dicot and monocotyledonous plants so that
they have been proposed as foliar markers of
the AM association (Wang et al. 2018). Blumenols therefore represent an extremely powerful
tool for high-throughput screening for a functional AM symbiosis and can be used instead of
laborious measurements of AM-induced transcripts or microscopic analyses.
Small RNA (sRNA) molecules are also
attractive candidates for long-distance signaling and have been targeted by -omics analyses
in arbuscular mycorrhizas. So far investigations
focused on roots revealed a large-scale reprograming of microRNAs (miRNAs) upon AM
colonization (Devers et al. 2011; Wu et al.
2016; Pandey et al. 2018). In a recent work, the
analysis of putative targets of selected miRNAs
revealed an involvement in P starvation, phytohormone signaling, and defense (Pandey et al.
2018).
In conclusion, all these studies convincingly demonstrate that AM fungi have a local
and systemic influence on their host plant,
since they lead to a deep reorganization of the
plant biology acting on multiple transcriptomic, regulatory, and metabolomic pathways.
V. The AM Symbiosis in the Light of
Natural Variation
Despite the low morphological variation and
their large host range, AM fungal species and
isolates show different efficiency in promoting
plant performance; on the other hand, the plant
genotype has an important role in determining
the extent of plant responsiveness to the AM
symbiosis (Smith et al. 2004). AM fungi can
present high functional diversity: even isolates
belonging to the same species can exert on a
specific host plant different growth effects,
which can vary in amplitude and direction
(promotion or inhibition) (Hart and Reader
2002; Munkvold et al. 2004; Feddermann et al.
2008; Antunes et al. 2011; Hong et al. 2012). An
extensive comparative study, considering
56 AM isolates belonging to 6 different families
and 17 genera inoculated on 3 different host
plants, revealed that the plant growth response
could not be predicted from AM species identity or morphological traits, such as extra- and
intraradical fungal volumes (Koch et al. 2017).
It can be hypothesized that the functional variability of the fungal symbionts may rely on other
factors such as qualitative and/or quantitative
differences in the production of signaling molecules, such as chitin oligomers, and/or effectors, which are crucial for triggering the
symbiotic program and dampening the plant
immune response, and in the expression and
functioning of transporters and/or metabolic
enzymes which control nutrient exchanges
and may guarantee, in the end, a highly compatible and efficient mutualistic symbiosis.
Since the mycorrhizal growth effect is also
dependent on the host plant, it is likely that
the host plant contributes, to some extent, to
the regulation of these fungal genetic determinants. The differential expression of some
secreted proteins, candidate effectors, on different host plants (Kamel et al. 2017; Zeng et al.
2018) already provides a support to this
hypothesis (Fig. 7.3).
Mateus et al. (2019) analyzed transcriptomes from five genetically different cultivars
of another important crop worldwide, cassava
(Manihot esculenta), each inoculated with two
different R. irregularis isolates to explore how
plant and fungal gene expression profiles are
affected by the intra-specific variability of the
partner organism. Interestingly, expression of
most plant genes responded in a different direction or magnitude depending on the plant
genotype. The abundance of several fungal
transcripts was also strongly influenced by the
genotype of the plant.
Comparative genomics and functional
genomics studies on several AM strains and
species will be instrumental to determine the
genetic, and possibly epigenetic, polymorphisms controlling the impact of specific AM
inocula on host plant performance. Hopefully,
evaluation of plant performance should not be
limited to the growth effect but also to other
traits provided by AM fungi, such as enhanced
tolerance to abiotic and abiotic stresses to cover
all the potential benefits of the symbiosis.
Exploring the genomic variations of AM
fungi is already giving important insights on
158
L. Lanfranco et al.
they have been proposed as foliar markers of
the AM association (Wang et al. 2018). Blumenols therefore represent an extremely powerful
tool for high-throughput screening for a functional AM symbiosis and can be used instead of
laborious measurements of AM-induced transcripts or microscopic analyses.
Small RNA (sRNA) molecules are also
attractive candidates for long-distance signaling and have been targeted by -omics analyses
in arbuscular mycorrhizas. So far investigations
focused on roots revealed a large-scale reprograming of microRNAs (miRNAs) upon AM
colonization (Devers et al. 2011; Wu et al.
2016; Pandey et al. 2018). In a recent work, the
analysis of putative targets of selected miRNAs
revealed an involvement in P starvation, phytohormone signaling, and defense (Pandey et al.
2018).
In conclusion, all these studies convincingly demonstrate that AM fungi have a local
and systemic influence on their host plant,
since they lead to a deep reorganization of the
plant biology acting on multiple transcriptomic, regulatory, and metabolomic pathways.
V. The AM Symbiosis in the Light of
Natural Variation
Despite the low morphological variation and
their large host range, AM fungal species and
isolates show different efficiency in promoting
plant performance; on the other hand, the plant
genotype has an important role in determining
the extent of plant responsiveness to the AM
symbiosis (Smith et al. 2004). AM fungi can
present high functional diversity: even isolates
belonging to the same species can exert on a
specific host plant different growth effects,
which can vary in amplitude and direction
(promotion or inhibition) (Hart and Reader
2002; Munkvold et al. 2004; Feddermann et al.
2008; Antunes et al. 2011; Hong et al. 2012). An
extensive comparative study, considering
56 AM isolates belonging to 6 different families
and 17 genera inoculated on 3 different host
plants, revealed that the plant growth response
could not be predicted from AM species identity or morphological traits, such as extra- and
intraradical fungal volumes (Koch et al. 2017).
It can be hypothesized that the functional variability of the fungal symbionts may rely on other
factors such as qualitative and/or quantitative
differences in the production of signaling molecules, such as chitin oligomers, and/or effectors, which are crucial for triggering the
symbiotic program and dampening the plant
immune response, and in the expression and
functioning of transporters and/or metabolic
enzymes which control nutrient exchanges
and may guarantee, in the end, a highly compatible and efficient mutualistic symbiosis.
Since the mycorrhizal growth effect is also
dependent on the host plant, it is likely that
the host plant contributes, to some extent, to
the regulation of these fungal genetic determinants. The differential expression of some
secreted proteins, candidate effectors, on different host plants (Kamel et al. 2017; Zeng et al.
2018) already provides a support to this
hypothesis (Fig. 7.3).
Mateus et al. (2019) analyzed transcriptomes from five genetically different cultivars
of another important crop worldwide, cassava
(Manihot esculenta), each inoculated with two
different R. irregularis isolates to explore how
plant and fungal gene expression profiles are
affected by the intra-specific variability of the
partner organism. Interestingly, expression of
most plant genes responded in a different direction or magnitude depending on the plant
genotype. The abundance of several fungal
transcripts was also strongly influenced by the
genotype of the plant.
Comparative genomics and functional
genomics studies on several AM strains and
species will be instrumental to determine the
genetic, and possibly epigenetic, polymorphisms controlling the impact of specific AM
inocula on host plant performance. Hopefully,
evaluation of plant performance should not be
limited to the growth effect but also to other
traits provided by AM fungi, such as enhanced
tolerance to abiotic and abiotic stresses to cover
all the potential benefits of the symbiosis.
Exploring the genomic variations of AM
fungi is already giving important insights on
158
L. Lanfranco et al.