essential for SNF establishment (Shimoda et al.
2012). Moreover, different calcium signatures
have been proposed to act in the two symbioses, with specific spiking patterns and differential responding cell types (Russo et al.
2013).
2. Symbiosis Establishment, Functioning, and
Senescence
IPD3/CYCLOPS targets a number of genes
through a cis-element called AM-CYC box
(Favre et al. 2014; Pimprikar et al. 2016). Interestingly, CYCLOPS also interacts with the gibberellin signaling repressor DELLA (Pimprikar
et al. 2016), and Medicago truncatula della
mutants are impaired in AM and SNF colonization (Floss et al. 2013; Jin et al. 2016). DELLA
proteins are repressors of gibberellic acid (GA)
signaling (Alvey and Harberd 2005), and GA
was shown to accumulate in Lotus japonicus
roots during mycorrhization (Takeda et al.
2015). Furthermore, DELLA proteins also control key developmental processes such as the
cell cycle (Gallego-Bartolome et al. 2012;
Daviere and Achard 2013). In M. truncatula,
DELLA1 and DELLA2 act redundantly to promote arbuscule development (Floss et al. 2013;
Foo et al. 2013; Yu et al. 2014). In short, the
CCaMK-CYCLOPS-DELLA pathway may act in
the adjustment of AM establishment based on
plant developmental and environmental stimuli
(Carbonnel and Gutjahr 2014; BreuillinSessoms et al. 2015; Nagata et al. 2015; Konvalinkova and Jansa 2016).
Among the CCaMK/CYCLOPS-regulated
transcription factors, the GRAS-domain proteins NSP1 (Nodulation Signalling Pathway 1)
and NSP2 play an essential role in Nod factor
signaling (Catoira et al. 2000; Kalo ´ et al. 2005;
Smit et al. 2005). Evidence suggests direct roles
of NSP1 and NSP2 also in Myc factor signaling:
with NSP2 being involved in NS-LCO-induced
lateral root growth (Maillet et al. 2011) and
NSP1 being required for the induction of three
mycorrhizal genes in response to NS-LCO
(Delaux et al. 2013). NSP1 and NSP2 interaction
is required for the induction of nodulationspecific promoters (Hirsch et al. 2009; Jin
et al. 2016) but not crucial for AM symbiosis,
suggesting that different GRAS transcription
factor complexes regulate distinct groups of
genes (Pimprikar and Gutjahr 2018).
RAM1 is an AM-specific transcription factor regulated by the CYCLOPS/DELLA complex
(Gobbato et al. 2012). RAM1 expression
restores arbuscule formation in cyclops mutants
(Pimprikar et al. 2016), and both RAM1 expression and arbuscule formation are rescued in
cyclops mutants by either overexpression of
DELLA1 (Floss et al. 2013; Park et al. 2015) or
GA treatment (Pimprikar et al. 2016). RAM1
forms a complex with NSP2 that was proposed
to regulate the expression of AM-specific genes,
similarly to the NSP1/NSP2 complex in SNF
(Gobbato et al. 2012).
Additional GRAS proteins were shown to
have a role in fungal colonization and arbuscule
development, such as RAD1 (Required for
Arbuscule Development 1), an interactor of
RAM1 and NSP2 (Xue et al. 2015), or DIP1
(Della Interacting Protein 1), a rice GRAS family interactor of the DELLA protein SLR1 (Yu
et al. 2014). Furthermore, two CAAT-box transcription factors, MtCbf3 and MtCbf4, were
involved in the pre-symbiotic stage (Hogekamp
et al. 2011; Czaja et al. 2012; Hogekamp and
Kuster 2013).
Lastly, recent studies revealed that the CSSP
also activates several other effectors, including
miRNA and small interfering RNA (siRNA)
with a critical role in transcriptional regulation
(Lelandais-Brie ´re et al. 2016; Bazin et al. 2017),
mRNA splicing, RNA-directed DNA methylation, and epigenetic functions (Ariel et al.
2015; Chekanova 2015).
Following this chemical courtship, the presymbiotic phase of AM development culminates in physical contact between symbionts,
with a hyphal tip touching the root surface
(Bonfante and Genre 2010). This crucial step
in AM symbiosis consists in the formation of
a large, swollen, and often branched hyphopodium, attached to the root epidermal surface.
Epidermal cell wall was proposed to stimulate
hyphopodium differentiation (Nagahashi and
Douds 1997) through specific physicochemical
signals. This idea arose from observations in
rice – where hyphopodia form on large lateral
7 Genetics and Genomics Decipher Partner Biology in Arbuscular Mycorrhizas
153
2012). Moreover, different calcium signatures
have been proposed to act in the two symbioses, with specific spiking patterns and differential responding cell types (Russo et al.
2013).
2. Symbiosis Establishment, Functioning, and
Senescence
IPD3/CYCLOPS targets a number of genes
through a cis-element called AM-CYC box
(Favre et al. 2014; Pimprikar et al. 2016). Interestingly, CYCLOPS also interacts with the gibberellin signaling repressor DELLA (Pimprikar
et al. 2016), and Medicago truncatula della
mutants are impaired in AM and SNF colonization (Floss et al. 2013; Jin et al. 2016). DELLA
proteins are repressors of gibberellic acid (GA)
signaling (Alvey and Harberd 2005), and GA
was shown to accumulate in Lotus japonicus
roots during mycorrhization (Takeda et al.
2015). Furthermore, DELLA proteins also control key developmental processes such as the
cell cycle (Gallego-Bartolome et al. 2012;
Daviere and Achard 2013). In M. truncatula,
DELLA1 and DELLA2 act redundantly to promote arbuscule development (Floss et al. 2013;
Foo et al. 2013; Yu et al. 2014). In short, the
CCaMK-CYCLOPS-DELLA pathway may act in
the adjustment of AM establishment based on
plant developmental and environmental stimuli
(Carbonnel and Gutjahr 2014; BreuillinSessoms et al. 2015; Nagata et al. 2015; Konvalinkova and Jansa 2016).
Among the CCaMK/CYCLOPS-regulated
transcription factors, the GRAS-domain proteins NSP1 (Nodulation Signalling Pathway 1)
and NSP2 play an essential role in Nod factor
signaling (Catoira et al. 2000; Kalo ´ et al. 2005;
Smit et al. 2005). Evidence suggests direct roles
of NSP1 and NSP2 also in Myc factor signaling:
with NSP2 being involved in NS-LCO-induced
lateral root growth (Maillet et al. 2011) and
NSP1 being required for the induction of three
mycorrhizal genes in response to NS-LCO
(Delaux et al. 2013). NSP1 and NSP2 interaction
is required for the induction of nodulationspecific promoters (Hirsch et al. 2009; Jin
et al. 2016) but not crucial for AM symbiosis,
suggesting that different GRAS transcription
factor complexes regulate distinct groups of
genes (Pimprikar and Gutjahr 2018).
RAM1 is an AM-specific transcription factor regulated by the CYCLOPS/DELLA complex
(Gobbato et al. 2012). RAM1 expression
restores arbuscule formation in cyclops mutants
(Pimprikar et al. 2016), and both RAM1 expression and arbuscule formation are rescued in
cyclops mutants by either overexpression of
DELLA1 (Floss et al. 2013; Park et al. 2015) or
GA treatment (Pimprikar et al. 2016). RAM1
forms a complex with NSP2 that was proposed
to regulate the expression of AM-specific genes,
similarly to the NSP1/NSP2 complex in SNF
(Gobbato et al. 2012).
Additional GRAS proteins were shown to
have a role in fungal colonization and arbuscule
development, such as RAD1 (Required for
Arbuscule Development 1), an interactor of
RAM1 and NSP2 (Xue et al. 2015), or DIP1
(Della Interacting Protein 1), a rice GRAS family interactor of the DELLA protein SLR1 (Yu
et al. 2014). Furthermore, two CAAT-box transcription factors, MtCbf3 and MtCbf4, were
involved in the pre-symbiotic stage (Hogekamp
et al. 2011; Czaja et al. 2012; Hogekamp and
Kuster 2013).
Lastly, recent studies revealed that the CSSP
also activates several other effectors, including
miRNA and small interfering RNA (siRNA)
with a critical role in transcriptional regulation
(Lelandais-Brie ´re et al. 2016; Bazin et al. 2017),
mRNA splicing, RNA-directed DNA methylation, and epigenetic functions (Ariel et al.
2015; Chekanova 2015).
Following this chemical courtship, the presymbiotic phase of AM development culminates in physical contact between symbionts,
with a hyphal tip touching the root surface
(Bonfante and Genre 2010). This crucial step
in AM symbiosis consists in the formation of
a large, swollen, and often branched hyphopodium, attached to the root epidermal surface.
Epidermal cell wall was proposed to stimulate
hyphopodium differentiation (Nagahashi and
Douds 1997) through specific physicochemical
signals. This idea arose from observations in
rice – where hyphopodia form on large lateral
7 Genetics and Genomics Decipher Partner Biology in Arbuscular Mycorrhizas
153