phase until plant penetration has occurred and
proliferation of the dikaryon is initiated (FlorParra et al. 2007; Heimel et al. 2010a; PerezMartin and Castillo-Lluva 2008; Scherer et al.
2006). Concomitant with the massive fungal
proliferation, plant tumors are formed in
which fungal hyphae differentiate into diploid,
melanized teliospores. Pathogenic development
is controlled by the bE/bW heterodimer,
encoded by the b mating-type locus. While
activity of the b-heterodimer is sufficient for
the activation of the pathogenic program
including filament formation and appressoriamediated plant penetration, proliferation of the
dikaryon in the host plant is dependent on the
functional modulation of the b-heterodimer by
the Clp1 protein. Clp1 physically interacts with
bW and with the master regulator downstream
of the b-heterodimer, Rbf1, ultimately reducing
activity of the mating-type signaling pathway to
overcome the G2 cell cycle arrest and to induce
mitotic growth of the dikaryon in planta (Heimel et al. 2010a; Scherer et al. 2006). The timing
of cell cycle release is mediated by Clp1 accumulation, which is stabilized via the interaction
with the Hac1 homolog Cib1. Cib1 is specifically expressed after successful plant penetration (Heimel et al. 2013; Heimel et al. 2010a),
and the UPR is continuously active during the
fungal/plant interaction. UPR activity is presumably connected to the strongly increased
expression of large numbers of effector genes
at this stage of the life cycle (Lanver et al. 2018)
and required for efficient secretion and processing of effector proteins including the wellcharacterized Pit2, Tin2, Cmu1, and Pep1 effectors (Hampel et al. 2016; Lo Presti et al. 2016;
Pinter et al. 2019). The stage-specific depletion
of Cib1 during biotrophic development
revealed that a functional UPR is not only
important for the establishment of a compatible
biotrophic interaction after plant penetration
but is continuously required for suppressing
the plant defense and maintenance of fungal
biotrophy (Schmitz et al. 2019a). Interestingly,
expression of pit2 and tin1-1 effector genes is
controlled directly by Cib1 binding to their
promoter regions, and abolishment of this regulation by targeted deletion of the Cib1 binding
site in the pit1/2 promoter leads to significantly
reduced virulence of U. maydis (Hampel et al.
2016). The interaction between Clp1 and Cib1
affects phosphorylation and stability of Cib1
and confers strongly increased ER stress resistance by modulated UPR gene expression.
Genome-wide analysis of Clp1-dependent
effects on UPR genes revealed a set of 65 UPR
core genes, which are induced by Cib1 and
differentially modulated in their expression by
Clp1. ChIP-seq (Chromatin Immunoprecipitation followed by massive parallel sequencing)
analysis identified a consensus UPRE motif
bound by Cib1 (TGACGTGG) (Hampel et al.
2016; Pinter et al. 2019), resembling the mammalian XBP1 binding motif (GATGACGTGG)
(Yamamoto et al. 2004) (see also Fig. 3.3). Systematic deletion of UPR core genes identified
the J-domain containing ER co-chaperone Dnj1
(DnaJ-like protein 1) as an important factor for
fungal virulence, ER stress resistance, and effector secretion (Lo Presti et al. 2016). Signal peptide peptidases (SPP) are aspartic proteases
catalyzing the intramembrane cleavage of type
II oriented transmembrane (TM) domains in
the ER membrane (Friedmann et al. 2004; Weihofen et al. 2002), including remnants of signal
peptides (after initial cleavage by the signal
peptidase), ER-TM containing transcription
factors, and high-affinity nutrient transporters
(Avci et al. 2014; Bat-Ochir et al. 2016). The
signal peptide peptidase Spp1 is a UPR core
gene in U. maydis and critical for fungal virulence. Spp1 is specifically required for plant
defense suppression, as evidenced by massive
accumulation of ROS in plant tissue infected by
Dspp1 mutants and strongly induced expression of salicylic acid marker genes. The absence
of Spp1 neither affects resistance to ER, cell
wall, or oxidative stress nor pathogenic development prior plant infection, and the
virulence-specific function of Spp1 is not
connected to previously reported physiological
roles of the conserved SPP protein family. Catalytic activity of Spp1 is essential for plant
defense suppression but not required for secretion or processing of effectors (Pinter et al.
2019), pointing towards a central role of Spp1dependently produced cleavage products that
are directly or indirectly involved in manipulating the plant immune system. Although the
3 Genetics of the Unfolded Protein Response in Fungi
61
proliferation of the dikaryon is initiated (FlorParra et al. 2007; Heimel et al. 2010a; PerezMartin and Castillo-Lluva 2008; Scherer et al.
2006). Concomitant with the massive fungal
proliferation, plant tumors are formed in
which fungal hyphae differentiate into diploid,
melanized teliospores. Pathogenic development
is controlled by the bE/bW heterodimer,
encoded by the b mating-type locus. While
activity of the b-heterodimer is sufficient for
the activation of the pathogenic program
including filament formation and appressoriamediated plant penetration, proliferation of the
dikaryon in the host plant is dependent on the
functional modulation of the b-heterodimer by
the Clp1 protein. Clp1 physically interacts with
bW and with the master regulator downstream
of the b-heterodimer, Rbf1, ultimately reducing
activity of the mating-type signaling pathway to
overcome the G2 cell cycle arrest and to induce
mitotic growth of the dikaryon in planta (Heimel et al. 2010a; Scherer et al. 2006). The timing
of cell cycle release is mediated by Clp1 accumulation, which is stabilized via the interaction
with the Hac1 homolog Cib1. Cib1 is specifically expressed after successful plant penetration (Heimel et al. 2013; Heimel et al. 2010a),
and the UPR is continuously active during the
fungal/plant interaction. UPR activity is presumably connected to the strongly increased
expression of large numbers of effector genes
at this stage of the life cycle (Lanver et al. 2018)
and required for efficient secretion and processing of effector proteins including the wellcharacterized Pit2, Tin2, Cmu1, and Pep1 effectors (Hampel et al. 2016; Lo Presti et al. 2016;
Pinter et al. 2019). The stage-specific depletion
of Cib1 during biotrophic development
revealed that a functional UPR is not only
important for the establishment of a compatible
biotrophic interaction after plant penetration
but is continuously required for suppressing
the plant defense and maintenance of fungal
biotrophy (Schmitz et al. 2019a). Interestingly,
expression of pit2 and tin1-1 effector genes is
controlled directly by Cib1 binding to their
promoter regions, and abolishment of this regulation by targeted deletion of the Cib1 binding
site in the pit1/2 promoter leads to significantly
reduced virulence of U. maydis (Hampel et al.
2016). The interaction between Clp1 and Cib1
affects phosphorylation and stability of Cib1
and confers strongly increased ER stress resistance by modulated UPR gene expression.
Genome-wide analysis of Clp1-dependent
effects on UPR genes revealed a set of 65 UPR
core genes, which are induced by Cib1 and
differentially modulated in their expression by
Clp1. ChIP-seq (Chromatin Immunoprecipitation followed by massive parallel sequencing)
analysis identified a consensus UPRE motif
bound by Cib1 (TGACGTGG) (Hampel et al.
2016; Pinter et al. 2019), resembling the mammalian XBP1 binding motif (GATGACGTGG)
(Yamamoto et al. 2004) (see also Fig. 3.3). Systematic deletion of UPR core genes identified
the J-domain containing ER co-chaperone Dnj1
(DnaJ-like protein 1) as an important factor for
fungal virulence, ER stress resistance, and effector secretion (Lo Presti et al. 2016). Signal peptide peptidases (SPP) are aspartic proteases
catalyzing the intramembrane cleavage of type
II oriented transmembrane (TM) domains in
the ER membrane (Friedmann et al. 2004; Weihofen et al. 2002), including remnants of signal
peptides (after initial cleavage by the signal
peptidase), ER-TM containing transcription
factors, and high-affinity nutrient transporters
(Avci et al. 2014; Bat-Ochir et al. 2016). The
signal peptide peptidase Spp1 is a UPR core
gene in U. maydis and critical for fungal virulence. Spp1 is specifically required for plant
defense suppression, as evidenced by massive
accumulation of ROS in plant tissue infected by
Dspp1 mutants and strongly induced expression of salicylic acid marker genes. The absence
of Spp1 neither affects resistance to ER, cell
wall, or oxidative stress nor pathogenic development prior plant infection, and the
virulence-specific function of Spp1 is not
connected to previously reported physiological
roles of the conserved SPP protein family. Catalytic activity of Spp1 is essential for plant
defense suppression but not required for secretion or processing of effectors (Pinter et al.
2019), pointing towards a central role of Spp1dependently produced cleavage products that
are directly or indirectly involved in manipulating the plant immune system. Although the
3 Genetics of the Unfolded Protein Response in Fungi
61