Contextual basis
The scientific literature on the life-history traits of AM fungi (i.e., the biological characteristics and features that influence their growth, reproduction, and survival) has predominantly centered on aspects related to plant growth and nutrition, largely through an agronomic lens. Although not explicitly reported as such, early studies employing experimental approaches to assess, for example, AM fungal root colonization, abundance of external hyphae, and spore counts for specific species under certain experimental conditions have yielded insights into AM fungal trait variation (Abbott, 1982; Reich, 1988; Jakobsen et al. , 1992a; Gazey et al. , 1992; Bever et al. , 1996). Given the wide variation observed, these and other seminal studies provided a foundation for further inquiry into the complex dynamics of AM fungal life-history traits and their broader implications to the AM symbiosis.
Studies of distinct traits within a taxonomic framework started with the comparison of mycelium form and function, and root colonization strategies among major families of the Glomeromycota. For example, (Doddet al. , 2000) compared the morphology and mycelial architecture of different AM fungal genera, discussing form and function, (Hart & Reader, 2002b) showed in a comparative study of 21 AM fungal isolates (i.e., an AM fungus isolated in the laboratory into pure culture but without genetic characterization, at which point it becomes a certified strain with a collection number) spanning 16 species from North America that the isolates of the Glomeraceae family, on average, colonized roots before those of Acaulosporaceae and Gigasporaceae families. Additionally, the proportion of fungal biomass in roots versus soil also diverged, on average, among isolates of those families. Those in the Glomeraceae exhibited high root colonization but low soil colonization, Gigasporaceae tended to have low root colonization but high soil colonization, while Acaulosporaceae displayed low colonization in both roots and soil. These findings revealed a strong association between AM fungal function and taxonomy for these fungi, as isolates from the main families could be differentiated based on colonization rate, biomass allocation, and the onset of sporulation. These observations were corroborated by subsequent studies, albeit using AM fungi from the same community and, possibly, the same isolations (Hart & Reader, 2002b, 2005; Maherali & Klironomos, 2007; Powell et al. , 2009; Sikeset al. , 2009). In fact, using the same data, Aguilar-Trigueros et al. 2019, showed that large-spore species produced, on average, fewer spores than small-spore species, suggesting that AM fungi experience similar resource allocation constraints during reproduction as plants seeds (Moles et al. , 2005). Results from these studies suggest differences between Glomeraceae and Gigasporaceae concerning life-history traits and their relationship with host benefits. However, new comparative studies that include more fungal species isolated from other ecological contexts are necessary to confirm these differences.
The patterns described above underscore the utility of employing a comparative framework to test hypotheses concerning AM fungal function by examining trait expression. For instance, based on soil mycelium production, Gigasporaceae would be expected to outperform Glomeraceae in nutrient uptake (Maherali and Klironomos, 2007). Alternatively, if early or extensive root colonization (with abundant coils/arbuscules) is more important for nutrient delivery to the host, then Glomeraceae could be more beneficial partners under nutrient limiting conditions (e.g., (Horsch et al. , 2023b). Despite inconsistencies among studies, which may to some extent be explained by variability in mycorrhizal dependency among hosts (Pringle & Bever, 2008; Sikes et al. , 2009), a meta-analysis (Yang et al. , 2017) suggested that, on average, Glomeraceae were better at acquiring P and reducing pathogen growth compared to other AM fungal families. Is it also of interest, that this family also appears to be the most abundant in many locations (Öpik et al. , 2010).
The preceding studies lacked a broader environmental perspective. For instance, considering diverse environmental conditions, such as varying soil types or climatic factors, could unveil how AM fungal traits respond and adapt. Currently, most data reporting the impact of different AM fungi on their host originate from short-term experiments, using fungal taxa that readily sporulate and are easily amenable to pure cultures (Ohsowski et al. , 2014) which may not reflect the reality in natural environments. The study by Sikes et al. (2009) investigating differences in plant pathogen protection between AM fungal taxa, as well as that of (Lerat et al. , 2003) on C-sink strength among different AM fungal families suggests that certain functional outcomes resulting from the symbiosis depend on the combination of plant and fungal traits (Johnson et al. , 1997). As such, considering fungal traits alone (i.e., in absence of plant and soil characteristics) may limit predictions of functional outcomes of the symbiosis. This brings an additional layer of complexity to the study of AM fungal ecophysiology, or trait-based ecology, as intricate relationships between fungal and plant traits are to be expected (Chagnon et al. , 2013).