Proposed trait-based frameworks for AM fungi
(Van Der Heijden & Scheublin, 2007) conducted the first comprehensive
review on AM fungal traits to predict plant growth and ecosystem
functioning. The authors provided a list of 13 AM fungal functional
traits, which they categorized into morphological traits (e.g.,hyphal length, rate and extent of root colonization, spore production)
and physiological traits (e.g. , fungal carbon acquisition, host
preference, nutrient uptake efficiency). Subsequently, (Behm & Kiers,
2014) noted substantial intraspecific trait variation among AM fungal
species (also see (Koch et al. , 2017)), complicating the
characterization of traits and their incorporation into functional trait
models. To address this issue, they proposed a five-part framework for
characterizing intraspecific trait variation of AM fungal species within
the context of nutrient cycling, based on experimental design and trait
measurement considerations. According to Behm and Kiers (2014), AM
fungal genetic units should be subjected to diverse environmental
conditions (e.g., host plants, soil nutrient concentrations). Subsequent
measurements would encompass the degree of variation, trait
reversibility, relationships among traits, the adaptive nature of
variation, and the potential for variation to evolve. Through these five
dimensions, researchers could map traits onto an evolutionary tree and
incorporate them into functional models for predicting nutrient cycling
dynamics.
Chaudhary et al., (2022) highlighted the challenges in defining traits
for organismal networks such as the mycorrhizal symbiosis. They proposed
a unified trait framework, complemented by a standardized vocabulary,
with the objective of establishing a clear connection between
trait-based mycorrhizal ecology, AM fungal niches and community assembly
rules. The authors categorized traits into three main groups:
morphological, physiological, and phenological. Within each of these
categories, they pinpointed distinctive mycorrhizal traits specific to
both the host plant (such as root:shoot ratio, growth form,
photosynthetic pathways) and the fungal partner (e.g., spore size,
hyphal length, and melanin content). Beyond these bifurcated traits by
plant or fungal traits, (Chaudhary et al. , 2022) introduced the
concept of mycorrhizal traits. These are unique attributes that emerge
during symbiosis and are co-dependent on both partners. They encompass
aspects such as root colonization-induced structures, plant mycorrhizal
response, and resource exchange rates. This novel framework provides an
enriched understanding of mycorrhizal ecology and serves as a basis for
the empirical framework proposed here.
(Chagnon et al. , 2013) put forth an AM fungal trait-based
framework building on Grime’s CSR (competitive, stress-tolerant,
ruderal) framework - which identifies stress, disturbance and
competition as the major filters driving trait selection and evolution
in natural communities. By allowing speculative connections to be made
regarding potential linkages between fungal traits (e.g., hyphal fusion,
sporulation phenology, carbon sink strength, growth rates, etc.) and
environmental filters (e.g., soil disturbances, scarce C transfer from
host, low soil pH), this framework could tentatively identify priority
traits for measurement, and combinations of host and fungal traits that
may lead to the highest mutual benefits. Building on the apparent
family-level conservatism of many traits or responses to environmental
filters, parallels were drawn between AM fungal major families and C, S
and R strategies. However, as stressed by Chagnon et al. (2013), this
family-to-strategy association is simplistic and unlikely to stand the
test of time. In addition, it several AM fungal families (e.g.,
Pacisporaceae, Entrophosporaceae, Diversisporaceae, or more basal
lineages in the Paraglomerales or the Archaeosporales). It also fails to
consider the relative distribution of different AM fungal families in
certain biomes or at certain latitudes. For example, Acaulosporais a common genus in the tropics, where it can be dominant both in
natural forests and under intensive land-use where ruderal traits are
crucial (e.g., (González-Cortés et al. , 2012). The primary
significance of the CSR framework in AM fungal trait-based ecology
should not be considered as merely a framework for associating families
with strategies. Instead, it should be recognized as a tool for
leveraging well-established life-history trade-offs in plant ecology to
pinpoint pertinent fungal traits that should be incorporated into our
research agenda.
We build upon prior frameworks, emphasizing that the discrepancies
observed among studies, stemming from non-standardized experimental
approaches and the absence of a comprehensive database on AM fungal
traits, represent significant barriers to achieving a more predictive
understanding of AM fungal ecology. Moreover, the validity and relevance
of the isolates and species employed in these studies are reliant on the
taxa available in culture collections or from a few natural communities.
A deliberate inclusion of numerous uncultured taxa, or other taxa
hitherto overlooked fungal mutualisms in conjunction with AM
fungi—such as Mucoromycotina, as suggested by (Hoysted et al. ,
2023) —remains an important task. Given the existing data showing
large variability in plant and soil responses to the AM symbiosis both
among and within AM fungal species, we must address these issues to
assess if, and to what extent, AM fungal traits determine growth
responses or effects on ecosystems.