presence of a signal peptide that guides proteins toward the endomembrane system for
secretion (Kloppholz et al. 2011; Tisserant
et al. 2013; Lin et al. 2014; Se ˛dzielewska Toro
and Brachmann 2016; Kamel et al. 2017; Zeng
et al. 2018) since the secretome includes proteins called effectors which are of crucial relevance in host-microbe interactions. Effectors
are microbial molecules that, once delivered to
the host cells, can manipulate cellular mechanisms often leading to an attenuation of innate
immune response or a promotion of nutrient
exchange and thus favoring host colonization
(Lo Presti et al. 2015).
AM fungi possess a rather rich secretome
with hundreds of candidate secreted proteins
which, in the different publications, may vary
in number according to the criteria used to
define them. Comparative analyses of genomes
and transcriptomic data from AM fungi showed
that many secreted proteins are conserved in
phylogenetically related AM species; however,
in analogy to other fungal groups with different
lifestyles/nutritional strategies (Schirawski
et al. 2010; Heard et al. 2015; Pellegrin et al.
2015), there is a prevalence of lineage-specific
proteins, suggesting specific biological roles.
Indeed, AM effectors have also been hypothesized to be important factors to control symbiotic efficiency and/or host preferences (Zeng
et al. 2018), two aspects of AM fungi biology
whose molecular mechanisms are still largely
unknown.
Interestingly, two studies have clearly
demonstrated that, while some secreted proteins showed similar gene expression levels in
different host plants, suggesting that they fulfill
conserved roles, a subset of them were differentially expressed depending on the host species (Kamel et al. 2017; Zeng et al. 2018). Hostspecifically expressed secreted proteins, candidate effectors, also have been observed for the
endophyte Piriformospora indica (Lahrmann
et al. 2013). Evidence that these secreted proteins can play a significant role in host specificity also comes from plant pathogens where
their evolution seems to be under host-directed
selection (Zhong et al. 2016).
On the other hand, a small set of secreted
proteins, also shared by distantly related AM
fungi (Rhizophagus irregularis and Gigaspora
rosea), showed similar expression patterns in
different host plants (Kamel et al. 2017). These
genes, described as the AM symbiotic core
secretome, encode proteins with unknown
function or proteases. The proteolytic activity
may play a role in the production of oligopeptides and amino acids with nutritional roles, the
inactivation of plant defense proteins (Jashni
et al. 2015), or the generation or turnover of
fungal/plant signaling proteins.
Induced expression in planta is a commonly applied additional predictive criterion
to identify effectors among secreted proteins.
Gene expression profiles from laser microdissected cells even allowed to identify a set of
genes most specifically expressed at the arbuscule stage (Zeng et al. 2018). Although the
majority of them are orphan genes, some
secreted proteins could be associated to lipid
signaling (which is of particular interest considering the finding of fatty acid auxotrophy of
AM fungi) or show homology, again, to endopeptidases (Zeng et al. 2018). But, so far, only
three AM effectors have been characterized in
detail. The first, called SP7, was shown to target
the host cell nucleus where it counteracts the
function of the pathogenesis-related transcription factor MtERF19 (Kloppholz et al. 2011).
The putative secreted protein SIS1 from R. irregularis was found among those genes upregulated in strigolactones-treated germinating
spores (Tsuzuki et al. 2016) and strongly
expressed in the intraradical mycelium, including arbuscules (Zeng et al. 2018), in line with its
predicted role in intraradical colonization
(Tsuzuki et al. 2016). Recently, a crinkler
(CRN) effector (RiCRN1) that belongs to a subfamily of secreted CRN proteins from R. irregularis was also characterized (Voß et al. 2018).
As CRNs were originally described in plant
pathogenic oomycetes (Schornack et al. 2009),
this finding extends the similarity between AM
fungi and plant pathogens also outside the fungal kingdom. Although not yet defined, the
mechanism of action of RiCRN1 does not
involve cell death processes as often described
for CRNs from oomycetes.
In all these three abovementioned cases,
host-induced gene silencing (HIGS) has been
148
L. Lanfranco et al.
secretion (Kloppholz et al. 2011; Tisserant
et al. 2013; Lin et al. 2014; Se ˛dzielewska Toro
and Brachmann 2016; Kamel et al. 2017; Zeng
et al. 2018) since the secretome includes proteins called effectors which are of crucial relevance in host-microbe interactions. Effectors
are microbial molecules that, once delivered to
the host cells, can manipulate cellular mechanisms often leading to an attenuation of innate
immune response or a promotion of nutrient
exchange and thus favoring host colonization
(Lo Presti et al. 2015).
AM fungi possess a rather rich secretome
with hundreds of candidate secreted proteins
which, in the different publications, may vary
in number according to the criteria used to
define them. Comparative analyses of genomes
and transcriptomic data from AM fungi showed
that many secreted proteins are conserved in
phylogenetically related AM species; however,
in analogy to other fungal groups with different
lifestyles/nutritional strategies (Schirawski
et al. 2010; Heard et al. 2015; Pellegrin et al.
2015), there is a prevalence of lineage-specific
proteins, suggesting specific biological roles.
Indeed, AM effectors have also been hypothesized to be important factors to control symbiotic efficiency and/or host preferences (Zeng
et al. 2018), two aspects of AM fungi biology
whose molecular mechanisms are still largely
unknown.
Interestingly, two studies have clearly
demonstrated that, while some secreted proteins showed similar gene expression levels in
different host plants, suggesting that they fulfill
conserved roles, a subset of them were differentially expressed depending on the host species (Kamel et al. 2017; Zeng et al. 2018). Hostspecifically expressed secreted proteins, candidate effectors, also have been observed for the
endophyte Piriformospora indica (Lahrmann
et al. 2013). Evidence that these secreted proteins can play a significant role in host specificity also comes from plant pathogens where
their evolution seems to be under host-directed
selection (Zhong et al. 2016).
On the other hand, a small set of secreted
proteins, also shared by distantly related AM
fungi (Rhizophagus irregularis and Gigaspora
rosea), showed similar expression patterns in
different host plants (Kamel et al. 2017). These
genes, described as the AM symbiotic core
secretome, encode proteins with unknown
function or proteases. The proteolytic activity
may play a role in the production of oligopeptides and amino acids with nutritional roles, the
inactivation of plant defense proteins (Jashni
et al. 2015), or the generation or turnover of
fungal/plant signaling proteins.
Induced expression in planta is a commonly applied additional predictive criterion
to identify effectors among secreted proteins.
Gene expression profiles from laser microdissected cells even allowed to identify a set of
genes most specifically expressed at the arbuscule stage (Zeng et al. 2018). Although the
majority of them are orphan genes, some
secreted proteins could be associated to lipid
signaling (which is of particular interest considering the finding of fatty acid auxotrophy of
AM fungi) or show homology, again, to endopeptidases (Zeng et al. 2018). But, so far, only
three AM effectors have been characterized in
detail. The first, called SP7, was shown to target
the host cell nucleus where it counteracts the
function of the pathogenesis-related transcription factor MtERF19 (Kloppholz et al. 2011).
The putative secreted protein SIS1 from R. irregularis was found among those genes upregulated in strigolactones-treated germinating
spores (Tsuzuki et al. 2016) and strongly
expressed in the intraradical mycelium, including arbuscules (Zeng et al. 2018), in line with its
predicted role in intraradical colonization
(Tsuzuki et al. 2016). Recently, a crinkler
(CRN) effector (RiCRN1) that belongs to a subfamily of secreted CRN proteins from R. irregularis was also characterized (Voß et al. 2018).
As CRNs were originally described in plant
pathogenic oomycetes (Schornack et al. 2009),
this finding extends the similarity between AM
fungi and plant pathogens also outside the fungal kingdom. Although not yet defined, the
mechanism of action of RiCRN1 does not
involve cell death processes as often described
for CRNs from oomycetes.
In all these three abovementioned cases,
host-induced gene silencing (HIGS) has been
148
L. Lanfranco et al.