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.