, P.O. Box 56, FIN-00014 University of
Helsinki, Finland. - E-mail: robin.sen@helsinki.fi
Boreal forest trees are highly dependent on root symbiotic
ectomycorrhizal (ECM) fungi for growth in low pH, ligninrich
podzolic soils. In Scots pine seedling microcosm
studies, extensive mycorrhizal fungal colonization of the
constituent upper humus (O) and underlying mineral (E
and B) horizons was visualised and horizon-specific ECM
identified following ITS-RFLP/sequence phylogenetics.
Soil N and P mobilisation and uptake was confirmed in
analyses of mycorrhizosphere compartment-specific gene
and/or enzyme expression and linked to organic acid
production. Bacterial biofilms in O horizon
mycorrhizospheres were shown to be mainly comprised of
forest soil specific gram -ve bacteria, Bacillus spp. and
Crenarchaea based on carbon profiling and 16S sequence
phylogenetics. Preferential organic acid utilisation by
bacteria in the external mycorrhizosphere highlights
mycorrhizal fungal involvement in weathering of E and B
horizon minerals. Mycorrhizospheres developed in the O
horizon did not host Pseudomonas fluorescens which was
further confirmed in the rapid loss of marker gene tagged
P. fluorescens introduced into mycorrhizal silver birch root
systems. However, Scots pine mycorrhizospheres
developed on petroleum contaminated soils do support
catabolic plasmid harbouring fluorescent pseudomonads
and increased hydrocarbon oxidation activity. Based on
these data it is hypothesised that, in functional terms, the
mycorrhizosphere behaves as an external rumen/gut of the
host tree.
131 - The Paxillus involutus / Betula pendula symbiosis:
Gene expression in ectomycorrhizal root tissue
T. Johansson * , A. Le Quéré, D.G. Ahrén, B. Söderström &
A. Tunlid
Microbial Ecology, Ecology Building, Lund University, S-
223 62 Lund, Sweden. - E-mail:
tomas.johansson@mbioekol.lu.se
Ectomycorrhizas (ECM) are symbiotic associations formed
between plants and soil fungi. To identify genes and
metabolic pathways specifically expressed in the
mycorrhizal root tissue, 3,555 Expressed Sequence Tags
(ESTs) were analyzed in a cDNA library constructed from
ECM formed between the basidiomycete Paxillus involutus
and Betula pendula (birch). In parallel, cDNA libraries
from saprophytically growing fungus (3,964 ESTs) and
from axenic plants (2,532 ESTs) were analyzed. By
assembly of all ESTs (10,051), 2,284 contigs were
identified, each representing a unique transcripts of either
fungal or plant origin. Of those, 650 contigs (28%) were
uniquely expressed in the mycorrhizal tissue. Based on
homology to sequence information in the GenBank (nr)
protein database a majority of contigs could be assigned
putative functional and metabolic roles. By comparing
redundancies between libraries, transcripts related to
protein synthesis were found down-regulated, whereas
transcripts related to cell rescue, defense, cell death and
ageing were up-regulated in the mycorrhizal root tissues as
compared to the free-living fungus. Furthermore, the
mycorrhizal root tissue displayed an up-regulation in
transcripts related to nucleotide metabolism and carbon
utilization as compared to the saprophytically growing
fungus, whereas transcripts related to amino-acid
metabolism and lipid, fatty-acid and isoprenoid metabolism
were down-regulated.
132 - Nutrient-regulated expression of functionally
diverse surface proteins in truffles
S. Ottonello * , B. Montanini & A. Bolchi
Dipartimento di Biochimica e Biologia Molecolare-
Università di Parma, Parco Area delle Scienze 23/A-43100
Parma, Italy. - E-mail: s.ottonello@unipr.it
Inorganic nitrogen is often the most limiting nutrient in the
rhizosphere and N-availability is one of the environmental
cues that influence ectomycorrhizae formation. This
presentation will focus on four distinct, N-status-regulated
surface protein genes from Tuber borchii. Two of them
code for transmembrane proteins involved in inorganic-N
internalization that resemble related transporters from other
(symbiotic and non-symbiotic) fungi. They specifically
respond to N-shortage, but do so at a surprisingly slow rate.
Much more intense responses (to both N and C starvation)
were measured for the genes encoding two other Surface
Proteins, which harbor secretion signal peptides at their Ntermini,
are both loosely associated to the cell wall, and
bear only a very restricted (TbSP1) or no (TbSP2)
resemblance to polypeptide sequences found in databases.
The TbSP2 gene, which contains starvation stress response
elements in its promoter, codes for a cysteine-rich, 11 kDa
structural protein of as yet unknown function. The product
of the TbSP1 gene, instead, is a calcium-activated
phospholipase A2 that is both secreted and cell wallassociated
in pre-symbiotic mycelia, but also accumulates
in symbiosis-engaged hyphae as well as in fruibodies.
Generalized surface remodeling and lipid-mediated events
thus appear to predominate in the Tuber response to
nutrient shortage. The results of ongoing experiments
aimed to understand the physiological significance of such
events will be discussed.
Book of Abstracts 43