The way forward on AM fungal traits research
Considering the above discussion and to overcome challenges related to
measurement of AM fungal traits, and to obtain a more accurate
understanding of AM fungal-plant interactions, we suggest the following
points for future research.
1) Create and maintain a centralized database of AM fungal
traits . This is a keystone task to integrate data and analyses to use
appropriate AM fungal traits to predict plant responses and ecosystem
processes. Databases are available on AM fungal traits but they are
limited to mycorrhizal type and intensity of root colonization primarily
in the early stages of plant development (Soudzilovskaia et al. ,
2020). We propose the development of a generic structure for the AM
fungal database for individual taxa stemming from the traits described
in Table 1 and that is consistent with the principles of the Observation
and Measurement Ontogeny (Madin et al. , 2007). The central
tabulation in this database is the taxonomy at the species and strain
level, and accession codes if available. Ancillary data are related to
a) the site of origin such as latitude and longitude, along with the
date and the observer b) its source either from field observations, a
culture collection or literature data, and c) metadata and information
about experimental treatments used to measure traits including, but not
limited to, the levels of replication and variation associated with each
estimate. A trait and measurement component integrates all information
related to a specific trait (spore, mycelium, arbuscules, vesicles) and
its measurement values and units. A trait database could facilitate the
synthesis of research findings and improve our understanding of the
functional diversity and ecological roles of AMF taxa.
2) Expand the scope of research to include a broader range of AM
fungi, with a particular focus on uncultured and underrepresented taxa .
Traits have been studied focusing on Glomeraceae, Acaulosporaceae, and
Gigasporaceae, all common components of AM fungal communities and
harboring 76% of the total number of species in the Glomeromycota.
Other families such as Diversisporaceae, Claroideoglomeraceae, and basal
families such as Paraglomeraceae and Archaeosporaceae are rarely
included in experiments and there is very little information on their
traits, despite being common components of AM fungal communities.
Glomeromycota comprise 13 families and 337 species but likely contain
many more (Öpik et al. , 2010). However, we estimate that onlyca. 88 species are represented in culture collections worldwide.
To ensure a more comprehensive understanding of AM fungal ecological
roles and interactions, it is essential to record traits from taxa in
these poorly studied families and attempt to increase taxonomic
diversity in culture collections.
3) Determine traits at fine levels of taxonomic resolution .
Results from distinct experimental approaches indicate that AM fungal
traits related with internal and external mycelium and sporulation time
exhibit some conservation at the family level, although variation within
these clades was also observed (Hart & Reader, 2002a; Maherali &
Klironomos, 2012). Seemingly inconsistent with this finding, high
intraspecific variability in root colonization, external mycelium length
and plant responses has been demonstrated for several species (Mensahet al. , 2015; Schoen et al. , 2021; Stahlhut et al. ,
2023). As argued above, there is a need to conduct additional
comparative studies using different species within the same genus to
investigate trait conservatism.
4) Measure and report AM fungal traits using standardized
experimental approaches . AM fungal traits reported in the literature
have been measured using different experimental approaches, which makes
comparisons difficult. We propose herein (Table 2) a set of minimum
parameters that can be used when studying AM fungal traits that can
minimize biases and account for the complex interplay between fungal and
plant traits. We expect that by adopting these standard approaches
database data supplied by different laboratories are more directly
comparable. An obvious research imperative will be to validate the
reproducibility of AM fungal trait measurements by different
teams/infrastructures but using the same starting inoculum material.
This reinforces the value of culture collections.
5) Determine the variability (plasticity) of AM fungal trait
expression . A gap in our understanding of AM fungal traits, partially
responsible for our inability to predict symbiotic function under field
conditions, lies in our knowledge of how consistent traits are under
varying environmental conditions. Experiments designed to test how
specific environmental factors impact AM fungal traits are needed.
Additionally, the scope of trait variations across different traits and
taxa could be studied to determine if some traits are more conserved
than others and/or vary more significantly in one taxon compared to
others.
6) Embrace AM fungal community diversity : A basic premise of
ecophysiology is that environmental filters will select for specific
traits/adaptations (Lambers et al. , 2008). Given that some traits
can be measured at the community level (e.g., hyphal nutrient
stoichiometry (Zhang et al. , 2023b)), it can be envisaged to
conduct experiments (physical disturbance, nutrient additions, drought
etc.) on whole natural AM fungal communities and look at correlations
between environmental filters and traits (Chagnon, 2023). Coupled with
rotating and static cores (Johnson et al. 2001), these experiments could
also assess AM fungal growth and mycorrhizal function. With synthesis
studies identifying major drivers of AM fungal community structure at
global scales (Davison et al., 2021), the next frontier is to move
beyond taxonomy and assess the functional biogeography (e.g., (Violleet al. , 2014) of AM fungi.
7) Utilize AM fungal isolates deposited in culture collections.Culture collections worldwide uphold a considerable variety of AM fungal
isolates in terms of physiology, genetics, taxonomy, and geographic
origin. These isolates, which are cultivated either in a mineral
substrate or in conjunction with root organ cultures, are well
characterized taxonomically, thereby representing important resources in
comparative studies of traits. These centers are instrumental in
training personnel, and specific workshops can be developed to provide
hands-on experience and theoretical knowledge about measurement of AM
fungal traits.
8) The use of microphotography and machine learning: The integration of
microphotography and machine learning algorithms could not only help
standardize and accelerate trait quantification but also eliminate the
subjectivity of the observer, a common issue entangled with our current
quantification approaches. Successful integration of the two can create
automations that will allow for large dataset acquisition, no longer
limited by space and time (e.g., continuous growth measurements of ERM
and its traits, or continuous progression of root colonization with the
help of fluorescent markers). These approaches can help reveal
behavioral patterns that have so far remained undetected due to
technical limitations.
As mentioned earlier, some ecologically significant functions of the AM
fungal symbiosis, such as the ability to promote host plant growth,
depend not only on the properties of an individual AM fungal isolate but
also on its interaction with the host. While we recognize that this may
limit the scope of some trait measurements, we consider that even though
a hypothesis-driven approach as outlined in this study for effectively
assessing AM fungal traits may not address all questions about the role
and impact of AM fungi in ecology, it represents a significant step
towards that goal.