Extending the root system: incorporating microbial communities
associated with roots
The root-associated microbiome, i.e., the diverse community of
micro-organisms tightly associated with roots (Aleklett and Hart 2013),
is of particular interest to biodiversity research given the important
functional roles played by root associated microbes in root and
rhizosphere processes. In this issue, several studies look into the
relationships between soil, rhizosphere and root endophytic microbial
community composition, root functional traits, and soil properties.
Mafa-Attoye et al. (2022) confirm a rhizosphere effect (i.e., microbial
changes in the soil directly around the roots mediated by root activity
(Hiltner 1904)), demonstrating substantial differentiation in terms of
community composition and functioning between rhizosphere and bulk soil
microbial communities (although only for bacterial and archaeal, but not
fungal communities) across four agro-ecological systems. In contrast,
Merino-Martín and collaborators (2022) only observed differences in
fungal, but not bacterial communities, between rhizosphere and bulk soil
along an elevation gradient. This rhizosphere effect is also partly
related to variation in inter- (Mafa-Attoye et al. 2022, Merino-Martín
et al. 2022) and intraspecific (Gagliardi et al. 2022, Leroy et al.
2022) variation in root morphological traits that are found here to
predict the community composition of the root-associated microbiome.
Despite these important contributions, important open questions remain.
For instance, the collaboration axis is a key dimension in the RES, but
it only explicitly accounts for symbiotic and not pathogenic
interactions, which may also drive (interspecific) differences in root
traits (Dai et al. 2022). Furthermore, the RES framework predicts a
greater abundance of symbiotic microbes in thick roots (i.e.,
representing the ‘outsourcing’ strategy) at the interspecific level, but
Gagliardi and co-workers (2022) found the opposite pattern
intra-specifically. Additional variables, such as plant ontogeny (see
Leroy et al. (2022)); the spatial scale at which plant-microbe
interactions are studied (see Merino-Martín et al. (2022)); and the
necessity to invest in plant root pathogen defense (Dai et al. 2022) may
provide valuable insights in unraveling the extent to which soil
properties, root traits, and microbial communities (and associated
functioning) interact (in)directly, and ultimately, shape plant
community diversity and ecosystem functioning.
The publications in this Special Issue Root traits and
functioning: from individual plants to ecosystems comprise a diversity
of plant belowground traits (Table 1) that are involved in the different
functional processes of plants (water and nutrient uptake; plant
defenses against below- and aboveground pathogens; anchorage;
competitive capacities; belowground symbioses) and ecosystems (soil
stability; belowground productivity; vegetation dynamics). Building on
different and novel methodological approaches (from meta-analyses and
machine-learning to greenhouse experiments and field study) and systems
(crops, agro-ecological settings and natural conditions), they expand
our global belowground datasets (Table 1). Together, they underwrite the
multidimensionality of the belowground world of plants across biomes
(from alpine tundra vegetation to tropical forests); taxonomic (from the
plant community to the intraspecific level) and spatial (from microbial
processes, to the whole plant and ecosystem) scales and maintain the
momentum of root ecology.