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).