used to specifically silence the fungal genes
during the symbiotic phase allowing the
description of an impaired colonization pattern. These examples also highlight how, in
the absence of protocols for stable genetic
transformation for AM fungi, genetic manipulation tools developed for the host plants can be
successfully applied to study the function of
AM fungal genes at least in the in planta phase.
The availability of genome and transcriptomic data also allowed the large-scale analysis
of Cu, Fe, and Zn transporters: beside an expansion of some gene families, it has been observed
that some genes were upregulated in the intraradical phase suggesting that metals are important for plant colonization (Tamayo et al. 2014),
in analogy to what has been observed for the
endophytic fungus Epichloe ¨ festucae (Johnson
et al. 2013).
Analogous investigations led to an inventory of conserved proteins of the RNA interference machinery (RNAi) in Rhizoglomus
irregulare with the discovery of putative
ancient events of horizontal gene transfer
involving two class I ribonuclease III proteincoding genes possibly from cyanobacterial genomes (Lee et al. 2018). From an evolutionary
perspective, this finding may reflect an ancient
symbiosis history of AM fungi with cyanobacteria. Remarkably, this type of interaction can
still be observed today between Geosiphon pyriforme, an AM fungus assigned to the basal
order Archaeosporales and the cyanobacterium
Nostoc punctiforme (Gehrig et al. 1996). The
presence of a RNA silencing machinery suggests that R. irregulare has the potential to
produce small RNAs and, hypothetically, to
use them also as effectors, in a process so called
cross-kingdom RNAi, as it has been shown in
the pathogenic interaction of Botrytis cinerea
and Arabidopsis (Wang et al. 2017). Some evidences in this direction have been recently
obtained (Silvestri et al. 2019). Further investigations are needed to verify whether also AM
fungi use RNA effectors to regulate plant processes; on the other hand, the inverse phenomenon, that is, the delivery of small RNA with
gene silencing purposes from the plant to the
fungus, is also likely to occur as again it has
been demonstrated in the Botrytis cinerea-Arabidopsis interaction (Cai et al. 2018). The success of the HIGS approach as a tool to silence
fungal genes in the AM symbiosis (Helber et al.
2011; Tsuzuki et al. 2016) is a strong clue
toward the occurrence of such a process.
IV. Molecular Tools Reveal Plant
Responses to AM Fungi
A. Cellular and Molecular Changes in the Host
Plant
In analogy to most root-microbe interactions,
AM establishment depends on finely tuned recognition processes (Bonfante and Genre 2015)
through signal release and perception between
both partners before their physical contact
(Gianinazzi-Pearson 1996).
Root exudates (Fig. 7.2) contain several
bioactive, low molecular weight compounds.
Among them, strigolactones are a class of terpenic compounds deriving from the carotenoid
metabolism. They are biosynthesized in the
root and actively transported to the shoot or
released in the rhizosphere (Gomez-Roldan
et al. 2008). Strigolactones, whose release
increases under phosphate starvation, rapidly
undergo spontaneous hydrolysis in water solutions, which limits their diffusion in the rhizosphere and makes them reliable signals of root
vicinity for several root-interacting organisms.
Indeed, strigolactones were originally identified
in plant root exudates as germination stimulants for parasitic plants of the family Orobanchaceae (Matusova et al. 2005). Later,
strigolactones have been reported to be indispensable for the establishment of AM (Akiyama
et al. 2005, 2010; Besserer et al. 2006, 2008) and
more recently also to be involved in symbiotic
nitrogen fixation (Soto et al. 2009; Foo and
Davies 2011).
Strigolactone perception triggers a cascade
of molecular and cellular events in AM fungi,
such as nuclear multiplication, mitochondrial
growth, and a fast increase of cytotoxic NADH
and ATP content, indicating respiration as one
of the primary metabolic targets (Akiyama and
Hayashi 2006). Such cellular responses associ7 Genetics and Genomics Decipher Partner Biology in Arbuscular Mycorrhizas
149
during the symbiotic phase allowing the
description of an impaired colonization pattern. These examples also highlight how, in
the absence of protocols for stable genetic
transformation for AM fungi, genetic manipulation tools developed for the host plants can be
successfully applied to study the function of
AM fungal genes at least in the in planta phase.
The availability of genome and transcriptomic data also allowed the large-scale analysis
of Cu, Fe, and Zn transporters: beside an expansion of some gene families, it has been observed
that some genes were upregulated in the intraradical phase suggesting that metals are important for plant colonization (Tamayo et al. 2014),
in analogy to what has been observed for the
endophytic fungus Epichloe ¨ festucae (Johnson
et al. 2013).
Analogous investigations led to an inventory of conserved proteins of the RNA interference machinery (RNAi) in Rhizoglomus
irregulare with the discovery of putative
ancient events of horizontal gene transfer
involving two class I ribonuclease III proteincoding genes possibly from cyanobacterial genomes (Lee et al. 2018). From an evolutionary
perspective, this finding may reflect an ancient
symbiosis history of AM fungi with cyanobacteria. Remarkably, this type of interaction can
still be observed today between Geosiphon pyriforme, an AM fungus assigned to the basal
order Archaeosporales and the cyanobacterium
Nostoc punctiforme (Gehrig et al. 1996). The
presence of a RNA silencing machinery suggests that R. irregulare has the potential to
produce small RNAs and, hypothetically, to
use them also as effectors, in a process so called
cross-kingdom RNAi, as it has been shown in
the pathogenic interaction of Botrytis cinerea
and Arabidopsis (Wang et al. 2017). Some evidences in this direction have been recently
obtained (Silvestri et al. 2019). Further investigations are needed to verify whether also AM
fungi use RNA effectors to regulate plant processes; on the other hand, the inverse phenomenon, that is, the delivery of small RNA with
gene silencing purposes from the plant to the
fungus, is also likely to occur as again it has
been demonstrated in the Botrytis cinerea-Arabidopsis interaction (Cai et al. 2018). The success of the HIGS approach as a tool to silence
fungal genes in the AM symbiosis (Helber et al.
2011; Tsuzuki et al. 2016) is a strong clue
toward the occurrence of such a process.
IV. Molecular Tools Reveal Plant
Responses to AM Fungi
A. Cellular and Molecular Changes in the Host
Plant
In analogy to most root-microbe interactions,
AM establishment depends on finely tuned recognition processes (Bonfante and Genre 2015)
through signal release and perception between
both partners before their physical contact
(Gianinazzi-Pearson 1996).
Root exudates (Fig. 7.2) contain several
bioactive, low molecular weight compounds.
Among them, strigolactones are a class of terpenic compounds deriving from the carotenoid
metabolism. They are biosynthesized in the
root and actively transported to the shoot or
released in the rhizosphere (Gomez-Roldan
et al. 2008). Strigolactones, whose release
increases under phosphate starvation, rapidly
undergo spontaneous hydrolysis in water solutions, which limits their diffusion in the rhizosphere and makes them reliable signals of root
vicinity for several root-interacting organisms.
Indeed, strigolactones were originally identified
in plant root exudates as germination stimulants for parasitic plants of the family Orobanchaceae (Matusova et al. 2005). Later,
strigolactones have been reported to be indispensable for the establishment of AM (Akiyama
et al. 2005, 2010; Besserer et al. 2006, 2008) and
more recently also to be involved in symbiotic
nitrogen fixation (Soto et al. 2009; Foo and
Davies 2011).
Strigolactone perception triggers a cascade
of molecular and cellular events in AM fungi,
such as nuclear multiplication, mitochondrial
growth, and a fast increase of cytotoxic NADH
and ATP content, indicating respiration as one
of the primary metabolic targets (Akiyama and
Hayashi 2006). Such cellular responses associ7 Genetics and Genomics Decipher Partner Biology in Arbuscular Mycorrhizas
149