plant surface microbiology.pdf

378
Uwe Nehls
monosaccharide transporter gene expression is a slow process, it could be
interpreted as an adaptation to the elevated hexose concentrations usually
only found at the plant/fungus interface, but not in the soil.
In yeast, sugar dependent induction/enhancement of gene expression is
controlled by two monosaccharide transporter-like proteins, RGT2 and SNF3,
sensing the external sugar concentration (Celenza et al. 1988; Özcan et al.
1996). These transporters have a C-terminal extension containing a conserved
amino acid motive thought to be involved in the transduction of the external
sugar signal. Due to their different glucose affinities, SNF3 senses low external
glucose concentrations while RGT2 senses high concentrations. In contrast to
yeast, the putative monosaccharide transporter RCO3 (Madi et al. 1997) that is
presumably also acting as monosaccharide sensor in Neurospora crassa,does
not contain any extension. The signal cascade, transforming the sugar signal
into modified gene expression, is not fully understood. To date, two elements
have been identified, the transcription factor RGT1 (Özcan et al. 1996) and a
signal transduction mediator, the SCF complex (Özcan and Johnston 1999).
Without the sugar signal, RGT1 is a repressor for glucose-induced genes while
activation via the SCF complex (in response to a sugar signal) modifies RGT1
function to that of a transcriptional activator (Johnston 1999).
The signal regulating the hexose-dependent, enhanced AmMst1 expression
is still unknown, but in contrast to yeast, it seems to be transmitted by an
internal and not an external sensor. Glucose analogues that are imported by
AmMst1 and phosphorylated, but not further metabolized, did not increase
the AmMst1 transcript level as glucose did (Wiese et al. 2000). Furthermore,
the result of these experiments makes it rather likely that the signal must be
generated downstream of hexokinase activity, in glycolysis or carbon storage
pathways.
While AmMst1 expression is an example of sugar-dependent enhancement
of gene expression in A. muscaria, a second gene (AmPAL) was identified that
revealed sugar-dependent gene repression (Nehls et al. 1999a). PAL is a key
enzyme of secondary metabolism and thus of the production of phenolic
compounds. ECM-forming fungi have been reported to use phenolic compounds
for both their own protection and that of their host against bacterial
or fungal attacks (Marx 1969; Chakravarty and Unestam 1987; Garbaye 1991).
In A. muscaria, the transcript of AmPAL was abundant in hyphae grown at
low external glucose concentrations, but exhibited a significant decrease in
hyphae cultured at glucose concentrations of above 2 mM (less than 1/30 of
the transcript level at low glucose). Unlike AmMst1, AmPAL-expression is
probably regulated by sugar phosphorylation via hexokinase as sugar sensor
(Nehls et al. 1999a).
Also in saprophytic ascomycetes the monosaccharide-dependent gene
repression is regulated via a hexokinase-dependent signaling pathway (Ronne
1995; Gancedo 1998). The molecular mechanism of signal initiation is still
unclear, but a hexokinase (in yeast mainly hex2) initiates the signal in