that is believed to trigger arbuscule degeneration by inducing hydrolase genes (Floss et al.
2017). In fact, arbuscule senescence is a regulated process where the host cell remains active
during and after arbuscule collapse and maintains the ability to be colonized again by a new
arbuscule.
B. Mycorrhizal Omics: From Local to Systemic
Responses
The AM symbiosis develops in roots where
extensive cellular reorganizations and specific
metabolic changes occur, which are mirrored
by local changes in the transcript profiles as it
has been demonstrated by transcriptomic analyses carried out on several plant species. The
root metabolome is also reprogrammed upon
mycorrhization. Laparre et al. (2014) detected
71 compounds exclusively present or more
abundant in M. truncatula roots colonized by
R. irregularis, including propionyl- and
butyryl-carnitines. Remarkably, the accumulation of carnitine, which is known to be involved
in lipid metabolism, could reflect changes in the
fungal metabolism and can be linked to the
described fatty acid auxotrophy of AM fungi.
An untargeted metabolomic analysis was
also recently performed on tomato mycorrhizal
roots with the aim to identify key metabolites
involved in the mycorrhiza-induced protection
against osmotic stresses (Rivero et al. 2018).
AM-colonized roots accumulated some amino
acids, lignans, oxylipins, and carotenoids,
among which some compounds are known to
be involved in plant stress adaptation (Havaux
2014). Interestingly, the protective effect of the
mycorrhizal symbiosis was higher than that
observed upon exogenous application of purified compounds highlighting that the AM symbiosis could be considered a more versatile
strategy for plant protection.
Even if AM colonization is physically confined to root tissues, epigeous portions of the
plants also experience physiological and metabolic changes. Indeed, transcriptomic analyses
revealed significant gene modulation in shoots
(Liu et al. 2007; Fiorilli et al. 2009; CervantesGa ´mez et al. 2016) and even fruits (Zouari
et al. 2014) of mycorrhizal plants, indicating
the occurrence of a long-distance systemic
response. Reorganization of the metabolic profiles was also observed in leaves of mycorrhizal
plants. A comparative multi-species metabolomic approach carried out on plants inoculated
with the same AM fungus revealed that,
although a core metabolome could be identified, leaves metabolic responses to arbuscular
mycorrhiza showed strong species specificity
(Schweiger et al. 2014).
It has been envisaged that these AMinduced changes at systemic level may have
an impact on the outcome of biotic and abiotic
interactions. Martinez-Medina et al. (2016)
have described how mycorrhizal fungi as beneficial microbes induce a priming status (i.e., the
induction of a physiological state in which a
plant is conditioned for the activation of
defenses against environmental challenges).
Interestingly, such priming status is raised
also by native microbiota which are associated
to tomato, including also AM fungi (Chialva
et al. 2018). As a consequence, mycorrhizal
plants acquire the so-called mycorrhizainduced resistance (MIR; Jung et al. 2012;
Cameron et al. 2013), thanks to which they
have been shown to alleviate the damage caused
by pathogen attacks. The dissection of the tripartite interaction among wheat, the AM fungus Funneliformis mosseae, and the bacterial
pathogen Xanthomonas translucens by using a
combined transcriptomic-proteomic-metabolomic approach revealed that AM symbiosis
does exert a positive effect on wheat growth
and productivity but also does provide protection against X. translucens (Fiorilli et al. 2018).
Indeed, induction of genes involved in a general
defense line (e.g., coding for pathogenesisrelated proteins, or leading to ROS formation)
was the result of the AM fungus presence at
local and systemic level, while specific defense
genes (encoding, e.g., a cytochrome P450
enzyme, involved in iron binding and with oxidoreductase activity) were detected exclusively
after the pathogen attack (Fiorilli et al. 2018).
It is worth to mention that studies on leaf
metabolome have also been instrumental for
the identification of blumenol-derived compounds which were detected in leaves of several
7 Genetics and Genomics Decipher Partner Biology in Arbuscular Mycorrhizas
157
2017). In fact, arbuscule senescence is a regulated process where the host cell remains active
during and after arbuscule collapse and maintains the ability to be colonized again by a new
arbuscule.
B. Mycorrhizal Omics: From Local to Systemic
Responses
The AM symbiosis develops in roots where
extensive cellular reorganizations and specific
metabolic changes occur, which are mirrored
by local changes in the transcript profiles as it
has been demonstrated by transcriptomic analyses carried out on several plant species. The
root metabolome is also reprogrammed upon
mycorrhization. Laparre et al. (2014) detected
71 compounds exclusively present or more
abundant in M. truncatula roots colonized by
R. irregularis, including propionyl- and
butyryl-carnitines. Remarkably, the accumulation of carnitine, which is known to be involved
in lipid metabolism, could reflect changes in the
fungal metabolism and can be linked to the
described fatty acid auxotrophy of AM fungi.
An untargeted metabolomic analysis was
also recently performed on tomato mycorrhizal
roots with the aim to identify key metabolites
involved in the mycorrhiza-induced protection
against osmotic stresses (Rivero et al. 2018).
AM-colonized roots accumulated some amino
acids, lignans, oxylipins, and carotenoids,
among which some compounds are known to
be involved in plant stress adaptation (Havaux
2014). Interestingly, the protective effect of the
mycorrhizal symbiosis was higher than that
observed upon exogenous application of purified compounds highlighting that the AM symbiosis could be considered a more versatile
strategy for plant protection.
Even if AM colonization is physically confined to root tissues, epigeous portions of the
plants also experience physiological and metabolic changes. Indeed, transcriptomic analyses
revealed significant gene modulation in shoots
(Liu et al. 2007; Fiorilli et al. 2009; CervantesGa ´mez et al. 2016) and even fruits (Zouari
et al. 2014) of mycorrhizal plants, indicating
the occurrence of a long-distance systemic
response. Reorganization of the metabolic profiles was also observed in leaves of mycorrhizal
plants. A comparative multi-species metabolomic approach carried out on plants inoculated
with the same AM fungus revealed that,
although a core metabolome could be identified, leaves metabolic responses to arbuscular
mycorrhiza showed strong species specificity
(Schweiger et al. 2014).
It has been envisaged that these AMinduced changes at systemic level may have
an impact on the outcome of biotic and abiotic
interactions. Martinez-Medina et al. (2016)
have described how mycorrhizal fungi as beneficial microbes induce a priming status (i.e., the
induction of a physiological state in which a
plant is conditioned for the activation of
defenses against environmental challenges).
Interestingly, such priming status is raised
also by native microbiota which are associated
to tomato, including also AM fungi (Chialva
et al. 2018). As a consequence, mycorrhizal
plants acquire the so-called mycorrhizainduced resistance (MIR; Jung et al. 2012;
Cameron et al. 2013), thanks to which they
have been shown to alleviate the damage caused
by pathogen attacks. The dissection of the tripartite interaction among wheat, the AM fungus Funneliformis mosseae, and the bacterial
pathogen Xanthomonas translucens by using a
combined transcriptomic-proteomic-metabolomic approach revealed that AM symbiosis
does exert a positive effect on wheat growth
and productivity but also does provide protection against X. translucens (Fiorilli et al. 2018).
Indeed, induction of genes involved in a general
defense line (e.g., coding for pathogenesisrelated proteins, or leading to ROS formation)
was the result of the AM fungus presence at
local and systemic level, while specific defense
genes (encoding, e.g., a cytochrome P450
enzyme, involved in iron binding and with oxidoreductase activity) were detected exclusively
after the pathogen attack (Fiorilli et al. 2018).
It is worth to mention that studies on leaf
metabolome have also been instrumental for
the identification of blumenol-derived compounds which were detected in leaves of several
7 Genetics and Genomics Decipher Partner Biology in Arbuscular Mycorrhizas
157