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.